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CYTOLOGY OF

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Originally appearing in Volume V21, Page 773 of the 1911 Encyclopedia Britannica.
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CYTOLOGY OF PLANTS The elementary unit of plant structure, as of animal structure, is the cell. Within it or its modifications all the vital phenomena of which living organisms are capable have their origin. Upon our knowledge of its minute structure or cytology, combined with a study of its physiological activities, depends the ultimate solution of all the important problems of nutrition and growth, reception and conduction of stimuli, heredity, variation, sex and reproduction. The Cell Theory.—For a general and historical account of the cell theory see CYTOLOGY. It is sufficient to note here that cells were first of all discovered in various vegetable tissues by Robert Hooke in 1665 (Micrographia); Malpighi and Grew (1674–1682) gave the first clear indications of the importance of cells in the building up of plant tissues, but it was not until the beginning of the 19th century that any insight into the real nature of the cell and its functions was obtained. Hugo von Mohl (1846) was the first to recognize that the essential vital constituent of the plant cell is the slimy mass—protoplasm—inside it, and not the cell wall as was formerly supposed. The nucleus was definitely recognized in the plant cell by Robert Brown in 1831, but its presence had been previously indicated by various observers and it had been seen by Fontana in some animal cells as early as 1781. The cell theory so far as it relates to plants was established by Schleiden in 1838. He showed that all the organs of plants are built up of cells, that the plant embryo originates from a single cell, and that the physiological activities of the plant are dependent upon the individual activities of these vital units. This conception of the plant as an aggregate or colony of independent vital units governing the nutrition, growth and reproduction of the whole cannot, however, be maintained. It is true that in the unicellular plants all the vital activities are performed by a single cell, but in the multicellular plants there is a more or less highly developed differentiation of physiological activity giving rise to different tissues or groups of cells, each with a special function. The cell in such a division of labour cannot therefore be regarded as an independent unit. It is an integral part of an individual organization and as such the exercise of its functions must be governed by the organism as a whole. General Structure and Differentiation of the Vegetable Cell.—The simplest cell forms are found in embryonic tissues, in reproductive cells and in the parenchymatous cells, found in various parts of the plant. The epidermal, conducting and strengthening tissues show on the other hand considerable modifications both in form and structure. The protoplasm of a living cell consists of a semifluid granular substance, called the cytoplasm, one or more nuclei, and some-times centrosomes and plastids. Cells from different parts of a plant differ very much in their cell-contents. Young cells arefull of cytoplasm, old cells generally contain a large vacuole or vacuoles, containing cell-sap, and with only a thin, almost invisible layer of cytoplasm on their walls. Chlorophyll grains, chromatophores, starch-grains and oil-globules, all of which can be distinguished either by their appearance or by chemical reagents, may also be present. Very little is known of the finer structure of the cytoplasm of a vegetable cell. It is sometimes differentiated into a clearer outer layer, of hyaloplasm, commonly called the ectoplasm, and an inner granular endoplasm. In some cases it shows, when submitted to a careful examination under the highest powers of the microscope, and especially when treated with reagents of various kinds, traces of a more or less definite structure in the form of a meshwork consisting of a clear homogeneous substance containing numerous minute bodies known as microsomes, the spaces being filled by a more fluid ground-substance. This structure, which is visible both in living cells and in cells treated by reagents, has been interpreted by many observers as a network of threads embedded in a homogeneous ground-substance. Biitschli, on the other hand, interprets it as a finely vacuolated foam-structure or emulsion, comparable to that which is observed when small drops of a mixture of finely powdered potash and oil are placed in water, the vacuoles or alveoli being spaces filled with liquid, the more solid portion representing the mesh-work in which the microsomes are placed. Evidence is not wanting, however, that the cytoplasm must be regarded as, fundamentally, a semifluid, homogeneous substance in which by its own activity, granules, vacuoles, fibrils, &c., can be formed as secondary structures. The cytoplasm is largely concerned in the formation of spindle fibres and centrosomes, and such structures as the cell membrane, cilia, or flagella, the coenocentrum, nematoplasts or vibrioids and physodes are also products of its activity. Protoplasmic Movements.—In the cells of many plants the cytoplasm frequently exhibits movements of circulation or rotation. The cells of the staminal hairs of Tradescantia virginica contain a large sap-cavity across which run, in all directions, numerous protoplasmic threads or bridges. In these, under favourable conditions, streaming movements of the cytoplasm in various directions can be observed. In other forms such as Elodea, Nitella, Chara, &c., where the cytoplasm is mainly restricted to the periphery of the sap vacuole and lining the cell wall, the streaming movement is exhibited in one direction only. In some cases both the nucleus and the chromatophores may be carried along in the rotating stream, but in others, such as Nitella, the chloroplasts may remain motionless hi a non-motile layer of the cytoplasm in direct contact with the cell wall.' Desmids, Diatoms and Oscillaria show creeping movements probably due to the secretion of slime by the cells; the swarm-spores and plasmodium of the Myxomycetes exhibit amoehoid movements; and the motile spores of Fungi and Algae, the spermatozoids of mosses, ferns, &c., move by means of delicate prolongations, cilia or flagella of the protoplast. Chromatophores.—The chromatophores or plastids are protoplasmic structures, denser than the cytoplasm, and easily distinguishable from it by their colour or greater refractive power. They are spherical, oval, fusiform, or rod-like, and are always found in the cytoplasm, never in the cell-sap. They appear to be permanent organs of the cell, and are transmitted from one cell to another by division. In young cells the chromatophores are small, colourless, highly refractive bodies, principally located around the nucleus. As the cell grows they may become converted into leucoplasts (starch-formers), chloroplasts (chlorophyll-bodies), or chromoplasts (colour-bodies). And all three structures may be converted one into the other (Schimper). The chloroplasts are generally distinguished by their green colour, which is due to the presence of chlorophyll; but in many Algae this is masked by another colouring matter—Phycoerythrin in the Florideae, Phycophaein in the Phaeophyceae, and Phycocyanin I 1 Ewart, On the Physics and Physiology of Protoplasmic Streaming in Plants. (Oxford, 1903), gives an excellent account of the phenomena of protoplasmic streaming with a full discussion of the probable causes to which it is due. in the Cyanophyceae. These substances can, however, be dissolved out in water, and the green colouring matter of the chloroplast then becomes visible. The chloroplast consists of two parts, a colourless ground substance, and a green colouring matter, which is contained either in the form of fibrils, or in more or less regular spherical masses, in the colourless ground-mass. The chloroplasts increase in number by division, which takes place in higher plants when they have attained a certain size, independent of the division of the cell. In Spirogyra and allied forms the chloroplast grows as the cell grows, and only divides when this divides. The division in all cases takes place by constriction, or by a simultaneous splitting along an equatorial plane. Chloroplasts are very sensitive to light and are capable in some plants of changing their position in the cell under the stimulus of a variation in the intensity of the light rays which fall upon them. In the chromatophores of many Algae and in the Liverwort Anthoceros there are present homogeneous, highly refractive, crystal-like bodies, called pyrenoids or starch-centres, which are composed of proteid substances and surrounded by an envelope of starch-grains. In Spirogyra the pyrenoids are distinctly connected by cytoplasmic strands to the central mass of cytoplasm, which surrounds the nucleus, and according to some observers, they increase exclusively by division, followed by a splitting of the cytoplasmic strands. Those chromatophores which remain colourless, and serve simply as starch-formers in parts of the plant not exposed to the light, are called leucoplasts or amyloplasts. They are composed of a homogeneous proteid substance, and often contain albuminoid or proteid crystals of the same kind as those which form the pyrenoid. If exposed to light they may become converted into chloroplasts. The formation of starch may take place in any part of the leucoplast. When formed inside it, the starch-grains exhibit a concentric stratification; when formed externally in the outer layers, the stratification is excentric, and the hilum occurs on that side farthest removed from the leucoplast. As the starch-grains grow, the leucoplasts gradually disappear. Chromoplasts are the yellow, orange or red colour-bodies found in some flowers and fruits. They arise either from the leucoplasts or chloroplasts. The fundamental substance or stroma is colourless and homogeneous. The colour is due to the presence of xanthophyll, or carotin or both. The colouring matters are not dissolved in the stroma of the chromoplast, but exist as amorphous granules, with or without the presence of a protein crystal, or in the form of fine crystalline needles, frequently curved and sometimes present in large numbers, which are grouped together in various ways in bundles and give the plastids their fusiform or triangular crystalline shape. Such crystalline plastids occur in many fruits and flowers (e.g. Tamus communis, Asparagus, Lonicera, berries of Solaneae, flowers of Cacalia coccinea, Tropaeolum, bracts of Strelitzia, &c.), and in the root of the carrot. In some cases the plastid disappears and the crystalline pigment only is left. In the red variety of Cucurbita pepo these crystals may consist of rods, thin plates, flat ribbons or spirals. Starch grains may often be seen in contact with the pigment crystals. The crystalline form appears to be due entirely to the carotin, which can be artificially crystallized from an alcohol or ether solution. In addition to the plastids, there are found in some plant-cells, e.g. in the epidermal cells of the leaf of species of Vanilla (Wakker), and in the epidermis of different parts of the flower of Funkia, Ornitkogalum, &e. (Zimmermann), highly refractive bodies of globular form, elaioplasts, which consist of a granular protein ground-substance containing drops of oil. They are stained deep red in dilute solution of alkamn. Substances contained in the Protoplasm.—Starch may be found in the chlorophyll bodies in the form of minute granules as the first visible product of the assimilation of carbon dioxide, and it occurs in large quantities as a reserve food material in the cells of various parts of plants. It is highly probable that starch is only produced as the result of the activity of chromatophores, either in connexion with chromoplasts, chloroplasts or leucoplasts. Starch exists, in the majority of cases, in the form of grains, which are composed of stratified layers arranged around a nucleus or hilum. The stratification, which may be concentric or excentric, appears to be due to a difference in density of the various layers. The outer layers are denser than the inner, the density decreasing more or less uniformly from the outside layers to the centre of hilum. The outermost, newly formed layer is composed of a more homogeneous, denser substance than theinner one, and can be distinguished in all starch-grains that are in process of development. The separate layers of the starch-grain are deposited on it by the activity of the chromatophore, and according to Meyer the grain is always surrounded by a thin layer of the chromatophore which completely separates it from the cytoplasm. The layers appear to be made up of elements which are arranged radially. These are, according to Meyer, acicular crystals, which he calls trichites. The starch grain may thus be regarded as a crystalline structure of the nature of a sphere-crystal, as has been suggested by many observers. Whether the formation of the starch grain is due to a secretion from the plastid (Meyer, 1895) or to a direct transformation of the proteid of the plastid (Timberlake, 1901) has not been definitely established. Aleurone.—Aleurone is a proteid substance which occurs in seeds especially those containing oil, in the form of minute granules or large grains. It may be in the form of an albumen crystal' some-times associated with a more or less spherical body—globoid--composed of a combination of an organic substance with a double phosphate of magnesium and calcium. Albumen crystals are also to be found in the cytoplasm, in leucoplasts and rarely in the nucleus. Glycogen, a substance related to starch and sugar, is found in the Fungi and Cyanophyceae as a food reserve. It gives a characteristic red-brown reaction with iodine solution. In the yeast cell it accumulates and disappears very rapidly according to the conditions of nutrition and is sometimes so abundant as to fill the cell almost entirely (Errera, 1882, 1895: Wager and Peniston, 1910). Volutin occurs in the cytoplasm of various Fungi, Bacteria, Cyanophyceae, diatoms, &c., in the form of minute granules which have a characteristic reaction towards methylene blue (Meyer). It appears to have some of the characteristics of nucleic acid, and according to Meyer may be a combination of nucleic acid with an unknown organic base. Numerous other substances are also found in the cytoplasm, such as tannin, fats and oil, resins, mucilage, caoutchouc, gutta-percha, sulphur and calcium oxalate crystals. The cell sap contains various substances in solution such as sugars, inulin, alkaloids, glucosides, organic acids and various inorganic salts. The colours of flowers are due to colouring matters contained in the sap of which the chief is anthocyanin. Reference must also be made here to the enzymes or unorganized ferments which occur so largely in the cytoplasm. It is probable that most, if not all, the metabolic changes which take place in a cell, such as the transformation of starch, proteids, sugar, cellulose; and the decomposition .of numerous other organic substances which would otherwise require a high temperature or powerful reagents is also due to their activity. Their mode of action is similar to that of ordinary mechanical catalytic agents, such as finely divided platinum (see Bayliss, The Nature of Enzyme Action, and J. R. Green, The Soluble Ferments). The Nucleus.—The nucleus has been demonstrated in all plants with the exception of the Cyanophyceae and Bacteria, and even here structures have been observed which resemble nuclei in some of their characteristics. The nucleus is regarded as a controlling centre of cell-activity, upon which the growth and development of the cell in large measure depends, and as the agent by which the transmission of specific qualities from one generation to another is brought about. If it is absent, the cell loses its power of assimilation and growth, and soon dies. Haherlandt has shown that in plant cells, when any new formation of membrane is to take place in a given spot, the nucleus is found in its immediate vicinity; and Klebs found that only that portion of the protoplasm of a cell which contains the nucleus is capable of forming a cell-wall; whilst Townsend has further shown that if the non-nucleated mass is connected by strands of protoplasm to the nucleated mass, either of the same cell or of a neighbouring cell, it retains the power of forming a cell-membrane. The Structure of the Nucleus.—In the living condition the resting nucleus appears to consist of a homogeneous ground sub-stance containing a large number of small chromatin granules and one or more large spherical granules—nucleoli--the whole being surrounded by a limiting membrane which separates it from the cytoplasm. When fixed and stained this granular mass is resolved into a more or less distinct granular network which consists of a substance called Linin, only slightly stained by the ordinary nuclear stains, and, embedded in it, a more deeply stainable substance called Chromatin. The nucleolus appears to form a part of the Linin network, but has usually also a strong affinity for nuclear stains. The staining reactions of the various parts of the nucleus depend to some extent upon their chemical It may also take place where rapid proliferation of the cell is constitution. The chromatin is practically identical with nuclein. This has a strong attraction for basic aniline dyes, and can usually be distinguished from other parts of the cell which are more easily coloured by acid anilines. Bit the staining reactions of nuclei may vary at different stages of their development; and it i$ probable that there is no method of staining which differentiates with certainty the various morphological constituents of the nucleus. Our knowledge of the chemical constitutions of the nucleus is due to the pioneer researches of Sir Lauder Brunton, Plosz, Miescher, Kossel and a host of more recent investigators. Nuclein is a complex albuminoid substance containing phosphorus and iron in organic combination (Macallum). It appears to be a combination of a protein with nucleic acid. Recent researches have shown that the nucleic acid can be broken up by chemical means into a number of different compounds or bases. The results at first obtained were very confusing and seemed to show that nucleic acid is very variable in constitution, but thanks to the work 4f Schmiedeberg and Stendel (Germany), Ivar Bang (Sweden) and Walter Jones and Levene (America), the confusion has been reduced to some sort of order, and it now seems probable that all ordinary nucleic acids yield two purine bases, adenine and guanine; two pyrimidine bases, cytosine and thymine and a hexose carbohydrate, the identity of which is uncertain., The Nucleolus.—In the majority of plant-nuclei, both in the higher and lower plants, there is found, in addition to the chromatin network, a deeply stained spherical or slightly irregular body (sometimes more than one) called the nucleolus (fig. 2, A to D). It is often vacuolar, sometimes granular, and in other cases it is a homogeneous body with no visible structure or differentiation. The special function of this organ has been a source of controversy during the past few years, and much uncertainty still exists as to its true nature. It forms a part of the limn or plastin network of the nucleus and may become impregnated with varying quantities of chromatin stored up for use in the formation of the chromosomes and other nuclear activities. The relation of the nucleolus to the chromosomes is clearly seen in the reconstruction of the daughter nuclei after division in the cells of the root-apex of Phaseolus (fig. 1, A to F). The chromosomes (fig. 1, A) unite to form an irregular mass (fig. 1, B) out of which is evolved the nucleolus and nuclear net-work (figs. 1, E, F) by a fusion of the chromosomes (fig. 1, C, D). Centrosome.—The centrosome is a minute homogeneous granule found in the cytoplasm of some cells in the neighbour-hood of the nucleus. It is generally surrounded by a granular or radiating cytoplasmic substance. In plant cells its presence has been demonstrated in the Thallophytes and Bryophytes. In the higher plants the structures which have been often de-scribed as centrosomes are too indefinite in their constitution to allow of this interpretation being placed upon them, and many of them are probably nothing more than granules of the fragmented nucleolus. The centrosomes in plants do not appear to be permanent organs of the cell. They are prominent during cell-division, but many disappear in the resting stage. They are more easily seen, when the nucleus is about to undergo mitosis, at the ends of the spindle, where they form the centres towards which the radiating fibres in the cytoplasm converge (see fig. 7, E G). The centrosome or centrosphere is usually regarded as the dynamic centre of the cell and a special organ of division; but its absence in many groups of plants does not lend support to this view so far as plant-cells are concerned. Nuclear Division.—The formation of new cells is, in the case of uninucleate cells, preceded by or accompanied by the division of the nucleus. In multinucleate cells the division of the nucleus is independent of the division of the cell. Nuclear division may be indirect or direct, that is to say it may either be accompanied by a series of complicated changes in the nuclear structures called mitosis or karyokinesis (fig. 2), or it may take place by simple direct division, amitosis, or fragmentation. Direct division is a much less common phenomenon than was formerly supposed to be the case. It occurs most frequently in old cells, or in cells which are placed under abnormal conditions. See Halliburton. Science Progress in the loth Century (1909), vol. iv. going on, as in the budding of the Yeast plant. It takes place in the internodal cells of Characeae; in the old internodal cells of 1:C 'r^idly' 1,.p-f. '1 E F Tradescantia; and in various other cells which have lost their power of division. It has been shown that, in cells of Spirogyra placed under special conditions, amitotic division can be induced, and that normal mitosis is resumed when they are placed again under normal conditions. Amitosis is probably connected by a series of intermediate gradations with karyokinesis. Mitosis.—In indirect nuclear division the nucleus undergoes a series of complicated changes, which result in an equal division of the chromatic substance between the two daughter nuclei. Four stages can be recognized. (1) Prophase.—The nucleus increases in size; the network disappears, and a much convoluted thread takes its place (fig. 2, B). The chromatin substance increases in amount; the thread stains more deeply, and in most cases presents a homogeneous appearance. This is commonly called the spirem-figure. The chromatin thread next becomes shorter and thicker, the nucleoli begin to disappear, and the thread breaks up into a number of segments—chromosomes—which vary in number in different species, but are fairly constant in the same species (fig. 2, C, D). Coincident with these changes the nuclear membrane disappears and a spindle-shaped or barrel-shaped group of threads makes its appearance in the midst of the chromosomes, the longitudinal axis of which is at right angles to the plane of the division (fig. 2, F). At each pole of this spindle figure there often occur fibres radiating in all directions into the cytoplasm, and sometimes a minute granular body, the centre-some, is also found there. (2) Metaphase.—The chromosomes pass to the equator of the spindle and become attached to the A spindle-fibres in such a way that they form a radiating star-shaped figure—Aster—when seen from the pole of the spindle. This is called the nuclear plate (fig. 2, E, F, G, H). As they pass into this position they undergo a longitudinal splitting by which the chromatin in each chromosome becomes divided into equal halves. (3) Anaphase.—The longitudinal division of the chromosomes is completed by the time they have' taken up their position in the nuclear plate, and the halves of the chromosomes then begin to move along the spindle-fibres to opposite poles of the spindle (fig. 2, I, J). Many observers hold the view that the chromosomes are pulled apart by the contraction of the fibres to which they are attached. (4) Telophase.—When they reach the poles the chromosomes group themselves again in the form of stars—Diaster--with spindle-fibres extending between them (fig. 2, K). The chromosomes then fuse together again to form a single thread (fig. 2, L), a nucleolus appears, a nuclear membrane is formed, and daughter nuclei are thus constituted which possess the same structure and staining reactions as the mother nucleus. The spindle figure is probably the expression of forces which are set up in the cell for the purpose of causing the separation of the daughter chromosomes. Hartog has endeavoured to show that it can only he formed by a dual force, analagous to that of magnetism, the spindle-fibs es being comparable to the lines of force in a magnetic field and possibly due to electrical differences in the cell. The spindle arises partly from the cytoplasm, partly from the nucleus, or it may be derived entirely from the nucleus—intranuclear spindle—as occurs in many of the lower plants (Fungi, &c.). The formation of the spindle begins in the prophases of division. A layer of delicate filamentous cytoplasm—kinoplasm—may collect around the nucleus, or at its poles, out of which the spindle is formed. As division proceeds, the filamentous nature of this cytoplasm becomes more prominent and the threads begin either to converge towards the poles of the nucleus, to form a bipolar spindle, or may converge towards, or radiate from, several different points, to form a multi-polar spindle. The wall of the nucleus breaks down, and the cytoplasmic spindle-fibres become mixed with those derived from the nuclear network. The formation of the spindle differs in details in different plants. The significance of this complex series of changes is very largely hypothetical. It is clear, however, that an equal quantitative division and distribution of the chromatin to the daughter cells is brought about; and if, as has been suggested, the chromatin consists of minute particles or units which are the carriers of the hereditary characteristics, the nuclear division also probably results in the equal division and distribution of one half of each of these units to each daughter cell. Reduction Divisions (Meiosis).—The divisions which take place leading to the formation of the sexual cells show a reduction in the number of chromosomes to one-half. This is a necessary consequence of the fusion of two nuclei in fertilization, unless the chromosomes are to be doubled at each generation. In the vascular cryptogams and phanerogams it takes place in the spore mother cells and the reduced number is found in all the cells of the gametophyte, the full number in those of the sporophyte. We know very little of the details of reduction in the lower plants, but it probably occurs at some stage in the life history of all plants in which sexual nuclear fusion takes place. The reduction is brought about simply by the segmentation of the spirem thread into half the number of segments instead of the normal number. In order to effect this the individual chromosomes must become associated in some way, for there is no diminution in the actual amount of nuclear substance, and this leads to certain modifications in the division which are not seen in the vegetative nuclei. The two divisions of the spore mother cell in which the reduction takes place, follow each other very rapidly and are known as Heterotype and Homotype (Flemming), or according to the terminology of Farmer and Moore (1905) as the meiotic phase. In the heterotype division the spirem thread is divided longitudinally before the segmentation occurs (fig. 2, B), and this is preceded by a peculiar contraction of the thread around the nucleolus which has been termed synapsis (fig. r, A). A second contraction may take place later, immediately preceding the segmentation of the thread. It has been suggested that synapsis may be connected with the early longitudinal splitting of the thread or with the pairing of the chromosomes, but it is possiblethat it may be connected with the transference of nucleolar substance to the nuclear thread. The segments of each chromosome are usually twisted upon each other and may be much contorted (fig. 2, C, D), and appearances are observed which suggest a second longitudinal division, but which are more (After Gregoire.) probably due to a folding of the segment by which the two halves come to lie more or less parallel to each other, and form variously shaped figures of greater or less regularity (fig. 2, E). The chromosomes now become attached to the spindle-fibres (fig. 2, F, G) and as the daughter chromosomes become pulled asunder they often appear more or less V-shaped so that each pair appears as a closed ring of irregular shape, the ends of the V's being in contact thus—<> (fig. 2, H. I, J, K). This V has been variously interpreted. Some observers consider that it represents a longitudinal half of the original segment of the spireme, others that it is a half of the segment produced by transverse division by means of which a true qualitative separation of the chromatin is brought about. The problem is a very difficult one and cannot be regarded as definitely settled, but it is difficult to understand why all this additional complexity in the division of the nucleus should be necessary if the final result is only a quantitative separation of the chromatin. It seems to be fairly well established that in the meiotic phase there is a true qualitative division brought about by the pairing of the chromosomes during synapsis, and the subsequent separation of whole chromosomes to the daughter nuclei. The method by which this is brought about is, however, the subject of much controversy. There are two main theories: (I) that the chromosomes which finally separate are at first paired side by side (Allen, Gregoire, Berghs, Strasburger and others), and (2) that they are joined together or paired end to end (Farmer and Moore, Gregory, Mottier and others). Good cytological evidence has been adduced in favour of both theories, but further investigation is necessary before any definite conclusion can be arrived at. The second or homotype division which immediately follows reverts to the normal type except that the already split chromosomes at once separate to form the daughter nuclei without the intervention of a resting stage. Cell Division.—W ith the exception of a few plants among the Thallophytes, which consist of a single multinucleate cell, Caulerpa, Vaucheria, &c., the division of the nucleus is followed by the division of the cell either at once, in uninucleate cells, or after a certain number of nuclear divisions, in multinucleate cells. This may take place in various ways. In the higher plants, after the separation of the daughter nuclei, minute granular swellings appear, in the equatorial region, on the connecting fibres which still persist between the two nuclei, to form what is called the cell-plate. These fuse together to form a membrane (fig. i, C, D) which splits into two layers between which the new cell-wall is laid down. In the Thallophytes the cytoplasm may be segmented by constriction, due to the in-growth of a new cell wall from the old one, as in Spirogyra and Cladophora, or by the formation of cleavage furrows in which the new cell-wall is secreted, as occurs in the formation of the spores in many Algae and Fungi. Cell budding takes place in yeast and in the formation of the conidia of Fungi. In a few cases both among the higher and the lower plants, of which the formation of spores in the ascus is a typical example, new cells are formed by the aggregation of portions of the cytoplasm around the nuclei which become delimited from the rest of the cell contents by a membrane. This is known as free cell formation. In Fucus and allied forms the spindle-fibres between the daughter nuclei disappear early and the new cell-wall is formed in the cytoplasm. Cell Membrane.—The membrane which surrounds the protoplasts in the majority of plants is typically composed of cellulose, together with a number of other substances which are known as pectic compounds. Some of these have a neutral reaction, others react as feeble acids. They can be distinguished by their insolubility in cuprammonia, which dissolves cellulose, and by their behaviour towards stains, some of which stain pectic substances but not cellulose. Cellulose has an affinity for acid stains, pectic substances for basic stains. The cell-membrane may become modified by the process of lignification, suberization, cuticularization or gelatinization. In the Fungi it is usually composed of a modified form of cellulose known as fungus cellulose, which, according to Mangin, consists of callose in combination either with cellulose or pectic compounds. The growth of the cell-wall takes place by the addition of new layers to those already formed. These layers arc secreted by the protoplasm by the direct apposition of substances on those already in existence; and they may go on increasing in thickness, both by apposition and by the intussusception of particles probably carried in through the protoplasmic fibres, which penetrate the cell-wall as long as the cell lives. The growth of the cell-wall is very rarely uniform. It is thickened more in some places than in others, and thus are formed the spiral, annular and other markings, as well as the pits which occur on various cells and vessels. Besides the internal or centripetal growth, some cell-walls are thickened on the outside, such as pollen grains, oospores of Fungi, cells of Peridineae, &c. This centrifugal growth must apparently take place by the activity of protoplasm external to the cell. The outer protective walls of the oospores of some Fungi are formed out of protoplasm containing numerous nuclei, which is at an early stage separated from the protoplasm of the oospore. In the Peridineae, Cell-walls may become modified by the impregnation of various substances. Woody or lignified cell-walls appear to contain sub-stances called conjoin and vanillin, in addition to various other compounds which are imperfectly known. Lignified tissues are coloured yellow by aniline sulphate or aniline chloride, violet with phloroglucin and hydrochloric acid, and characteristic reactions are also given by mixtures containing phenol, indol, skatol, thailin, sulphate, &c. (see Zimmermann's Microtechnique). Staining reagents can also be used to differentiate lignified cell-walls. Cuticularized or suberized cell-walls occur especially in those cells which per-form a protective function. They are impervious to water and gases. Both cuticularized and suberized membranes are insoluble in cuprammonia, and are coloured yellow or brown in a solution of chlor-iodide of zinc. It is probable that the corky or suberized cells do not contain any cellulose (Gilson, Wisselingh) ; whilst cuticularized cells are only modified in their outer layers, cellulose inner layers being still recognizable. The suberized and cuticularized cell-walls appear to contain a fatty body called suberin, and such cell-walls can be stained red by a solution of alcanin, the lignified and cellulose membranes remaining unstained. Fertilization.—The formation of the zygote or egg-cell takes place usually by the fusion of the contents of two cells, and always includes, as an essential feature, the _egg fusion of two germ nuclei. In many of the lower plants the fusing cells gametes — are precisely similar so far as size and general appearance are concerned; and the whole contents of the two cells fuse together, cytoplasm with cytoplasm, nucleus with nucleus, nucleolus with nucleolus and plastid with plastid. The gametes may be motile (some Algae) or non-motile, as in Spirogyra, Mucci'', Basidiobolus, &c. In many of the lower plants and in all higher plants there is a difference in size in the fusing cells, the male cell being the smaller. The reduction in size is due to the absence of cytoplasm, which is in some cases so small in amount that the cell consists mainly of a nucleus. In all cases of complete sexual differentiation the egg-cell is quies- cent; the male cell may be motile or non-motile. In many of the Fungi the non-motile male cell or nucleus is carried by means of a fertilizing tube actually into the interior of the egg-cell, and is extruded through the apex in close proximity to the egg nucleus. In the Florideae, Lichens and Laboulbeniaceae the. male cell is a non-motile spermatium, which is carried to the female organ by movements in the water. In Monoblepharis, one of the lower Fungi, in some Algae, in the Vascular Crypto-' grams, in Cycads (Zamia and Cycas), and in Ginkgo, an isolated genus of Gymnosperms, the male cell is a motile spermatozoid with two or more cilia. In the Algae, such as Fucus, Volvo; Oedogonium, Bulbochaete, and in the Fungus Monoblepharis, the spermatozoid is a small oval or elongate cell containing nucleus, cytoplasm and sometimes plastids. In the Characeae, the Vascular Cryptogams, in Zamia and Cycas, and in Ginkgo, the spermatozoids are more or less highly modified cells witl_• two or more cilia, and resemble in many respects, both in their ,I I (From Wilson. After Guignard and Mottier.) a, Antipodal cell; sp, polar nuclei; pt, pollen tube. A, Two vermiform nuclei in the embryo sac; one approaching the egg-nucleus, the other uniting with the upper polar nucleus. B, Union of the vermiform nuclei with the egg-nucleus and the two polar nuclei. C, Fusion of the germ nuclei in the egg-cell. structure and mode of formation, the spermatozoids of animals. In Characeae and Muscineae they are of elongate spiral form, and consist of an elongate dense nucleus and a small quantity of cytoplasm. At the anterior end are attached two cilia or flagella. In the Vascular Cryptogams the structure is much the same, but a more or less spherical mass of cytoplasm remains attached to the posterior spirals, and a large number of cilia are grouped along the cytoplasmic anterior portion of the spiral. In Zamia (fig. 4, A), Cycas and Ginkgo they consist of large spherical or oval cells with a coiled band of cilia at one end, and a large nucleus which nearly fills the cell. They are carried by the pollen tube to the apex of the prothallus, where they are extruded, and by means of their cilia swim through a small quantity of liquid, contained in a slight depression to the oosphere. In the other Phanerogams the male cell, which is non-motile; is carried to the oosphere by means of a pollen tube. In the spermatozoids of Chara, Vascular Cryptogams, and in those of Cycas, Zamia and Ginkgo, the cilia arise from a centrosome-like body which is found on one side of the nucleus of the spermatozoid mother-cell. This body has been called a blepharoplast, and in the Pteridophytes, Cycads and Ginkgo it gives rise to the spiral band on which the cilia are formed. Belajeff regards it as a true centrosome; but this is doubtful, for while in some cases it appears to be connected with the division of the cell, in others it is independent of it. The egg-cell or oosphere is a large cell containing a single large nucleus, and in the green plants the rudiments of plastids. In plants with multinucleate cells, such as Albugo, Peronospora and Vaucheria, it is usually a uninucleate cell differentiated by separation of the nuclei from a multinucleate cell, but in Albugo bliti it is multinucleate, and in Sphaeroplea it may contain more than one nucleus. In some cases the region where the penetration of the male organ takes place is indicated on the oosphere by a hyaline receptive spot (Oedogonium, Vaucheria, &c.), or by a receptive papilla consisting of hyaline cytoplasm (Peronosporeae). Fertilization is effected by the union of two nuclei in all those cases which have been carefully investigated. Even in the multinucleate oosphere of Albugo bliti the nuclei fuse in pairs; and in the oospheres of Sphaeroplea, which may contain more than one nucleus, the egg nucleus is formed by the fusion of one only of these with the spermatozoid nucleus (Klebahn). In the higher Fungi nuclear fusions take place in basidia or asci which involve the union of two (fig. 7, A) nuclei, which may be regarded as physiologically equivalent to a sexual fusion. The union of the germ nuclei has now been observed in all the main groups of Angiosperms, Gymnosperms, Ferns, Mosses, Algae and Fungi, and presents a striking resemblance in all. In nearly all cases the nuclei appear to fuse in the resting stage (fig. 3, C). In many Gymnosperms the male nucleus penetrates the female nucleus before fusing with it (Blackman, Ikeno). In other cases the two nuclei place them-selves side by side, the nuclear membrane between them disappears, and the contents fuse together—nuclear thread with nuclear thread, and nucleolus with nucleolus—so completely that the separate constituents of the nuclei are not visible. It was at one time thought that the centrosomes played an important part in the fertilization of plants, but recent researches seem to indicate that this is not so. Even in those cases where the cilia band, which is the product of the centrosome-like body or blepharoplast, enters the ovum, as in Zamia (c in fig. 4, B, C, D), it appears to take no part in the fertilization phenomena, nor in the subsequent division of the nucleus. During the process of fertilization in the Angiosperms it has been shown by the researches of Nawaschin and Guignard that in Lilium and Fritillaria both generative nuclei enter the embryo sac, one fusing with the oosphere nucleus, the other with the polar nuclei (fig. 3, A, B ). A double fertilization thus takes place. Both nuclei are elongated vermiform structures, and as they enter the embryo sac present a twisted appearance like a spermatozoid without cilia (fig. 3, A, B). It has since been shown by other observers that this double fertilization occurs in many other Angiosperms, both Dicotyledons and Monocotyledons, so that it is probably of general occurrence throughout the group (see ANGIOSPERMS). The Nucleus in Relation to Heredity.—There is a certain amount of cytological evidence to show that the nucleus is largely concerned with the transmission of hereditary characters. Whether this is entirely confined to the nucleus is, however, not certain. The strongest direct evidence seems to be that the nuclear substances are the only parts of the cells which are always equivalent in quantity, and that in the higher plants and animals the male organ or spermatozoid is composed almost entirely of the nucleus, and that the male nucleus is carried into the female cell without a particle of cytoplasm.) Since, however, the nucleus of the' female cell is always accompanied by a larger or smaller quantity of cytoplasm, and that in a large majority of the power plants and animals the male cell also contains cytoplasm, it cannot yet be definitely stated that the cytoplasm does not play some part in the process. On the other hand, the complex structure of the nucleus with its separate units, the chromosomes, and possibly even smaller units represented by the chromatin granules, and the means taken through the complex phenomena of mitosis to ensure that an exact and equal division of the chromosomes shall take place, emphasizes the importance of the nucleus in heredity. Further, it is only in the nucleus and in its chromosomes that we have any visible evidence to account for the Mendelian .segregation of characters in hybrids which are known to occur. Visible differences in the chromosomes have even been observed, especially in insects, which are due apparently to an unequal division by which an additional or accessory chromosome is produced, or in some cases one or two extra chromosomes which differ in size from the others. These differences indicate a separation of different elements in the formation of the chromosomes and have been definitely associated with the determination of sex. It is possible, however, that the segregation of characters in the gametes may depend upon something far more subtle and elusive than the chromosomes or even of possible combinations of units within the chromosomes, but so far as we can see at present these are the only structures in the cell with which it can be satisfactorily associated. Boveri in fact has put forward the view that the chromosomes are elementary units which maintain an organic continuity and independent existence in the cell. The cytological evidence for this appears to be made stronger for animal than for plant cells. From numerous investigations which have been made to trace the chromosomes through the various stages of the nuclear ontogeny of plant cells, it appears that the individuality and continuity of the chromosomes can only be conceived as possible if we assume the existence of something like chromosome centres in the resting nucleus around which the chromosomes become organized for purposes of division. Rosenberg (1909) adduces evidence for 1 Strasburger (1909) states very definitely that he has observed the entrance of the male nucleus into the egg without a trace of cytoplasm. [After Webber.) tion in Zamia. the existence of chromosomes or " prochromosomes " in resting nuclei in a large number of plants, but most observers consider that the chromosomes during the resting stage become completely resolved into a nuclear network in which no trace of the original chromosomes can be seen. Special Cell-Modifications for the Reception of Stimuli.—In studying the physiology of movement in plants certain modifications of cell-structure have been observed which appear to have been developed for the reception of the stimuli by which the response to light, gravity and contact are brought about. Our knowledge of these structures is due mainly to Haberlandt. Organs which respond to the mechanical stimulus of contact are found to possess special contrivances in certain of their cells—0) sensitive spots, consisting of places here and there on the epidermal cells where the wall is thin and in close contact with protoplasmic projections. These occur on the tips of tendrils and on the tentacles of Drosera; (2) sensitive papillae found on the irritable filaments of certain stamens; and (3) sensitive hairs or bristles on the leaves of Dionaea muscipula and Mimosa pudica—all of which are so constructed that any pressure exerted on them at once reacts on the protoplasm. Response to the action of gravity appears to be associated with the movements of starch grains in certain cells—statolith cells—by which pressure is exerted on the cytoplasm and a stimulus set up which results in the geotropic response. The response to the action of light in diatropic leaves is, according to Haberlandt, due to the presence of epidermal cells which are shaped like a lens, or with lens-shaped thickenings of the cuticle, through which convergence of the light rays takes place and causes a differential illumination of the lining layer of protoplasm on the basal walls of the epidermal cells, by which the stimulus resulting in the orientation of the leaf is brought about. Fig. 5, A, shows the A PIG. 5. B A, Epidermal cells of Saxifraga hirsutum. B, of Tradescantia fluminensis. convergence of the light to a bright spot on the basal walls of the epidermal cells of Saxifraga hirsutum and fig. 5, B, shows a photograph taken from life through the epidermal cells of Tradescantia fluminensis. Notwithstanding the fact, however, that these cells are capable of acting as very efficient lenses the explanation given by Haberlandt has not been widely accepted and evidence both morphological and physiological has been brought forward against it. The presence of an eye-spot in many motile unicellular Algae and swarm spores is also probably concerned with the active response to light exhibited by these organisms. In Euglena viridis, which has been most carefully studied in this respect, the flagellum which brings about the movement bears near its base a minute spherical or oval refractive granule or swelling which is located just in the hollow of the red pigment-spot (fig. 6) ; and it has been suggested that the association of these two is analogous to the association of the rods and cones of the animal eye with their pigment layer, the light absorbed by the red pigment-spot setting up changes which react upon the refractive granule and being transmitted to the flagellum bring about those modifications in its vibrations by which the direction of movement of the organism is regulated. The Nuclei of the Lower Plants.—It is only in comparatively recent times that it has been possible to determine with any degree of certainty that the minute deeply stainable bodies described more especially by Schmitz (1879) in many Algae and Fungi could be regarded as true nuclei. The researches of the last twenty years have shown that the structure of the nucleus and the phenomena of nuclear division in these lower forms conforms in all essential details to those in the higher plants. Thus in the Basidiomycetes (fig. 7) the nuclei possess all the structures found in the higher plants, nuclear membrane, chromatin network and nucleolus (fig. 7, B), and in the processof division, chromosomes, nuclear spindle and centrosomes are to be seen (fig. 7, C–G). The investigations of Dangeard, Harper, Blackman, Miss Fraser and many others have also • r (From the Journal of the Linnean Society, "Zoology" vol. xxvil.) shown that in the Ascomycetes, Rust Fungi, &c., the same structure obtains so far as all essential details are concerned. The only groups of plants in which typical nuclei have not been found are the Cyanophyceae, Bacteria and Yeast Fungi. D E (From the Annals of Botany, vols. vii. and viii.) A to D, Amanita muscarius; E to G, Mycena galericulatus. A, Basidium with two nuclei. B, single nucleus due to the fusion of the two pre-existing nuclei. C, Nuclear thread segmenting. D, Nuclear cavity with chromosomes. E, Chromosomes on the spindle. F, Separation of the chromosomes into two groups. G, Chromosomes grouped at opposite ends of the spindle to form the daughter nuclei. A C In the Cyanophyceae the contents of the cell are differentiated into a central colourless region, and a peripheral layer containing the chlorophyll and other colouring matters together with granules of a reserve substance called cyanophycin. Chromatin is contained in the central part together with granules known as volutin, the function of which is unknown. The central body probably plays the part of a nucleus and some observers consider that it has the characters of a typical nucleus with mitotic division. But this is very doubtful. The central body seems to consist merely of a spongy mass of slightly stainable substance, more or less impregnated with chromatin, which divides by constriction. At a certain stage in the division figures are produced resembling a mitotic phase (fig. 8, I), which are not, in A B (From Prot. Roy. Soc., vol. lxxii.) A and B, Tolypothrix lanata: (I) Young, (2) Old cells. C, Oscillaria limos¢: transverse microtome section. the opinion of the writer, to be interpreted as a true mitosis. It is interesting to note that in many species the formation of new cell-walls is initiated before any indication of nuclear division is to be seen. The bacteria, in most cases, have no definite nucleus or central body. The chromatin is distributed throughout the cytoplasm in the form of granules which may be regarded as a distributed nucleus corresponding to what Hertwig has designated, in protozoa, chromidia. In the yeast cell the nucleus is represented by a homogenous granule, probably of a nucleolar nature, surrounded and perhaps to some extent impregnated by chromatin and closely connected with a vacuole which often has chromatin at its periphery, and contains one or more volutin granules which appear to consist of nucleic acid in combination with an unknown base. Some observers consider that the yeast nucleus possesses a typical nuclear structure, and exhibits division by mitosis, but the evidence for this is not very satisfactory. Tissues.—The component parts of the tissues of which plants are composed may consist of but slightly modified cells with copious protoplasmic contents, or of cells which have been modified in various ways to perform their several functions. In some the protoplasmic contents may persist, in others they disappear. The formation of the conducting tubes or secretory sacs which occur in all parts of the higher plants is due either to the elongation of single cells or to the fusion of cells together in rows by the absorption of the cell-walls separating them. Such cell-fusions may be partial or complete. Cases of complete fusion occur in the formation of laticiferous vessels, and in the spiral, annular and reticulate vessels of the xylem. Incomplete fusion occurs in sieve tubes. Tubes formed by the elongation of single cells are found in bast fibres, tracheides, and especially in laticiferous cells. Laticiferous Tissue.—The laticiferous tissue consists of a network of branching or anastomosing tubes which contain a coagulable fluid known as latex. These tubes penetrate to all parts of the plant and occur in all parts of the root, stem and leaves. A protoplasinic lining is found on their walls which contains nuclei. Thewalls are pitted, and protoplasmic connexions between the laticiferous tubes and neighbouring parenchyma-cells have been seen. There are two types of laticiferous tissue—non-articulate and articulate. The non-articulate tissue which occurs in Euphorbiaceae, Apocynaceae, Urticaceae, Asclepiadaceae, consists of long tubes, equivalent to single multinucleate cells, which ramify in all directions throughout the plant. Laticiferous vessels arise by the coalescence of originally distinct cells. The cells not only fuse together in longitudinal and transverse rows, but put out transverse projections, which fuse with others of a similar nature, and thus form an anastomosing network of tubes which extends to all parts of the plant. They are found in the Compoditae (Cichoriaceae), Campanulaceae, Papaveraceae, Loheliaceae, Papayaceae, in some Aroideae and Musaceae, and in Euphorbiaceae (Manihot, Ilevea). The nuclei of the original cells persist in the protoplasmic membrane. The rows of cells from which the laticiferous vessels are formed can be distinguished in many cases in the young embryo while still in the dry seed (Scott), but the latex vessels in process of formation are more easily seen when germination has begun. In the process cf cell-fusion the cell-wall swells slightly and then begins to dissolve gradually at some one point. The opening, which is at first very small, increases in size, and before the cross-wall has entirely disappeared the contents of the two cells become continuous (Scott). The absorption of the cell-walls takes place very early in the germinating seedling. Sieve Tubes.—The sieve tubes consist of partially fused rows of cells, the transverse cr lateral walls being perforated by minute openings, through which the contents of the cells are connected with each other, and which after a certain time become closed by the formation of callus on the sieve plates. The sieve tubes contain a thin lining layer of protoplasm on their walls, but no nuclei, and the cell sap contains albuminous substances which are coagulable by heat. Starch grains are sometimes present. In close contact with the segments of the sieve tubes are companion ceils which communicate with the sieve tubes by delicate protoplasmic strands; they can be distinguished from ordinary parenchymatous cells by their small size and dense protoplasm. Companion cells are not found in the Pteridophyta and Gymnosperms. In the latter their place is taken by certain cells of the medullary rays and bast parenchyma. The companion cells are cut off from the same cells as those which unite to form the sieve tube. The mode of formation of the sieve plate is not certainly known; but from the fact that delicate connecting threads of protoplasm are present between the cells from their first development it is probable that it is a special case of the normal protoplasmic continuity, the sieve pores being produced by a secondary enlargement of the minute openings through which these delicate strands pass. According to Lecointe, the young wall consists partly of cellulose and partly of a substance which is not cellulose, the latter existing in the form of slight depressions, which mark the position of the future pores. As the sieve plate grows these non-cellulose regions swell and gradually become converted into the same kind of mucous substance as that contained in the tube; the two cells are thus placed in open communication. If this is correct it is easy to see that the changes which take place may be initiated by the original delicate protoplasmic strands which pass through the cell-wall. (For further information regarding tissues, see the section on Anatomy above.) Protoplasmic Continuity.—Except in the unicellular plants the cell is not an independent unit. Apart from their dependence in various ways upon neighbouring cells, the protoplasts of all plants are probably connected together by fine strands of protoplasm which pass through the cell-wall (Tangl, Russow, Gardiner, Kienitz-Gerloff and others) (fig. 9). In Pinus the presence of connecting threads has recently been demonstrated throughout all the tissues of the plant. These protoplasmic strands are, except in the case of sieve tubes, so delicate that special methods have to be employed to make them visible. The basis of these methods consists in causing a swelling of the cell-wall by means of sulphuric acid or zinc chloride, and subsequent staining with Hoffmann's blue or other aniline dyes. The a results so far obtained show that the connecting threads may be either " pit-threads " which traverse the (After Gardiner.) closing membrane of the pits in the FIG. 9.—Continuity of cell-walls (fig. 9, B), or "wall protoplasm of cells of Tamus communis and eiido threads " which are present perm in the perof f Lilium Martagon n wall of the cell (fig. 9, A). Both (B). pit-threads and wall-threads may occur in the same cell, but more often the threads are limited to the pits. The pit-threads are larger and stain more readily than the wall-threads. The threads vary in size in different plants. They are very thick in Viscum album, and are well seen in Phaseolus multi, florus and Lilium Martagon. They are present from the beginning of the development of the cell-wall, and arise from the spindle fibres, all of which may be continued as connecting threads (endosperm of Tamus communis), or part of them may be overlaid by cellulose lamellae (endosperm of Lilium Martagon), or they may be all overlaid as in pollen mother-cells and pollen grains of Helleborus foetidus. The presence of these threads between all the cells of the plant shows that the plant body must be regarded as a connected whole; the threads themselves probably play an important part in the growth of the cell-wall, the conduction of food and water, the process of secretion and the transmission of impulses.
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