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PHYSIOLOGY OF

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Originally appearing in Volume V21, Page 754 of the 1911 Encyclopedia Britannica.
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PHYSIOLOGY OF PLANTS The so-called vegetable physiology of a generation ago was in grrear of animal, and particularly of human, physiology, thestudy of the latter being followed by many more observers, and from its relative degree of advancement being the more capable of rapid development. It was fully recognized by its followers that the dominating influence in the structure and working of the body was the protoplasm, and the division of labour which it exhibited, with the accompanying or resulting differentiation into various tissues, was the special subject of investigation. Many who followed the study of vegetable structure did not at that time give an equal prominence to this view. The early histological researches of botanists led them to the recognition of the vegetable cell, and the leading writers in the middle of the 19th century pointed out the probable identity of Von Mohl's " protoplasm " with the " sarcode " of zoologists. They laid great stress on the nitrogenous nature of protoplasm, and noted that it preceded the formation of the cell-membrane. But by the ordinary student of thirty years later their work was to some extent overlooked, and the cell-wall assumed a prominence to which it was not entitled. The study of the differentiation of protoplasm was at that time seldom undertaken, and no particular attention was paid either to fixing it, to enable staining methods to be accurately applied to it, or to studying the action of chemical reagents upon it. It is only comparatively recently that the methods of histological investigation used by animal physiologists have been carefully and systematically applied to the study of the vegetable organ-isms. They have, however, been attended with wonderful results, and have revolutionized the whole study of vegetable structure. They have emphasized the statements of Von Mohl, Cohn, and other writers alluded to, that the protoplasm is here also the dominant factor of the body, and that all the peculiarities of the cell-wall can only be interpreted in the light of the needs of the living substance. The Nature of the Organization of the Plant, and the Relations of the Cell-Membrane and the Protoplasm.—This view of the structure of the plant and this method of investigation lead us to a greatly modified conception of its organization, and afford more completely an explanation of the peculiarities of form found in the vegetable kingdom. The study of simple organisms, many of which consist of nothing but a little mass of protoplasm, exhibiting a very rudimentary degree of differentiation, so far as our methods enable us to determine any at all, shows that the duties of existence can be discharged in the absence of any cell-wall. Those organisms which possess the latter are a little higher in the scale of life than those which remain unclothed by it, but a comparison of the behaviour of the two quickly enables us to say that the membrane is of but secondary importance, and that for those which possess it, it is nothing more than a protective covering for the living substance. Its physical properties, permeability by water, extensibility and elasticity, receive their interpretation in the needs of the latter. We come, accordingly, to regard it as practically an exoskeleton, and its functions as distinctly subordinate to those of the protoplasm which it clothes. If we pass a little higher up the scale of life we meet with forms consisting of two or more cells, each of which contains a similar minute mass of living substance. A study of them shows that each is practically independent of the others; in fact, the connexion between them is so slight that they can separate and each become free without the slightest disadvantage to another. So long as they are connected together mechanically they have apparently the power of influencing one another in various ways, and of passing liquid or gaseous materials from one to another. The conjoined organism is, in fact, a colony or association of the protoplasmic units, though each unit retains its independence. When we pass, again, from these to examine more bulky, and consequently more complex, plants, we find that the differences which can be observed between them and the simple lowly forms are capable of being referred to the increased number of the protoplasmic units and the consequent enlarged bulk of the mass or colony. Every plant is thus found to be composed of a number of these protoplasmic units, or, as they may preferably be termed, protoplasts, all of which are at first exactly alike in appearance and in properties. This is evident in the case; of such plants as have a body consisting of filaments or plates of cells, and is little less conspicuous in tho'se whose mass is but small, though the cells are evidently capable of computation in three dimensions. It does not at first appear to be the same with the bulkier plants, such as the ordinary green herbs, shrubs or trees, but a study of their earlier development indicates that they do not at the outset differ in any way from the simple undifferentiated forms. Each commences its existence as a simple naked protoplast, in the embroyo-sac or the archegonium, as the case may be. After the curious fusion with another similar protoplast, which constitutes what we call fertilization, the next stage in complexity already noted may be observed, the protoplasm becoming clothed by a cell-membrane. Very soon the single cell gives rise to a chain of cells, and this in turn to a cell mass, the individual units of which are at first quite uniform. With increase of number, however, and consequently enlargement of bulk in the colony, differentiation becomes compulsory. The requirements of the several protoplasts must be met by supplies from without, and, as many of them are deep seated, varieties of need arise, so that various members of the colony are set apart for special duties, masses of them being devoted to the discharge of one function, others to that of another, and so on. Such limitations of the powers and properties of the individuals have for their object the well-being of the community of which those individuals are constituents. Physiological and Morphological Differentiation.—The first indication of this differentiation in the vegetative body of the plant can be seen not only in the terrestrial green plants which have been particularly referred to, but also in the bulkier sea-weeds. It is an extension of the first differentiation which was observable in the simple protoplasts first discussed, the formation, that is, of a protective covering. Fucus and its allies, which form conspicuous members of the larger Algae, have their external cells much smaller, more closely put together, and generally much denser than the rest of their tissue. In the lowly as well as the higher green plants we have evidence of special= ization of the external protoplasts for the same purpose, which takes various shapes and shows different degrees of completeness, culminating in the elaborate barks which clothe our forest trees. The second prominent differentiation which presents itself takes the form of a provision to supply the living substance with water. This is a primal necessity of the protoplast, and every cell gives evidence of its need by adopting one of the various ways in which such need is supplied. What little differentiation can be found to exist in the protoplasm of the simple unicellular organism shows the importance of an adequate water-supply, and indeed, the dependence of life upon it. The naked cells which have been alluded to live in water, and call therefore for no differentiation in connexion with this necessity; but those which are surrounded by a cell-wall always develop within themselves a vacuole or cavity which occupies the greater part of their interior, and the hydrostatic pressure of whose contents keeps the protoplasm in contact with the membrane, setting up a condition of turgidity. The need for a constant supply of water is partly based upon the constitution of protoplasm, so far as we know it. The apparently structureless substance is saturated with it; and if once a cell is completely dried, even at a low temperature, in the enormous majority of cases its life is gone and the restoration of water fails to enable it to recover. Besides this intimate relationship, however, we can point to other features of the necessity for a constantly renewed water supply. The protoplasm derives its food from substances in solution in the water; the various waste products which are incident to its life are excreted into it, and so removed from the sphere of its activity. The raw materials from which the food is constructed are absorbed from the exterior in solution in water, and the latter is the medium through which the gaseous constituents necessary for life reach the protoplasm. Moreover, growth is essentiallydependent upon water-supply. There is little wonder, then, that in a colony of protoplasts such as constitute a large plant a considerable degree of differentiation is evident, bearing upon the question of water supply. Certain cells of the exterior are set apart for absorption of water from the soil, this being the source from which supplies are derived. Others are devoted to the work of carrying it to the protoplasts situated in the interior and at the extremities of the plant, a conducting system of considerable complexity being the result. Other collections of cells are in many cases set apart for giving rigidity and strength to the mass of the plant. It is evident that as the latter increases in bulk, more and more attention must be paid to the dangers of uprooting by winds and storms. Various mechanisms have been adopted in different cases, some connected with the subterranean and others with the sub-aerial portions of the plant. Another kind of differentiation in such a cell-mass as we are dealing with is the setting apart of particular groups of cells for various metabolic purposes. We have the formation of numerous mechanisms which have arisen in connexion with the question of food supply, which may not only involve particular cells, but also lead to differentiation in the protoplasm of those cells, as in the development of the chloroplastids of the leaves and other green parts. The inter-relations of the members of a large colony of protoplasts such as constitute a tree, demand much adjustment. Relations with the exterior are continually changing, and the needs of different regions of the interior are continually varying, from time to time. Two features which are essentially protoplasmic assume a great importance when we consider these relations. They are the power of receiving impressions or stimuli from the exterior, and of communicating with each other, with the view of co-ordinating a suitable response. We have nothing structural which corresponds to the former of these. In this matter, differentiation has proceeded very differently in animals and plants respectively, no nerves or sense organs being structurally recognizable. Communication between the various protoplasts of the colony is, however, carried on by means of fine protoplasmic threads, which are continuous through the cell-walls. All the peculiarities of structure which we encounter consequently support the view with which we started, that the protoplasm of the plant is the dominant factor in vegetable structure, and that there need be but one subject of physiology, which must embrace the behaviour of protoplasm wherever found. There can be no doubt that there is no fundamental difference between the living substance of animals and plants, for many forms exist which cannot be referred with certainty to either kingdom. Free-swimming organisms without cell-membranes exist in both, and from them series of forms can be traced in both directions. Cellulose, the material of which vegetable cell-walls are almost universally composed, at any rate in their early condition, is known to occur, though only seldom, among animal organisms. Such forms as Volvox and the group of the Myxomycetes have been continually referred to both kingdoms, and their true systematic position is still a subject of controversy. All physiology, consequently, must be based upon the identity of the protoplasm of all living beings. - This method of study has to a large extent modified our ideas of the relative importance of the parts of such an organism as a large tree. The interest with which we regard the latter no longer turns upon the details of the structure of its trunk, limbs and roots, to which the living substance of the more superficial parts was subordinated. Instead of regarding these as only ministering to the construction of the bulky portions, the living protoplasts take the first place as the essential portion of the tree, and all the other features are important mainly as ministering to their individual well-being and to their multiplication. The latter feature is the growth of the tree, the well-being of the protoplasts is its life and health. The interest passes from the bulky dense interior, with the elaborate features of its cell-walls, to the superficial parts, where its life is in evidence. We see herein the reason for the great subdivision of the body, with its finely cut twigs' and their ultimate expansions, the leaves, and we recognize that this subdivision is only an expression of the need to place the living substance in direct relationship with the environment. The formation and gradually increasing thickness of its bark are explained by the continually increasing need of adequate protection to the living cortex, under the strain of the increasing framework which the enormous multiplication of its living protoplasts demands, and the development of which leads to continual rupture of the exterior. The increasing development of the wood as the tree grows older is largely due to the demands for the conduction of water and mineral matters dissolved in it, which are made by the increased number of leaves which from year to year it bears, and which must each be put into communication with the central mass by the formation of new vascular bundles. Similar considerations apply to the peculiar features of the root-system. All these points of structure can only be correctly interpreted after a consideration of the needs of the individual protoplasts, and of the large colony of which they are members. Gaseous Interchanges and their Mechanism.—Another feature of the construction of the plant has in recent years come into greater prominence than was formerly the case. The organism is largely dependent for its vital processes upon gaseous inter-changes. It must receive a large constituent of what ultimately becomes its food from the air which surrounds it, and it must also take in from the same source the oxygen of its respiratory processes. On the other hand, the aerial environment presents considerable danger to the young and tender parts, where the protoplasts are most exposed to extremes of heat, cold, wet, &c. These must in some way be harmonized. No doubt the primary object of the cell-wall of even the humblest protoplast is pro- tection, and this too is the meaning of the coarser tegumentary employed in the evaporation of the water, the leaf being thus structures of a bulkier plant. These vary considerably in I kept cool. Whether the leaf is brightly or only moderately completeness with its age; in its younger parts the outer cells wall undergoes the change known as cuticularization, the material being changed both in chemical composition and in physical properties. The corky layers which take so prominent a share in the formation of the bark are similarly modified and subserve the same purpose. But these protective layers are in the main impermeable by gases and by either liquid or vapour, and prevent the access of either to the protoplasts which need them. Investigations carried out by Blackman, and by Brown and Escombe, have shown clearly that the view put forward by Boussingault, that such absorption of gases takes place through the cuticular covering of the younger parts of the plant, is erroneous and can no longer be supported. The difficulty is solved by the provision of a complete system of minute intercellular spaces which form a continuous series of delicate canals between the cells, extending throughout the whole substance of the plant. Every protoplast, except in the very young regions, has part of its surface abutting on these, so that its wall is accessible to the gases necessary for its vital processes. There is no need for cuticularization here, as the external dangerous influences do not reach the interior, and the processes of absorption which Boussingault attributed to the external cuticularized cells can take place freely through the, delicate cell-walls of the interior, saturated as these are with water. This system of channels is in communication with the outer atmosphere through numerous small apertures, known as stomata, which are abundant upon the leaves and young twigs, and gaseous interchange between the plant and the air is by their assistance rendered constant and safe. This system of intercellular spaces, extending throughout the plant, constitutes a reservoir, charged with an atmosphere which differs somewhat in its composition from the external air, its gaseous constituents varying from time to time and from place to place, in consequence of the interchanges between itself and the protoplasts. It constitutes practically the exterior environment of the protoplasts, though it is ramifying through the interior of the plant. The importance of this provision in the case of aquatic vascular plants of sturdy bulk is even greater than in that of terrestrial organisms, as their environment offers considerable obstacles to the renewal of the air in their interior. They are without stomata on their submerged portions, and the entry of gases can only take place by diffusion from the water through their external cells, which are not cuticularized. Those which are only partially submerged bear stomata on their exposed portions, so that their environment approximates towards that of a terrestrial plant, but the communication even in their case is much less easy and complete, so that they need a much larger reservoir of air in their interior. This is secured by the development of much larger intercellular spaces, amounting to lacunae or passages of very considerable size, which are found ramifying in different ways in their interior. Transpiration.—In the case of terrestrial plants, the continual renewal of the water contained in the vacuoles of the protoplasts demands a copious and continuous evaporation. This serves a double purpose, bringing up from the soil continually a supply of the soluble mineral matters necessary for their metabolic processes, which only enter the plant in solutions of extreme dilution, and at the same time keeping the plant cool by the process of evaporation. The latter function has been found to be of extreme importance in the case of plants exposed to the direct access of the sun's rays, the heat of which would rapidly cause the death of the protoplasts were it not employed in the evaporation of the water. Brown and Escombe have shown that the amount of solar energy taken up by a green leaf may often be fifty times as much as it can utilize in the constructive processes of which it is the seat. If the heat were allowed to accumulate in the leaf unchecked, they have computed that its temperature would rise during bright sunshine at the rate of more than 2° C. per minute, with of course very rapidly fatal results. What is not used in the constructive processes is illuminated, the same relative proportions of the total energy absorbed are devoted to the purposes of composition and construction respectively. This large evaporation, which constitutes the so-called transpiration of plants, takes place not into the external air but into this same intercellular space system, being possible only through the delicate cell-walls upon which it abuts, as the external coating, whether bark, cork or cuticle, is impermeable by watery vapour. The latter ultimately reaches the external air by diffusion through the stomata, whose dimensions vary in proportion as the amount of water in the epidermal cells becomes greater or less. Mechanism and Function of Stomata.—it is not quite exact to speak of either the gaseous interchanges or the transpiration as taking place through the stomata. The entry of gases into, and exit from, the cells, as well as the actual exhalation of watery vapour from the latter, take place in the intercellular space system of which the stomata are the outlets. The opening and closing of the stomata is the result of variation in the turgidity of their guard cells, which is immediately affected by the condition of turgidity of the cells of the epidermis contiguous to them. The amount of watery vapour in the air passing through a stoma has no effect upon it, as the surfaces of the guard cells abutting on the air chamber are strongly cuticularized, and there-fore impermeable. The only way in which their turgidity is modified is by the entry of water into them from the contiguous cells of the general epidermis and its subsequent withdrawal through the same channel. This opening and closing of the stomata must be looked upon as having a direct bearing only on the emission of watery vapour. There is a distinct advantage in the regulation of this escape, and the mechanism is directly connected with the greater or smaller quantity of water in the plant, and especially in its epidermal cells. This power of varying the area of the apertures by which gases enter the internal reservoirs is not advantageous to the gaseous interchanges—indeed it may be directly the reverse. It may lead to an incipient asphyxiation, as the supply of oxygen may be greatly interfered with and the escape of carbon dioxide may be almost stopped. It may at other times lead to great difficulties in the supply of the gaseous constituents which are used in the manufacture of food. The importance of transpiration, is, however, so great, that these risks must be run. The Ascent of Water in Trees.—The supply of water to the peripheral protoplasts of a tree is consequently of the first importance. The means by which such a supply is ensured are by no means clearly understood, but many agencies are probably at work. The natural source of the water is in all cases the soil, and few plants normally obtain any from elsewhere. The water of the soil, which in well-drained soil is met with in the form of delicate films surrounding the particles of solid matter, is absorbed into the plant by the delicate hairs borne by the young roots, the entry being effected by a process of modified osmosis. Multitudes of such hairs on the branches of the roots cause the entry of great quantities of water, which by a subsequent similar osmotic action accumulates in the cortex of the roots. The great turgidity which is thus caused exerts a considerable hydrostatic pressure on the stele of the root, the vessels of the wood of which are sometimes filled with water, but at other times contain air, and this often under a pressure less than the ordinary atmospheric pressure. This pressure of the turgid cortex on the central stele is known as root pressure, and is of very considerable amount. This pressure leads to the filling of the vessels of the wood of both root and stem in the early part of the year, before the leaves have expanded, and gives rise to the exudation of fluid known as bleeding when young stems are cut in early spring. Root pressure is one of the forces co-operating in the forcing of the water upwards. The evaporation which is associated with transpiration is no doubt another, but by themselves they are insufficient to explain the process of lifting water to the tops of tall trees. There is at present also a want of agreement among botanists as to the path which the water takes in the structural elements of the tree, two views being held. The older is that the water travels in the woody cell-walls of the vascular bundles, mainly under the action of the forces of root pressure and transpiration, and that the cavities of the vessels contain only air. The other is that the vessels are not empty, but that the water travels in their cavities, which contain columns of water in the course of which are large bubbles of air. On this view the water flows upwards under the influence of variations of pressure and tension in the vessels. These forces however fail to furnish a complete explanation of the ascent of the current, and others have been thought to supplement them, which have more or less weight. Westermaier and Godlewski put forward the view that the living cells of the medullary rays of the wood, by a species of osmosis, act as a kind of pumping apparatus, by the aid of which the water is lifted to the top of the tree, a series of pumping-stations being formed. Though this at first met with some acceptance, Strasburger showed that the action goes on in great lengths of stem the cells of which have been killed by poison or by the action of heat. More recently, Dixon and Joly in Dublin and Askenasy in Germany have suggested the action of another force. They have shown that columns of water of very small diameter can so resist tensile strain that they can be lifted bodily instead of flowing along the channel. They suggest that the forces causing the movement are complex, and draw particular attention to the pull upwards in consequence of disturbances in the leaves. In these we have (I) the evaporation from the damp delicate cell-walls into the intercellular spaces; (2) the imbibition by the cell-wall of water from the vacuole; (3) osmotic action, consequent upon the subsequent increased concentration of the cell sap, drawing water from the wood cells or vessels which abut upon the leaf parenchyma. They do not, of course, deny the co-operation of the other forces which have been suggested, except so far as these are inconsistent with the motion of the water in the form of separate columns rather than a flowing stream. This view requires the existence of certain anatomical arrangements to secure the isolation of the separate columns, and cannot be said to be fully established. Nature of the Food of Plants.—The recognition of the fundamental identity of the living substance in animals and plants has directed attention to the manner in which plants are nourished, and especially to the exact nature of their food. The idea was till recently currently accepted, that anything which plants absorbed from without, andwhich went to build up their organic substance, or to supply them with energy, or to exert some beneficial influence upon their metabolism, constituted their food. Now, as the materials which plants absorb are carbon dioxide from the air, and various inorganic compounds from the soil, together with water, it is clear that if this view is correct, vegetable protoplasm must be fed in a very different way from animal, and on very different materials. A study of the whole vegetable kingdom, however, negatives the theory that the compounds absorbed are in the strict sense to be called food. Fungal and phanerogamic parasites can make no use of such substances as carbon dioxide, but draw elaborated products from the bodies of their hosts. Those Fungi which are saprophytic can only live when supplied with organic compounds of some complexity, which they derive from decomposing animal or vegetable matter. Even in the higher flowering plants, in which the processes of the absorption of substances from the environment has been most fully studied, there is a stage in their life in which the nutritive processes approximate very closely to those of the group last mentioned. When the young sporophyte first begins its independent life—when, that is, it exists in the form of the embryo in the seed—its living substance has no power of utilizing the simple inorganic compounds spoken of. Its nutritive pabulum is supplied to it in the shape of certain complex organic substances which have been stored in some part or other of the seed, sometimes even in its own tissues, by the parent plant from which it springs. When the tuber of a potato begins to germinate the shoots which it puts out derive their food from' the accumulated store of nutritive material which has been laid up in the cells of the tuber. If we examine the seat of active growth in a young root or twig, we find that the cells in which the organic substance, the protoplasm, of the plant is being formed and increased, are not supplied with carbon dioxide and mineral matter, but with such elaborated material as sugar and proteid substances, or others closely allied to them. Identity of the Food of Animals and Plants.—It is evidently to the actual seats of consumption of food, and of consequent nutrition and increase of living substance, that we should turn when we wish to inquire what are the nutritive materials of plants. If we go back to the first instance cited, the embryo in the seed and its development during germination, we can ascertain what is necessary for its life by inquiring what are the materials which are deposited in the seed, and which become exhausted by consumption as growth and development proceed. We find them to consist of representatives of the great classes of foodstuffs on which animal protoplasm is nourished, and whose presence renders seeds such valuable material for animal consumption. They are mainly carbohydrates such as starch and sugar, proteids in the form of globulins or albumoses, and in many cases fats and oils, while certain other bodies of similar nutritive value are less widely distributed. The differences between the nutritive processes of the animal and the plant are not therefore fundamental, as they were formerly held to be. The general vegetable protoplasm has not the capacity of being nourished by inorganic substances which are denied to the living substance of the animal world. Differences connected with the mode of supply of nutritive material do exist, but they are mainly correlated with the structure of the organisms, which makes the method of absorption different. The cell-walls of plants render the entry of solid material into the organism impossible. The food must enter in solution in order to pass the walls. Moreover, the stationary habit of plants, and the almost total absence of locomotion, makes it impossible for them to seek their food. The Special Apparatus of Plants for constructing Food: The explanation of the apparent difference of food supply is very simple. Plants are furnished with a constructive mechanism by which they are enabled to fabricate the food on which they live from the inorganic, gaseous and liquid matters which they absorb. The fact of such absorption does not render these substances food; they are taken in not as food, but as raw materials to be subjected to the action of this constructive mechanism, and by it to be converted into substances that can nourish protoplasm, both vegetable and animal. It is sometimes forgotten, when discussing questions of animal nutrition, that all the food materials of all living organisms are prepared originally from inorganic substances in exactly the same way, in exactly the same place, and by the same machinery, which is the chlorophyll apparatus of the vegetable kingdom. A consideration of these facts emphasizes still more fully the view with which we set out, that all living substance is fundamentally, the same, though differentiated both anatomically and physiologically in many directions and in different degrees. All is nourished alike on materials originally prepared by a mechanism attached to the higher vegetable organism, and capable of being dissociated, in theory at least, from its own special means of nutrition, if by the latter term we understand the appropriation by the protoplasm of the materials so constructed. The chlorophyll apparatus of plants demands a certain description. It consists essentially of a number of minute corpuscles or plastids, the protoplasmic substance of which is impregnated with a green colouring matter. These bodies, known technically as chloroplasts, are found embedded in the protoplasm of the cells of the mesophyll of foliage leaves, of certain of the cells of some of the leaves of the flower, and of the cortex of the young twigs and petioles. Usually they are absent from the cells of the epidermis, though in some of the lower plants they are met with there also. The plastids are not rigidly embedded in the cytoplasm, but are capable of a certain amount of movement therein. Each is a small protoplasmic body, in the meshes of whose substance the green colouring matter chlorophyll is contained in some form of solution. Various solvents, such as benzene, alcohol and chloroform, will dissolve out the pigment, leaving the plastid colourless. Chlorophyll is not soluble in water, nor in acids or alkalies without decomposition. These plastids are especially charged with the duty of manufacturing carbohydrates from the carbon dioxide which the air contains, and which is absorbed from it after it has entered the intercellular passages and has so reached the cells containing the plastids. This action is found to take place only in the presence of light, preferably moderate sunlight. The reason for the distribution of the chloroplasts described above is consequently seen. The relation of the chlorophyll to light has been studied by many observers. If a solution of the pigment is placed in the path of a beam of light which is then allowed to fall on a prism, the resulting spectrum will be found to be modified. Instead of presenting the appearance of a continuous band in which all the colours are represented, it is interrupted by seven vertical dark spaces. The rays which in the absence of the solution of chlorophyll would have occupied those spaces have no power to pass through it, or in other words chlorophyll absorbs those particular rays of light which are missing. The absorption of these rays implies that the pigment absorbs radiant energy from the sun, and gives us some explanation of its power of constructing the carbohydrates which has been mentioned as the special work of the apparatus. The working of it is not at all completely understood at present, nor can we say exactly what is the part played by the pigment and what is the role of the protoplasm of the plastid. It is not certain either whether the action of the chlorophyll apparatus is confined to the manufacture of carbohydrates or whether it is concerned, and if so how far, with the construction of proteids also. As the action of the chlorophyll apparatus is directly dependent upon light, and the immediate result of its activity is the building up of complex compounds, it has become usual to speak of the processes it sets up under the name of photosynthesis. Pkotosynthesis.—In the presence of light and when the plant is subjected to a suitable temperature, photosynthesis commences, provided that the plant has access to air containing its normal amount of carbon dioxide, about 3 parts, or rather less, in 10,000. The process involves the iqter-action of water also, and this, as we have seen, is always present in the cell. In addition, certain in-organic salts, particularly certain compounds of potassium, are apparently necessary, but they seem to take no part in the chemical changes which take place. The original hypothesis of Baeyer suggested that the course of events is the following: the carbon dioxide is decomposed into carbon monoxide and oxygen, while water is simultaneously split up into hydrogen and oxygen; the hydrogen and the carbon monoxide unite to form formaldehyde and the oxygen is exhaled. This explanation is unsatisfactory from many points of view, but till quite recently no acceptable alternative has been advanced. There is no evidence that carbon monoxide is ever produced, indeed there are strong reasons for disbelieving in its occurrence. The formation of formaldehyde has till recently not been satisfactorily proved, though it has been obtained from certain leaves by distillation. Certain Algae have been found capable of forming nutritive carbohydrates in darkness, when supplied with a compound of this body with sodium-hydrogen-sulphite. But it is certain that it can only be present in a cell in very small amount at any moment, for an extremely dilute solution acts as a poison to protoplasm. If formed, as it probably is, it is immediately changed into some more complex combination, and so rendered incapable of exerting its poisonous action. Baeyer's hypothesis was entertained by botanists partly because it explained the gaseous interchanges accompanying photosynthesis. These show that a definite intake of carbon dioxide is always accompanied by an exhalation of an equal volume of oxygen. Recent investigations have confirmed Baeyer's view of the formation of formaldehyde, but a different explanation has been recently advanced. The first chemical change suggested is an interaction between carbon dioxide and water, under the influence of light acting through chlorophyll, which leads to the simultaneous formation of formaldehyde and hydrogen peroxide. The formaldehyde at once undergoes a process of condensation or polymerization by the protoplasm of the plastid, while the hydrogen peroxide is said to be decomposed into water and free oxygen by another agency in the cell, of the nature of one of the enzymes of which we shall speak later. Polymerization of the aldehyde was also a feature of Baeyer's hypothesis, so that this view does not very materially differ from those he advanced. More emphasis is, however, now laid on the action of the plastid in polymerization, while the initial stages are still not definitely explained. The steps which lead from the appearance of formaldehyde to that of the first well-defined carbohydrate are again matters of speculation. There are many possibilities, but no definite body of simpler composition than a sugar has so far been detected. Noris the nature of the first formed sugar certain; the general opinion has been that it is a simple hexose such as glucose or fructose, CsH12Os. Brown and Morris in 1892 advanced strong reasons for thinking that cane-sugar, C12H22O11, is the first carbohydrate synthesized, and that the hexoses found in the plant result from the decomposition of this. The whole story of the different sugars existing in the plant—their relations, and their several functions—requires renewed investigation. The first visible carbohydrate formed, one which appears so rapidly on the commencement of photosynthesis as to have been regarded as the first evidence of the setting up of the process, is starch. This is met with in the form of small granular specks in the substance of the chloroplast, specks which assume a blue colour when treated with a solution of iodine. Its very prompt appearance, as soon as the apparatus became active, led to the opinion formerly held, that the work of the latter was complete only when the starch was formed. We have seen that the starch is preceded by the formation of sugar, and its appearance is now interpreted as a sign of surplus manufacture. As much sugar as is produced in excess of the immediate requirements of the cell is converted into the insoluble form of starch by the plastidsof the chlorophyll apparatus, and is so withdrawn from the sphere of action, thereby enabling the construction of further quantities of sugar to take place. The presence of too much sugar in solution in the sap of the cell inhibits the activity of the chloroplasts; hence the necessity for its removal. Starch, indeed, wherever it appears in the plant seems to be a reserve store of carbohydrate material, deposited where it is found for longer or shorter periods till it is needed for consumption. The readiness with which it is converted into sugar fits it especially to be a reserve or stored material. Proteid Formation.—We have seen that it has been suggested that the chlorophyll apparatus may perhaps be concerned in the manufacture of proteids as well as of carbohydrates. If not, there must exist in the green plant, side by side with it, another mechanism which is concerned with the manufacture of the complex compounds in which nitrogen is present. The independence of the two is suggested by the fact that fungi can live, thrive and grow in nutritive media which contain carbohydrates together with certain salts of ammonia, but which are free from proteids. It is certain that their protoplasm cannot be nourished by inorganic compounds of nitrogen, any more than that of animals. We must therefore surmise their possession of a mechanism which can construct proteids, if supplied with these compounds of nitrogen together with sugar. The probability is that this mechanism is to be found in green plants in the leaves—at any rate there is a certain body of evidence pointing in this direction. It may be, however, that there is no special mechanism, but that this power is a particular differentiation of a physiological kind, existing in all vegetable protoplasm, or in that of certain cells. The idea of an identity of protoplasm does not involve a denial of special powers developed in it in different situations, and the possession of such a power by the vegetable cell is not more striking than the location of the powers of co-ordination and thought in the protoplasm of cells of the human brain. But if we accept either view we have still to examine the process of construction in detail, with a view to ascertaining the stages by which proteid is built up. Here unfortunately we find ourselves in the region of speculation and hypothesis rather than in that of fact. The nitrogen is absorbed by the plant in some form of combination from the soil. The green plant prefers as a rule nitrates of various metals, such as calcium, magnesium or potassium. The fungus seems to do better when supplied with compounds of ammonia. The nitrogen of the atmosphere is not called into requisition, except by a few plants and under special conditions, as will be explained later. The fate of these inorganic compounds has not been certainly traced, but they give rise later on to the presence in the plant of various amino acid amides, such as leucin, Wycin, asparagin, &c. That these are stages on the way to proteids as been inferred from the fact that when proteids are split up by various means, and especially by the digestive secretions, these nitrogen-containing acids are among the products which result. While we know little of the processes of proteid-construction, we are almost completely in the dark also as to what are the particular proteids which are first constructed. Opinions are conflicting also as to the conditions under which proteids are formed. There is a certain amount of evidence that at any rate in some cases light is necessary, and that the violet rays of the spectrum are chiefly concerned. But the subject requires elucidation from both chemical and biological points of view. The normal green plant is seen thus to be in possession of a complete machinery for the manufacture of its own food. The way in which such food when manufactured is incorporated into, and enabled to build up, the living substance is again hidden in obscurity. This is, however, also the case with the nutrition of animal protoplasm. The building up and nutrition of the living substance by the foods manufactured or absorbed is properly spoken of as the assimilation of such food. Up to very recently the original absorption and subsequent treatment of the carbon dioxide and the compounds of nitrogen has been called by the same term. We frequently find the expression used, "the `assimilation' of carbon dioxide, or of nitrogen." As this is not the incorporation of either into the living substance, but is only its manufacture into the complex substances which we find in the plant, it seems preferable to limit the term " assimilation " to the processes by which foods are actually taken into the protoplasm. Symbiosis.—Though green plants thus possess a very complete mechanism for the manufacture of their different foodstuffs, it is not always exercised to the fullest extent. Many of them are known to supplement it, and some almost entirely to replace it, by absorbing the food they need in a fully prepared condition from their environment. It may be that they procure it from decomposing organic matter in the soil, or they may get it by absorption from other plants to which they attach themselves, or they may in rare cases obtain it by preying upon insect life. The power of green plants, not even specialized in any of these directions, to absorb certain carbohydrates, particularly sugars, trom the soil was demonstrated by Acton in 1889. Similar observations have been made in the case of various compounds of nitrogen, though these have not been so complex as the proteids. It was formerly the custom to regard as parasites all those plants which inserted roots or root-like organs into the tissues of other plants and absorbed the contents of the latter. The most conspicuous case, perhaps, of all these is the mistletoe, which flourishes luxuriantly upon the apple, the poplar and other trees. Bonnier has drawn attention to the fact that the mistletoe in its turn, remaining green in the winter, contributes food material to its host when the latter has lost its leaves. The relationship thus existing he showed to be mutually beneficial, each at one time or another supplying the necessities of the other. Such a relationship is known as symbiosis, and the large majority of the cases of so-called parasitism among green plants can be referred to it. Bonnier showed that the same relationship could be proved in the cases of such plants as the rattle (Rhinanthus), the eye-bright (Euphrasie), and other members of the Natural Orders, Scrophulariaceae and Santalaceae, which effect a union between their roots and the roots of other plants growing near them. The union taking place underground, while the bulk of both partners in the symbiosis rises into the air, renders the association a little difficult to see, but there is no doubt that the plants in question do afford each other assistance, forming, as it were, a kind of partnership. The most pronounced case of parasitism, that of Cuscuta, the dodder, which infests particularly clover fields, appears to differ only in degree from those mentioned, for the plant, bare of leaves as it is, yet contains a little chlorophyll. The advantages it can offer to its host are, however, infinitesimal when compared with the injury it does it. Many other cases of symbiosis have been investigated with some completeness, especially those in which lower plants than the Phanerogams are concerned. The relations of the Alga and the Fungus, which have formed a close associationship in the structure known as the Lichen, were established many years ago. Since about 188o our knowledge of the species which can enter into such relationships has been materially extended, and the fungal constituents of the Lichens are known to include Basidiomycetes as well as Ascomycetes. Mycorhizas.—The most interesting cases, however, in which Fungi form symbiotic relationships with green plants have been discovered in connexion with forest trees. The roots of many of the latter, while growing freely in the soil are found to be surrounded with a dense feltwork of fungal mycelium, which sometimes forms a mass of considerable size. The plants showing it are not all forest trees, but include also some Pteridophytes and some of the prothallia of the Ferns, Club-mosses, Liverworts and Horsetails. The true nature of the relationship was first recognized by Pfeffer in 1877, but few cases were known till recent years. Very complete examination, however, has now been made of many instances, and the name mycorhiza has been given to the symbiotic union. Two classes are recognized. In the first, which are called ectotropie, the fungal filaments form a thick felt or sheath round the root, either completely enclosing it or leaving the apex free. They seldom penetrate the living cells, though they do so in a few cases. The root-hairs penetrate between masses of the hyphae of the Fungus. This type of mycorhiza is found among the Poplars, Oaks q.nd Fir trees. The other type is called endctropic. The fungal filaments either penetrate the epidermis of the root, or enter it from the stem and ramify in the interior. Some make their way through the cells of the outer part of the cortex towards the root-tip, and form a mycelium or feltwork of hyphae, which generally occupies two or three layers of cells. From this branches pass into the middle region of the cortex and ramify through the interior half of its cells. They often cause a considerable hypertrophy of the tissue. From the outer cortical mycelium, again, branches pass through the epidermis and grow out in the soil, In such cases the roots of the plants are usually found spreading in soils which contain a large amount of humus, or decaying vegetable matter. The organic compounds of the latter are absorbed by the protruding fungal filaments, which take the place of root-hairs, the tree ceasing to develop the latter. The food so absorbed passes to the outer cortical mycelium, and from this to the inner hyphae, which appear to be the organs of the interchange of substance, for they are attracted to the neighbourhood of the nuclei of the cells, which they enter, and in which they form agglomerations of interwoven filaments. The prothalli of the Pterido-phytes, which form similar symbioses, show a somewhat different mode of arrangement, the Fungi occupying the external or the lower layers of the thalloid body. The discovery of the widespread occurrence of this mycorhizal symbiosis must be held to be one of the most important results of research upon the nutritive processes of plants during the closing decade of the 19th century. Among green plants the symbionts include Conifers, Orchids, Heaths, Oaks, Poplars and Beeches, though all do not derive equal advantages from the association. Monotropas afford an extreme case of it, having lost their chlorophyll almost entirely, and come to depend upon the Fungi for their nutriment. The fungal constituents vary considerably. Each species of green plant may form a mycorhiza with two or three different Fungi, and a single species of Fungus may enter into symbiosis with several green plants. The Fungi that have been discovered taking part in the union include Eurotium, Pythium, Boletus, Agaricus, Lactarius, Penicillium and many others of less frequent occurrence. All the known species belong to the Oomycetes, the Pyrenomycetes, the Hymenomycetes or the Gasteromycetes. The habit of forming mycorhizas is found more frequently in warm climates than cold; indeed, the percentage of the flora exhibiting this peculiarity seems to increase with a certain regularity from the Arctic Circle to the equator. Fixation of Nitrogen.—Another, and perhaps an even more important, instance of symbiotic association has come to the front during the same period. It is an alliance between the plants of the Natural Order Leguminosae and certain bacterium-like forms which find a home within the tissues of their roots. The importance of the symbiosis can only be understood by considering the relationship in which plants stand with regard to the free nitrogen of the air. Long ago the view that this gas might be the source of the combined nitrogen found in different forms within the plant, was critically examined, particularly by Boussingault, and later by Lawes and Gilbert and by Pugh, and it was ascertained to be erroneous, the plants only taking nitrogen into their substance when it is presented to their roots in the form of nitrates of various metals, or compounds of ammonia. Many writers in recent years, among whom may be named especially Hellriegel and Wilfarth, Lawes and Gilbert, and Schleesing and Laurent, have shown that the Leguminosae as a group form conspicuous exceptions to this rule. While they are quite capable of taking up nitrates from the soil where and so long as these are present, they can grow and thrive in soil which contains no combined nitrogen at all, deriving their supplies of this element in these cases from the air. The phenomena have been the subject of very careful and critical examination for many years, and may be regarded as satisfactorily established. The power of fixing atmospheric nitrogen by the higher plants seems to be confined to this solitary group, though it has been stated by various observers with more or less emphasis that it is shared by others. Frank has claimed to have found oats, buckbeans, spurry, turnips, mustard, potatoes and Norway maples exercising it; Nobbe and others have Imputed its possession to Elaeagnus. There is little direct evidence pointing to this extension of the power, and many experimenters directly contradict the statements of Frank. The power exercised by the Leguminosae is associated with' the presence of curious tubercular swellings upon their roots, which are developed at a very early age, as they are cultivated in ordinary soil. If experimental plants are grown in sterilized soil, these swellings do not appear, and the plant can then use no atmospheric nitrogen. The swellings have been found to be due to a curious hypertrophy of the tissue of the part, the cells being filled with an immense number of minute bacterium-like organisms of V, X or Y shape. The development of these structures has been studied by many observers, both in England and on the continent of Europe. They appear to be present in large numbers in the soil, and to infect the Leguminous plant by attacking its root-hairs. One of these hairs can be seen to be penetrated at a particular spot, and the entering body is then found to grow along the length of the hair till it reaches the cortex of the root. It has the appearance of a delicate tube which has granular contents, and is provided withan apex that appears to be open. The wall of the tube is very thin and delicate, and does not seem to be composed of cellulose or any modification of it. Careful staining shows that the granular substance of the interior really consists of a large number of delicate rod-like bodies. As the tube grows down the hair it maintains its own independence, and does not fuse with the contents of the root-hair, whose protoplasm re-mains quite distinct and separate. After making its way into the interior, the intruder sets up a considerable hyper trophy of the tissue, causing the formation of a tubercle, which soon shows a certain differentiation, branches of the vascular bundles of the root being supplied to it. The rod-like bodies from the interior of the tube, which has considerable resemblance to the zoogloea of many Bacteria, are liberated into the interior of the cells of the tubercle and fill it, increasing by a process of branching and fission. When this stage is reached the invading tubes and their ramifications frequently disappear, leaving the cells full of the bacterioids, as they have been called. When the root dies later such of these as remain are discharged into the soil, and are then ready to infect new plants. In some cases the zoogloea thread or tube has not been seen, the organ-ism consisting entirely of the bacterioids. This peculiar relationship suggests at once a symbiosis, the Fungus gaining its nutriment mainly or entirely from the green plant, while the latter in some way or other is able to utilize the free nitrogen of ;he air. The exact way in which the utilization or fixation of the nitrogen is effected remains undecided. Two views are still receiving ertain support, though the second of them appears the more probable. These are: (1) That the green plant is so stimulated by the symbiotic association which leads to the hypertrophy, that it is able to fix the nitrogen or cause it to enter into combination. (2) That the fixation of the gas is carried out by the fungal organism either in the soil or in the plant, and the nitrogenous substance so produced is absorbed by the organism, which is in turn consumed by the green plant. Certain evidence which supports this view will be referred to later. Whichever opinion is held on this point, there seems no room for doubt that the fixation of the nitrogen is concerned only with the root, and that the green leaves take no part in it. The nodules, in particular, appear to play the important part in the process. Mar-shall Ward has directed attention to several points of their structure which bear out this view. They are supplied with a regular system of conducting vascular bundles communicating with those of the roots. Their cells during the period of incubation of the symbiotic organism are abundantly supplied with starch. The cells in which the fungoid organism is vigorously flourishing are exceedingly active, showing large size, brilliant nuclei, protoplasm and vacuole, all of which give signs of intense metabolic activity. The sap in these active tissues is alkaline, which has been interpreted as being in accordance with Lcew's suggestion that the living protoplasm in presence of an alkali and free nitrogen can build up ammonium nitrate, or some similar body. It is, however, at present entirely unknown what substances are formed at the expense of the atmospheric nitrogen. The idea that the atmospheric nitrogen is gradually being made use of by plants, although it is clearly not easily or commonly utilized, has been growing steadily. Besides the phenomena of the symbiosis just discussed, certain experiments tend to show that we have a constant fixation of this gas in the soil by various Bacteria. Researches which have been carried out since 1885 by Berthelot, Andree, Laurent and Schleesing, and more recently by Kossowitsch, seem to establish the fact, though the details of the process remain undiscovered. Berthelot imputes it to the action of several species of soil Bacteria and Fungi, including the Bacterium of the Leguminosae, when the latter is cultivated free from its ordinary host. Laurent and Schlcesing affirm that the free nitrogen of the air can be fixed by a number of humble green plants, principally lowly green Algae. They must be exposed freely to light and air during the process, or they fail to effect it. Frank has stated that Penicilli-rm cladiosporioides can flourish in a medium to which no nitrogen but that of the atmosphere has access. Kossowitsch claims to have proved that fixation of nitrogen takes place under the influence of a symbiosis of certain Algae and soil Bacteria, the process being much facilitated by the presence of sugar. The Algae include Nostoc, Cystococcus, Cylindrospermum and a few other torms. In the symbiosis the Algae are supplied with nitrogen by the bacteria, and in turn they construct carbohydrate material, part of which goes to the microbes. This is supported by the fact that if the mixed culture is placed in the light there is a greater fixation than when it is left in darkness. If there is a plentiful supply of carbon dioxide, more nitrogen is fixed. Nitrification and Denitrification in the Soil.—Another aspect of the nitrogen question has been the subject of much investigation and controversy since 1877. The round of changes which nitrogenous organic matter undergoes in the soil, and how it is ultimately made use of again by plants, presents some curious features. We have seen that when nitrogenous matter is present in the condition of humus, some plants can absorb it by their roots or by the aid of mycorhizas. But the changes in it in the usual course of nature are much more profound than these. It becomes in the soil the prey of various microbes. Ammonia appears immediately as a product of the disruption of the nitrogen-containing organic molecule. Later, oxidation processes take place, and the ammonia gives rise to nitrates, which are absorbed by plants. These two processes go on successively rather than simultaneously, so that it is only towards the end of the decomposition of the organic matter that nitrification of the ammonia which is formed is set up. In this process of nitrification we can distinguish two phases, first the formation of nitrites, and secondly their oxidation ton rates. The researches of Waring-ton in England and Winogradsky on the Continent have satisfactorily shown that two distinct organisms are concerned in it, and that probably more than one species of each exists. One of them comprising the genera Nitrosomonas and Nitrosococcus, has the power of oxidizing salts of ammonium to the condition of compounds of nitrous acid. When in a pure culture this stage has been reached no further oxidation takes place. The oxidation of the nitrites into nitrates is effected by another organism, much smaller than the first. The name Nitrobacter has been given to this genus, most of our knowledge of which is due to the researches of Winogradsky. The two lands of organism are usually both present in the same soil, those of the second type immediately oxidizing the nitrites which those of the first form from ammonium salts. The Nitro-batter forms not only cannot oxidize the latter bodies, but they are very injuriously affected by the presence of free ammonia. When cultivated upon a suitable nutritive material in the laboratory, the organism was killed by the presence of .015 % of this gas, and seriously inconvenienced by one-third as much. Except in this respect, however, the two classes show great similarity. A very interesting peculiarity attaching to them is their distaste for organic nutriment. They can be cultivated most readily on masses of gelatinous silica impregnated with the appropriate compounds of nitrogen, and their growth takes place most copiously in the absence of light. They need a little carbonate 'in the nutrient material, and the source of the carbon which is found in the increased bulk of the plant is partly that and partly the carbon dioxide of the air. We have in these plants a power which appears special to them, in the possession of some mechanism for the construction of organic substance which differs essentially from the chlorophyll apparatus of green plants, and yet brings about substantially similar results. The steps by which this carbon dioxide is built up into a compound capable of being assimilated by the protoplasm of the cells are not known. The energy for the purpose appears to be supplied by the oxidation of the molecules containing nitrogen, so that it is dependent upon such oxidation taking place. Winogradsky has investigated this point with great care, and he has come to the conclusion that about 35 milligrammes of nitrogen are oxidized for each miiligramme of carbon absorbed and fixed. Deposition and Digestion of Reserve Materials in Plants and Animals.—As we have seen, the tendency of recent research is to prove the identity of the mode of nutrition of vegetable and animal organisms. The material on which they feed is of the same description and its treatment in the body is precisely similar. In both groups we find the presence of nutritive material in two forms, one specially fitted for transport, the other for storage. We have seen that in the plant the processes of construction go on in the seats of manufacture faster than those of consumption. We have the surplus sugar, for instance, deposited as starch in the chloroplasts themselves. The manufacture goes on very actively so long as light shines upon the leaves, and we find towards night a very great surplus stored in the cells. This excess of manufacture is one of the features of plant life, and is exhibited, though in various degrees, by all green plants. The accumulated material is made to minister to the need of the plant in various ways; it may be by increasing the bulk of the plant, as by the formation of the wood of the trunk, branches and roots; or it may be by laying up a store of nutritive materials for purposes of propagation, as in tubers, corms, seeds, &c. In any case the surplus is continuously being removed from the seats of its construction and deposited for longer or shorter periods in other parts of the structure, usually near the regions at which its ultimate consumption will take place. We have the deposition of starch, aleurone grains, amorphous proteids, fats, &c., in the neighbourhood of growing points, cambium rings and phellogens; also the more prolonged storage in tubers, seeds and other reproductive bodies. Turning to the animal, we meet with similar pro-visions in the storage of glycogen in the liver and other parts, of fat in various internal regions, and so on. In both we find the reserve of food, so far as it is in excess of immediate need, existing in two conditions, the one suitable for transport, the other for storage, and we see continually the transformation of the one into the other. The formation of the storage form at the expense of the travelling stream is due to the activity of some protoplasmic structure—it may be a plastid or the general protoplasm of the cell—and is a process of secretion. The converse process is one of a true 'digestion, which deserves the name no less because it is intracellular. We find processes of digestion strictly comparable to those of the alimentary canal of an animal in the case of the insectivorous Nepenthes, Drosera and other similar plants, and in the saprophytic Fungi. Those which now concern us recall the utilization of the glycogen of the liver, the stored fats and proteids of other parts of the animal body being like them intracellular. Enzymes.—The agents which effect the digestive changes in plants have been studied with much care. They have been found to be mainly enzymes, which are in many cases identical with those of animal origin. A vast number of them have been discovered and investigated, and the majority call for a brief notice. Their number, indeed, renders it necessary to classify them, and rather to look at groups of them than to examine them one by one. They are usually classified according to the materials on which they work, and we may here notice especially four principal groups, the members of which take part in the digestion of reserve materials as well as in the processes of external digestion. These decompose respectively carbohydrates, glucosides, proteids and fats or oils. The action of the enzyme in nearly every case is one of hydration, the body acted on being made to take up water and to undergo a subsequent decomposition. Among those which act on carbohydrates the most important are; the two varieties of diastase, which convert starch into maltose or malt sugar; inulase, which forms fructose from inulin; invertase, which converts cane sugar into glucose (grape sugar) and fructose; glucase or maltase, which produces grape sugar from maltose; and cytase, which hydrolyses cellulose. Another enzyme which does not appear to be concerned with digestion so directly as the others is pectase, which forms vegetable jelly from pectic substances occurring in the cell-wall. The enzymes which act upon glucosides are many; the best known are emulsin and myrosin, which split up respectively amygdalin, the special glucoside of certain plants of the Rosaceae; and sinigrin, which has a wide distribution among those of the Cruciferae. Others of less frequent occurrence are erythrozym, rhamnase and gaultherase. The proteolytic enzymes, or those which digest proteids, are usually divided into two groups, one which breaks down ordinary proteids into diffusible bodies, known as peptones, which are them-selves proteid in character. Such an enzyme is the pepsin of the stomach of the higher animals. The other group attacks these peptones and breaks them down into the amino-acids of which we have spoken before. This group is represented by the erepsin of the pancreas and other organs. A third enzyme, the trypsin of the pancreas, possesses the power of both pepsin and erepsin. The relationships existing between these enzymes are still the subjects of experiment, and we cannot regard them as exhaustively examined. It is not quite certain whether a true pepsin exists in plants, but many trypsins have been discovered, and one form of erepsin, at least, is very widespread. Among the trypsins we have the paladin of the Papaw fruit (Carica Papaya), the bromelin of the Pine-apple, and the enzymes present in many germinating seeds, in the seedlings of several plants, and in other parts. Another enzyme, rennet, which in the animal body is proteolytic, is frequently met with in plants, but its function has not been ascertained. The digestion of fat or oil has not been adequately investigated, but its decomposition in germinating seeds has been found to be due to an enzyme, which has been called lipase. It splits it into a fatty acid and glycerine, but seems to have no further action. The details of the further transformations have not yet been completely followed. Oxidases.—Another class of enzymes has been discovered in both animals and plants, but they do not apparently take any part in digestion. They set up a process of oxidation in the substances which they attack, and have consequently been named oxidases. Very little is known about them. In many cases the digestion of reserve food materials is effected by the direct action of the protoplasm, without the intervention of enzymes. This property of living substance can be proved in the case of the cells of the higher plants, but it is especially prominent in many of the more lowly organisms, such as the Bacteria. The processes of putrefaction may be alluded to as affording an instance of such a power in the vegetable organisms. At the same time it must be remembered that the secretion of enzymes by Bacteria is of widespread occurrence. Supply and Distribution of Energy in Plants.—It is well known that one of the conditions of life is the maintenance of the process which is known as respiration. It is marked by the constant and continuous absorption of a certain quantity of oxygen and by the exhalation of a certain volume of carbon dioxide and water vapour. There is no direct connexion between the two, the oxygen is absorbed almost immediately by the protoplasm, and appears to enter into some kind of chemical union with it. The protoplasm is in a condition of instability and is continually breaking down to a certain extent, giving rise to various substances of different degrees of complexity, some of which are again built up by it into its own substances, and others, more simple in composition, are given off. Of these carbon dioxide and water are the most prominent. These respiratory processes are associated with the liberation of energy by the protoplasm, energy which it applies to various purposes. The assimilation of complex foods consequently may be regarded as supplying the protoplasm with a potential store of energy, as well as building up its substance. Whenever complex bodies are built up from simple ones we have an absorption of energy in some form and its conversion into potential energy; whenever decomposition of complex bodies into simpler ones takes place we have the liberation of some or all of the energy that was used in their construction. Since about 188o considerable attention has been directed to the question of the supply, distribution and expenditure of energy in the vegetable kingdom. This is an extremely important question, since the supply of energy to the animal world has been found to depend entirely upon the vegetable one. The supply of energy to the several protoplasts which make up the body of a plant is as necessary as is the transport to them of the food they need; indeed, the two things are inseparably connected. The source of energy which is the only one accessible to the ordinary plant as a whole is the radiant energy of the rays of the sun, and its absorption is mainly due to the properties of chlorophyll. This colouring matter, as shown by its absorption spectrum, picks out of the ordinary beam of light a large proportion of its red and blue rays, together with some of the green and yellow. This energy is obtained especially by the chloroplastids, and part of it is at once devoted to the construction of carbohydrate material, being thus turned from the kinetic to the potential condition. The other constructive processes, which are dependent partly upon the oxidation of the carbohydrates so formed, and therefore upon an expenditure of part of such energy, also mark the storage of energy in the potential form. Indeed, the construction of protoplasm itself indicates the same thing. Thus even inthese constructive processes there occurs a constant passage of energy backwards and forwards from the kinetic to the potential condition and vice versa. The outcome of the whole round of changes, how-ever, is the fixation of a certain part of the radiant energy absorbed by the chlorophyll.' The rays of the visible spectrum do not supply all the energy which the plant obtains. It has been suggested by several botanists, with considerable plausibility, that the ultra-violet or chemical rays can be absorbed and utilized by the protoplasm without the intervention of any pigment such as chlorophyll. There is some evidence pointing to the existence of this power in the cells of the higher plants. Again, we have evidence of the power of plants to avail themselves of the heat rays. There is, no doubt, a direct interchange of heat between the plant and the air, which in many cases results in a gain of heat by the plant. Indeed, the tendency to absorb heat in this way, either from the air or directly from the sunlight, has already been pointed out as a danger which needs to be averted by transpiration. There is probably but little transformation of one form of kinetic energy into another in the plant. It has been suggested that the red pigment Anthocyan, which is found very commonly in young developing shoots, petioles and midribs, effects a conversion of light rays into heating ones, so facilitating the metabolic processes of the plant. This is, however, rather a matter of speculation. The various electrical phenomena of plants also are obscure. Certain plants possess another source of energy which is common to them and the animal world. This is the absorption of elaborated compounds from their environment, by whose decomposition the potential energy expended in their construction can be liberated. Such a source is commonly met with among the Fungi, the insectivorous plants, and such of the higher plants as have a saprophytic habit. This source is not, however, anything new, for the elaborated compounds so absorbed have been primarily constructed by other plants through the mechanism which has just been described. The question of the distribution of this stored energy to the separate protoplasts of the plant can be seen to be the same problem as the distribution of the food. The material and the energy go together, the decomposition of the one in the cell setting free the other, which is used at once in the vital processes of the cell, being in fact largely employed in constructing protoplasm or storing various products. The actual liberation in any cell is only very gradual, and generally takes the form of heat. The metabolic changes in the cells, however, concern other decompositions side by side with those which involve the building up of protoplasm from the products of which it feeds. So long as food is supplied the living substance is the seat of transformations which are continually proceeding, being partially decomposed and again constructed, the new food being incorporated into it. The changes involve a continual liberation of energy, which in most cases is caused by the respiration of the protoplasm and the oxidation of the substances it contains. The need of the protoplasm for oxygen has already been spoken of : in its absence death soon supervenes, respiration being stopped. Respiration, indeed, is the expression of the liberation of the potential energy of the protoplasm itself. It is not certain how far substances in the protoplasm are directly oxidized without entering into the composition of the living substance, though this appears to take place. Even their oxidation, however, is effected by the protoplasm acting as an oxygen carrier. The supply of oxygen to a plant is thus seen to be as directly connected with the utilization of the energy of a cell as is that of food concerned in its nutrition. If the access of oxygen to a protoplast is interfered with its normal respiration soon ceases, but frequently other changes supervene. The partial asphyxiation or suffocation stimulates the protoplasm to set up a new and perhaps supplementary series of decompositions, which result in the liberation of energy just as do those of the respiratory process. One of the constant features of respiration—the exhalation of carbon dioxide —can still be observed. This comes in almost all such cases from the decomposition of sugar, which is split up by the protoplasm into alcohol and carbon dioxide. Such decompositions are now generally spoken of as anaerobic respiration. The decomposition of the complex molecule of the sugar liberates a certain amount of energy, as can be seen from the study of the fermentation set tip by yeast, which is a process of this kind, in that it is intensified by the absence of oxygen. The liberated energy takes the form of heat, which raises the temperature of the fermenting wort. It has been ascertained that in many cases this decomposition is effected by the secretion of an enzyme, which has been termed zymase. This body has been prepared from active yeast, and from fruits and other parts which have been kept for some time in the absence of oxygen. The protoplasm appears to be able also to bring about the change without secreting any enzyme. Expenditure of Energy by Plants.—The energy of the plant is, as we have seen, derived originally from the kinetic radiant energy of the sun. In such cells as are capable of absorbing it, by virtue of their chlorophyll apparatus, the greater part of it is converted into the potential form, and by the transport from cell to cell of the compounds constructed every part of the plant is put into possession of the energy it needs. The store of energy thus accumulated and distributed has to subserve various purposes in the economy of the plant. A certain part of it is devoted to the maintenance of 752 the framework of the fabric of the cell, and the construction of a continuously increasing skeleton; part is used in maintaining the normal temperature of the plant, part in constructing various sub-stances which are met with in the interior, which serve various purposes in the working of the vital mechanism. A great part again is utilized in that increase of the body of the plant which we call growth. Growth, as usually spoken of, includes two essentially different processes. The first of these, which may be regarded as growth proper, is the manufacture of additional quantities of living sub-stance. The second, which is usually included in the term, is the increase of such accessories of living substance as are necessary for its well-being. These include cell walls and the various stored products found in growing cells. There is clearly a difference between these two categories. The formation of living substance is a process of building up from simple or relatively simple materials; the construction of its cellulose framework and supporting substance is done by the living substance after its own formation is completed, and is attended by a partial decomposition of such living substance. Growth is always going on in plants while they are alive. Even the oldest trees put out continually new leaves and twigs. It does not, of course, follow that increase of bulk is always conspicuous; in such trees death is present side by side with life, and the one often counterbalances the other. As, however, we can easily see that the constructive processes are much greater than those which lead to the disappearance of material from the plant-body, there is generally to be seen a conspicuous increase in the substance of the plant. This is, in nearly all cases, attended by a permanent change in form. This is not perhaps so evident in the case of axial organs as it is in that of leaves and their modifications, but even in them it can be detected to a certain extent. In the lowliest plants growth may be co-extensive with the plant-body; in all plants of any considerable size, however, it is localized in particular regions, and in them it is associated with the formation of new protoplasts or cells. These regions have been called growing points. In such stems and roots as increase in thickness there are other growing regions, which consist of cylindrical sheaths known as cambium layers or phellogens. By the multiplication of the protoplasts in these merismatic areas the substance of the plant is in-creased. In other words, as these growing regions consist of cells, the growth of the entire organ or plant will depend upon the behaviour of the cells or protoplasts of which the merismatic tissues are composed. The growth of such a cell will be found to depend mainly upon five conditions: (i) There must be a supply of nutritive or plastic materials, at the expense of which the increase of its living substance can take place, and which supply the needed potential energy. (2) There must be a supply of water to such an extent as to set up a certain hydrostatic pressure in the cell, for only turgid cells can grow. (3) The supply of water must be associated with the formation of osmotic substances in the cell, or it cannot be made to enter it. (q) The cell must have a certain temperature, for the activity of a protoplast is only possible within certain limits, which differ in the case of different plants. (5) There must be a supply of oxygen to the growing cell, for the protoplast is dependent upon this gas for the performance of its vital functions, and particularly for the liberation of the energy which is demanded in the constructive processes. This is evident from the consideration that the growth of the cells is attended by the growth in surface of the cell wall, and as the latter is a secretion from the protoplasm, such a decomposition cannot readily take place unless oxygen is admitted to it. When these conditions are present, the course of the growth of a cell appears to be the following: The young cell, immediately it is cut off from its fellow, absorbs water, in consequence of the presence in it of osmotically active substances. With the water it takes in the various nutritive substances which the former contains in solution. There is set up at once a certain hydrostatic pressure, due to the turgidity which ensues upon such absorption, and the extensible cell wall stretches, at first in all directions. The growth or increase of the protoplasm at the expense of the nutritive matter for a time keeps pace with the increased size of the cell, but by and by it be-comes vacuolated as more and more water is attracted into the interior. Eventually the protoplasm usually forms only a lining to the cell wall, and a large vacuole filled with cell sap 'occupies the centre. The growth of the protoplasm, though considerable, is therefore not commensurate with the increase in the size of the cell. The stretching of the cell wall by the hydrostatic pressure is fixed by a secretion of new particles and their deposition upon the original wall, which as it becomes slightly thicker is capable of still greater extension, much in the same way as a thick band of indiarubber is capable of undergoing greater stretching than a thin one. The increase in surface of the cell wall is thus due—firstly to the stretching caused by turgidity, and secondly to the formation and deposition of new substance upon the old. When the limit of extensibility is reached the cell wall increases in thickness from the continuation of the latter of the two processes. The rate of growth of a cell varies gradually throughout its course; it begins slowly, increases to a maximum, and then becomes slower till it stops. The time during which these regular changes in the rate can be observed is generally spoken of as the grand period of growth.[PHYSIOLOGY If we consider the behaviour of a growing organ such as a root, we find that, like a cell, it shows a grand period of growth. Just behind its apex the cells are found to be all in process of active division. Growth is small, and consists mainly in an increase of the quantity of protoplasm, for the cells divide again as soon as they have reached a certain size. As new cells are continually formed in the merismatic mass those which are farthest from the apex gradually cease to divide and a different process of growth takes place in them, which is associated more particularly with the formation of the vacuoles, consequent upon the establishment of considerable hydro-static pressure in them, thus causing the bulk of the cells to be greatly enlarged. Here it is that the actual extension in length of the root takes place, and the cells reach the maximum point of the grand period. They then gradually lose the power of growth, the oldest ones or those farthest from the apex parting with it first, and they pass gradually over into the condition of the permanent tissues. The same order of events may be ascertained to take place in the stem; but in this region it is complicated by the occurrence of nodes and internodes, growth in length being confined to the latter, many of which may be growing simultaneously. The region of growth in the stem is, as a rule, much longer than that of the root. The growth of the leaf is at first apical, but this is not very prolonged, and the subsequent enlargement is due to an intercalary growing region near the base. The turgidity in the cells of a growing member is not uniform, but shows a fairly rhythmical variation in its different parts. If the member is one which shows a difference of structure on two sides, such as a leaf, the two sides frequently show a difference of degree of turgidity, and consequently of rate of growth. If we consider a leaf of the common fern we find that in its young condition it is closely rolled up, the upper or ventral surface being quite concealed. As it gets older it gradually unfolds and expands into the adult form. This is due to the fact that while young the turgidity and consequent growth are greater in the dorsal side of the leaf, so that it becomes rolled up. As it develops the maximum turgidity and growth change to its upper side, and so it becomes unfolded or expanded. These two conditions are generally described under the names of hyponasty and epinasty respectively. Cylindrical organs may exhibit similar phenomena. One side of a stem may be more turgid than the opposite one, and the maximum turgidity, with its consequent growth, may alternate between two opposite sides. The growing apex of such a stem will alternately incline, first to one side and then to the other, exhibiting a kind of nodding movement in the two directions. More frequently the region of maximum turgidity passes gradually round the growing zone. The apex in this case will describe a circle, or rather a spiral, as it is elongating all the time, pointing to all points of the compass in succession. This continuous change of position has been called circumnutation, and is held to be universal in all growing cylindrical organs. The passage of the maximum turgidity round the stem may vary in rapidity in different places, causing the circle to be replaced by an ellipse. The bending to two sides alternately, described above, often called simple nutation, may be regarded as only an extreme instance of the latter. Nervous System of Plants.—So far we have considered the plant almost exclusively as an individual organism, carrying out its own vital processes, and unaffected by its surroundings except in so far as these supply it with the materials for its well-being. When we consider, however, the great variability in those surroundings and the consequent changes a plant must encounter, it appears obvious that interaction and adjustment between the plant and its environment must be constant and well balanced. That such adjustment shall take place postulates on the part of the plant a kind of perception or appreciation of the changing conditions which affect it. Careful examination soon shows an observer that such perceptions exist, and that they are followed by certain purposeful changes in the plant, sometimes mechanical, sometimes chemical, the object being evidently to secure some advantage for the plant, to ward off some danger, or to extricate it from some difficulty. We may speak, indeed, of the plant as possessed of a rudimentary nervous system, by the aid of which necessary adjustments are brought about. The most constantly occurring changes that beset a plant are connected with illumination, temperature, moisture, and contact with foreign bodies. Setting aside other susceptibilities, we have evidence that most plants are sensitive to all these. If a growing stem receives stronger illumination on one side than another, its apex slowly turns from the vertical in the direction of the light source, continuing its change of position until it is in a direct line with the incident rays. If a root is similarly illuminated, a similar change of direction of growth follows, but in this case the organ grows away from the light. These movements are spoken of as heliotropic and apheliotropic curvatures. The purpose of the movements bears out the contention that the plant is trying to adjust itself to its environment. The stem, by pointing directly to the light source, secures the best illumination possible for all of its leaves, the latter being distributed symmetrically around it. The root is made to press its way into the darker cracks and crannies of the soil, so bringing its root-hairs into better contact with the particles round which the hygroscopic water hangs. Leaves respond in another way to the same influence, placing themselves across the path of the beam of light. Similar sensitivenesses can be demonstrated in other cases. When a root comes in contact at its tip with some hard body, such as might impede its progress, a curvature of the growing part is set up, which takes the young tip away from the stone, or what-not, with which it is in contact. When a sensitive tendril comes into contact with a foreign body, its growth becomes so modified that it twines round it. Many instances might be given of appreciation of and response to other changes in the environment by the growing parts of plants; among them we may mention the opening and closing of flowers during the days of their expansion. One somewhat similar phenomenon, differing in a few respects, marks the relation of the plant to the attraction of gravity. Observation of germinating seedlings makes it clear that somehow they have a perception of direction. The young roots grow vertically downwards, the young stems vertically upwards. Any attempt to interfere with these directions, by placing the seedlings in abnormal positions, is frustrated by the seedlings themselves, which change their direction of growth by bringing about curvatures of the different parts of their axes, so that the root soon grows vertically downward again and the stem in the opposite direction. Other and older plants give evidence of the same perception, though they do not respond all in the same way. Speaking generally, stems grow upwards and roots downwards. But some steins grow parallel to the surface of the soil, while the branches both of stems and roots tend to grow at a definite angle to the main axis from which they come. These movements are spoken of as different kinds of geotropic curvatures. This power of perception and response is not by any means confined to the growing organs, though in these it is especially striking, and plays a very evident part in the disposition of the growing organs in advantageous positions. It can, however, be seen in adult organs, though instances are less numerous. When the pinnate leaf of a Mimosa pudica, the so-called sensitive plant, is pinched or struck, the leaf droops rapidly and the leaflets become approximated together, so that their upper surfaces are in contact. The extent to which the disturbance spreads depends on the violence of the stimulation—it may be confined to a few leaflets or it may extend to all the leaves of the plant. The leaves and leaflets of many plants, e.g. the telegraph plant, Desmodium gvra;zs, behave in a similar way under the stimulus of approaching darkness. A peculiar sensitiveness is manifested by the leaves of the so-called insectivorous plants. In the case of Dionaea muscipula we find a two-lobed lamina, the two lobes being connected by a midrib, which can play the part of a kind of hinge. Six sensitive hairs spring from the upper surface of the lobes, three from each; when one of these is touched the two lobes rapidly close, bringing their upper surfaces into contact and imprisoning any-thing which for the' moment is between them. The mechanism is applied to the capture of insects alighting on the leaf. Drosera, another of this insectivorous group, has leaves which are furnished with long glandular tentacles. When these are excited by the settling of an insect on the leaf they slowly bend over and imprison the intruder, which is detained there mean-while by a sticky excretion poured out by the glands. In both these cases the stimulation is followed, not only by movement, but by the secretion of an acid liquid containing a digestive juice, by virtue of which the insect is digested after being killed. The purposeful character of all these movements or changes of position indicates that they are of nervous origin. We have in them evidence of two factors, a perception of some features of the environment and following this, after a longer or shorter interval, a response calculated to secure some advantage to the responding organ. We find on further investigation that these two conditions are traceable to different parts of the organs concerned. The perception of the changes, or, in other words, the reception of the stimulus, is associated for example, with the tips of roots and the apices of stems. The first recognition of a specially receptive part was made by Charles Darwin, who identified the perception of stimulation with the tip of the young growing root. Amputation of this part involved the cessation of the response, even when the conditions normally causing the stimulation were maintained. Francis Darwin later demonstrated that the tips of the plumules of grasses were sensitive parts. The responding part is situated some little distance farther back, being in fact the region where growth is active. This bending part has been proved to be insensitive to the stimuli. There is consequently a transmission of the stimulus from the sensitive organ to a kind of motor mechanism situated some little way off. We find thus three factors of a nervous mechanism present, a receptive, a conducting, and a responding part. The differentiation of the plant's substance so indicated is, however, physiological only; there is no histological difference between the cells of these regions that can be associated with the several properties they possess. Even the root tip, which shows a certain differentiation into root cap and root apex, cannot be said to be a definite sense organ in the same way as the sense organs of an animal. The root is continually growing and so the sensitive part is continually changing its composition, cells being formed, growing and becoming permanent tissue. The cells of the tip at any given moment may be sensitive, but in a few days the power of receiving the stimulus has passed to other and younger cells which then constitute the tip. The power of appreciating the environment is therefore to be associated with the protoplasm only at a particular stage of its development and is transitory in its character. What the nature of the stimulation is we are not able to say. The protoplasm is sensitive to particular influences, perhaps of vibration, or of contact or of chemical action. We can imagine though perhaps only vaguely, the way in which light, temperature, moisture, contact, &c., can affect it. The perception of direction or the influence of gravity presents greater difficulty, as we have no clear idea of the form which the force of gravity takes. Recently some investigations by Haberlandt, Noll, Darwin and others have suggested an explanation which has much to recommend it. The sensitive cells must clearly be influenced in some way by weight—not the weight of external organs but of some weight within them. This may possibly be the cell sap in their interior, which must exercise a slightly different hydrostatic pressure on the basal and. the lateral walls of the cells. Or more probably it may be the weight of definite particulate structures in their vacuoles. Many experimehts point to certain small grains of starch which are capable of displacement as the position of the cell is altered. Such small granules have been observed in the sensitive cells, and there is an evident correlation between these and the power of receiving the geotropic stimulus. It has been shown that if the organ containing them is shaken for some time, so that the contact between them and the protoplasm of the cells is emphasized, the stimulus becomes more efficient in producing movement. This reduces the stimulus to one of contact, which is in harmony with the observations made upon roots similarly stimulated from the exterior. The stimulating particles, whether starch grains in all cases, or other particles as well, have been termed statoliths. We have spoken of the absence of structural differentiation in the sense organs. There is a similar difficulty in tracing the paths by which the impulses are transmitted to the growing and curving regions. The conduction of such stimulation to parts removed some distance from the sense organ suggests paths of transmission comparable to those which transmit nervous impulses in animals. Again, the degree of differentiation is very slight anatomically, but delicate protoplasmic threads have been shown to extend through all cell-walls, connecting together all the protoplasts of a plant. These may well serve as conductors of nervous impulses. The nervous mechanism thus formed is very rudimentary, but in an organism the conditions of whose life render locomotion impossible great elaboration would seem superfluous. There is, however, very great delicacy of perception or appreciation on the part of the sense organ, stimuli being responded to which are quite incapable of impressing themselves upon the most highly differentiated animal. The power of response is seen most easily in the case of young growing organs, and the parts which show the motor mechanism are mainly the young growing cells. We do not find their behaviour like that of the motor mechanism of an animal. The active contraction of muscular tissue has no counterpart in the plant. The peculiarity of the protoplasm in almost every cell is that it is especially active in the regulation of its permeability by water. Under different conditions it can retain it more strongly or allow it to escape more freely. This regulation of turgor is as characteristic of vegetable protoplasm as contraction is of muscle. The response to the stimulus takes the form of increasing the permeability of particular cells of the growing structures, and so modifying the degree of the turgidity that is the precursor of growth in them. The extent of the area affected and of the variation in the turgor depends upon many circumstances, but we have no doubt that in the process of modifying its own permeability by some molecular change we have the counterpart of muscular contractibility. The response made by the adult parts of plants, to which reference has been made, is brought about by a mechanism similar in nature though rather differently applied. If the leaf of Mimosa or Desmodium be examined, it will be seen that at the base of each leaflet and each leaf, just at the junction with the respective axes, is a swelling known as a pulvinus. This has a relatively large development of succulent parenchyma on its upper and lower sides. In the erect position of the leaf the lower side has its cells extremely turgid, and the pulvinus thus forms a cushion, holding up the petiole. On stimulation these cells part with their water, the lower side of the organ becomes flaccid and the weight of the leaf causes it to fall. The small pulvini of the leaflets, by similar changes of the distribution of turgidity, take up their respective positions after receiving the stimulus. In some cases the two sides of the pulvini vary their turgidity in turns; in others only the lower side becomes modified. Similar turgescence changes, taking place with similar rapidity in the midrib of the leaf of Dionaea, explain'the closing of the lobes upon their hinge. More slowly, but yet in the same way, we may note the change in turgidity of certain cells of the Droscra tentacles, as they close over the imprisoned insect. Organic Rhythm.—It is a remarkable fact that during the process of growth we meet with rhythmic variation of such turgidity. The existence of rhythm of this kind has been observed and studied with some completeness. It is the immediate cause of the phenomena of circumnutation, each cell of the circumnutating organ showing a rhythmic enlargement and decrease of its dimensions, due to the admission of more and less water into its interior. The restraint of the protoplasm changes gradually and rhythmically. The sequence of the phases of the rhythm of the various cells are co-ordinated to produce the movement. Nor is it only in growing organs that the rhythm can be observed, for many plants exhibit it during a much longer period than that of growth. It is easy to realize how such a rhythm can be modified by the reception of stimuli, and can consequently serve as the basis for the movement of the stimulated organ. This rhythmic affection of vegetable protoplasm can be observed in very many of its functions. What have been described as " periodicities," such as the daily variations of root-pressure, afford familiar instances of it. It reminds us of a similar property of animal protoplasm which finds its expression in the rhythmic beat of the heart and other phenomena.
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