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MORPHOLOGY OF

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Originally appearing in Volume V21, Page 777 of the 1911 Encyclopedia Britannica.
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MORPHOLOGY OF PLANTS The term morphology, which was introduced into science by Goethe (1817), designates, in the first place, the study of the form and composition of the body and of the parts of which the body may consist; secondly, the relations of the parts of the same body; thirdly, the comparison of the bodies or parts of the bodies of plants of different kinds; fourthly, the study of the development of the body and of its parts (ontogeny); fifthly, the investigation of the historical origin and descent of the body and its parts (phylageny); and, lastly, the consideration of the relation of the parts of the body to their various functions, a study that is known as organography. It is this last department of morphology that was the first to be pursued. The earliest scientific result of the study of plants was the recognition of the fact that the various parts of the body are associated with the performance of different kinds of physiological work; that they are, in fact, organs discharging special functions. The origin of the organography of the present day may be traced back to Aristotle, who described the parts of plants as " organs, though very simple ones." It was not until many centuries had passed that the parts began to be regarded from the point of view of their essential nature and of their mutual relations; that is, morphologically instead of organographically. Joachim Jung, in his Isagoge phytoscopica (1678), recognized that the plant-body consists of certain definite members, root, stem and leaf, and defined them by their different form and by their mutual relations. This point of view was further developed in the following century by Caspar Friedrich Wolff (Theoria generationis, 1759), who first followed the development of the members at the growing-point of the stem. He observed that the " appendicular organs," as he called the leaves, are developed in the same way, whether they be foliage-leaves, or parts of the flower, and stated his conclusions thus: " In the entire plant, whose parts we wonder at as being, at the first glance, so extraordinarily diverse, I finally perceive and recognize nothing beyond leaves and. stem (for the root may be regarded as a stem). Consequently all parts of the plant, except the stem, are modified leaves." Similar views were arrived at by Goethe, though by the deductive rather than the inductive method, and were propounded in his famous pamphlet, Versuch die Metamorphose der Pflanzen zu erklaren (1790), from which the following is a quotation: " The underlying relationship between the various external parts of the plant, such as the leaves, the calyx, the corolla, the stamens, which develop one after the other and, as it were, out of one another, has long been generally recognized by investigators, and has in fact been specially studied; and the operation by which one and the same organ presents itself to us in various forms has been termed Metamorphosis of Plants." Pure Morphology.—Thus it became apparent that the many and various organs of plants are, for the most part, different forms of a small number of members of the body, which have been distinguished as follows, without any reference to function. The thallus (thallome) is a plant-body which is not differentiated into the members root, stem and leaf; it is the morphologically simplest body, such as is of common occurrence in the lower plants (e.g. Thallophyta). In a differentiated body the stem (caulome) is an axis capable of bearing leaves and (directly or indirectly) the proper reproductive organs. The leaf (phyllome) is an appendicular member only borne by a stem, but differing from it more or less obviously in form and development, though co-ordinate with it in complexity of structure. The root is an axis which never bears either leaves or the proper reproductive organs (whether sexual or asexual) of the plant. The hair (trichome) is a superficial appendage of simple structure, which may be borne by any of the other members. The emergence is also an appendicular member of more complex structure than the hair (e.g. the prickles of the rose). Further, it has been found convenient to designate the leaf-bearing stem as a whole by the term shoot, so that the body may, as Sachs suggested, be primarily analysed into shoot and root. At the present time some objection is being taken to this purely morphological conception of the body and its parts as being too abstract. It is urged that the various parts are, as a matter of fact, organs; and that it is therefore inadmissible to ignore their functions, as is done in the foregoing definitions. To this it may be replied that pure morphology and organography are not alternatives, but are two complementary and equally necessary modes of considering the composition of the plant-body. Moreover, the abstract terms " stem," " leaf," " root," &c., are absolutely indispensable; and are continually used in this sense by the most ardent organographers. It has not yet been suggested that they should be replaced by organographical terms; were this accomplished, descriptive botany would become impossible. It is also urged against these definitions that they are not of universal applicability; that there are exceptional structures which cannot be brought within the limits of any one of them. But admitting the validity of this criticism, and even going so far as to question the possibility of ever devising absolutely inclusive and, at the same time, exclusive definitions, no sufficient reason is adduced for giving up all attempt at morphological analysis. Homology.—All members belonging to the same morphological category are said to be homologous, however diverse their functions. Thus, in a phanerogam, the sepals, petals, stamens and foliage-leaves all come under the category leaf, though some are parts of the perianth, others are spore-bearing organs (sporophylls), and others carry on nutritive processes. The homology of members was based, in the first instance, upon similarity of development and upon similar relations to the other parts of the body, that is, upon ontogeny. But since the general adoption of the theory of evolution, similarity of descent, that is of phylogeny, has come to form an essential part of this conception; in other words, in order that their homology may be established the parts compared must be proved to be homogenetic. The introduction of the phylogenetic factor has very much increased the difficulty of determining homologies; for the data necessary for tracing phylogeny can only be obtained by the study of a series of allied, presumably ancestral, forms. One of the chief difficulties met with in this line of research, which is one of the more striking developments of modern morphology, is that of distinguishing between organs which are " reduced," and those which are really " primitive." The object of the phylogenetic study of any organ is to trace it back to its primitive form. But, as will be pointed out later, organs are often found to have undergone " degeneration " or " reduction," and such reduced or degenerate structures may easily be mistaken for primitive structures, and so the investigator may be misled. The effect of the phylogenetic factor in homology may be illustrated in the following cases. The leaves of the true mossesand those of the club-mosses (Lycopodium, Selaginella) being somewhat alike in general appearance and in ontogeny, might be, and indeed have been, regarded as homologous on that ground. However, they belong respectively to two different forms in the life-history of the plants; the leaves of the mosses are borne by the gametophyte, those of the club-mosses by the sporophyte. In accordance with the prevalent antithetic view of the alternation of generations in these plants (see PLANTS, REPRODUCTION OF), the forms distinguished as sporophyte and gametophyte are not homogenetic; consequently their leaves are not homologous, but are only functionally similar (homoplastic; see infra). Another effect is that different degrees of homology have to be recognized, just as there are different degrees of relationship or affinity between individual plants. When two organs can be traced along the same line of descent to one primitive form, that is when they are found to be monophyletic, their homology is complete; when, however, they are traceable to two primitive forms, though these forms belong to the same morphological series, they are polyphyletic and therefore only incompletely homologous. For instance, all the leaves of the Bryophyta are generally homologous inasmuch as they are all developments of the gametophyte. But there is reason to believe that they have been differentiated quite independently in various groups, such as the Marchantiaceae, the Jungermanniaceae, and the mosses proper; consequently their phylogeny is not the same, they are polyphyletic, and therefore they are not completely homologous, but are parallel developments. Analogy.—Considering the parts of the body in relation to their functions, that is as organs, they are found to present peculiarities of form and structure which are correlated with the functions that they have to discharge; in other words, the organ shows adaptation to its functions. All organs performing the same function and showing similar adaptations are said to be analogous or homoplastic, whatever their morphological nature may be; hence organs are sometimes both homologous and analogous, sometimes only analogous. The tendrils of a vetch and of a cucumber are analogous, and also homologous because they both belong to the category leaf; but they are only analogous to the tendrils of the vine and of the passion-flower, which belong to the category stem. Metamorphosis.—It has already been pointed out that each kind of member of the body may present a variety of forms. For example, a stem may be a tree-trunk, or a twining stem, or a tendril, or a thorn, or a creeping rhizome, or a tuber; a leaf may be a green foliage-leaf, or a scale protecting a bud, or a tendril, or a pitcher, or a floral leaf, either sepal, petal, stamen or carpel (sporophyll); a root may be a fibrous root, or a swollen tap-root like that of the beet or the turnip. All these various forms are organs discharging some special function, and are examples of what Wolff called " modification," and Goethe " metamorphosis." It may be inquired what meaning is to be attached to these expressions, and what are the conditions and the nature of the changes assumed by them. The leaf of the higher plants will be taken as the illustrative case because it is the most " plastic of the members, the one, that is, which presents the greatest variety of adaptations, and because it has been most thoroughly studied. In this, as in all morphological inquiries, two lines of investigation have to be followed, the phylogenetic and the ontogenetic. Beginning with its phylogeny, it appears, so far as present knowledge goes, that the differentiation of the shoot of the sporophyte into stem and leaf first occurred in the Pteridophyta; and, in accordance with the views of Bower (Origin of a Land-Flora), the primitive leaf was a reproductive leaf, a sporophyll, from which the foliage-leaf was derived by progressive sterilization. From the nature of the case, this view is not, and could not be, based upon actual observation, nor is it universally accepted; however, it seems to correspond more closely than any other to the facts of comparative morphology. It was formerly assumed, and the view is still held, that the foliage-leaf was the primitive form from which all others were derived, mainly on the ground that, in ontogeny, the foliage-leaf generally precedes the sporophyll. The phylogeny of the various floral leaves, for instance, was generally traced as follows: foliage-leaf, bract, sepal, petal, stamen and carpel (sporophylls)—in accordance with what Goethe termed " ascending metamorphosis." Recent researches, however, more especially those of Celakovsky, tend to prove that the perianth-leaves have been derived from the stamens (i.e. from sporophylls); that is, they are the result of " descending metamorphosis." Moreover there is the fact that the flowers of nearly all the primitive phanerogams, such as the Gymnosperms, consist solely of sporophylls, having no perianth. There is thus a considerable body of evidence to support Bower's view of the primitive nature of the sporophyll. Accepting this view of the phylogeny of the leaf, the perianth-leaves (sepals and petals) and the foliage-leaves may be regarded as " modified " or " metamorphosed " sporophylls; that is, as leaves which are adapted to functions other than the bearing of spores. The sepals are generally organs for the protection of the flower-bud; the petals, for attracting insects by their conspicuous form and colour; the foliage-leaves, for the assimilation of carbon dioxide and other associated functions. But this phylogenetic differentiation of the organs was not what Wolff and Goethe had in mind; what they contemplated was an onto-genetic change, and there is abundant evidence that such changes actually occur. Taking first the conversion of members of one morphological category into those of another, this has been actually observed, though rarely. Goebel (Organography) gives several instances of the conversion of the root into a shoot in ferns, and a few in phanerogams (Listera ovata, Neottia nidusavis, Anthurium longifolium). Much more common is the conversion of one form of a member into another form. The most varied changes of this kind have been described, and are generally familiar as " monstrosities "; the study of them constitutes, under the name of teratology, a distinct department of biology. A simple case is that of " double " flowers, in which the number of the petals is increased by the " metamorphosis " of stamens; or again the conversion of floral leaves into green leaves, a change known as " chloranthy." These changes may be brought about by external causes, such as the attacks of insects or of fungi, alterations in external conditions, &c., or by some unexplained internal disturbance of the morphological equilibrium. They can also be effected experimentally. Goebel has shown that if the developing foliage-leaves of the fern Onoclea struthiopteris be removed as they are formed, the subsequently developed sporophylls assume more or less completely the habit of foliage-leaves, and may be sterile. Similarly bud-scales can be caused to develop into foliage-leaves, if the buds to which they belong are caused to grow out in the year of their formation by the removal of the existing foliage-leaves. Useful and suggestive as they often are, teratological facts played, at one time, too large a part in the framing of morphological theories; for it was thought that the " monstrous " form gave a clue to the essential nature of the organ assuming it. There is, however, no sufficient reason for regarding the monstrous form as necessarily primitive or ancestral, nor even as a stage in the ontogeny of the organ. For when the older morphologists spoke of a stamen as a " metamorphosed " leaf, it was implied that it originated as a foliage-leaf and subsequently became a stamen. As a matter of fact, a stamen is a stamen and nothing else, from the very beginning. The development of the organ is already determined at its first appearance upon the growing-point; though, as already explained, the normal course of its ontogeny may be interfered with by some abnormal external or internal condition. The word " metamorphosis " cannot, in fact, be used any longer in its original sense, for the change which it implied does not normally occur in ontogeny, and in phylogeny the idea is more accurately expressed by the term " differentiation." However, it may still be useful in describing " monstrosities," and perhaps also those cases in which an organ serves first one purpose and then another, as when a leafy shoot eventually becomes a thorn, or the base of a foliage-leaf becomes a bud-scale. Dijerentiation.—Any account of the general morphology of living organisms is incomplete if it does not include some attempt at an explanation of its causation; though such an attempt cannot be carried far at the present time. A survey of the vegetable kingdom indicates that evolution has proceeded, on the whole, from the simple to the complex; at the same time, as has been already mentioned, evidence of reduction or degeneration in common. Thus in the series Bryophyta, Pteridophyta, Phanerogamia, whilst the sporophyte presents progressive development, the gametophyte presents continuous reduction. Evolution means the gradual development of " highly organized " from " lowly organized " forms; that is, of forms in which the " physiological division of labour " is more complete, from those in which it is less complete; of forms possessing a variety of organs, from forms possessing but few. Differentiation means the development and the specialization as organs of various parts of the body. It presents itself in two aspects: there is morphological differentiation, which can be traced in the distinction of the members of the body, root, stem, leaf, &c.; there is physiological differentiation, which can be traced in the adaptation of these members to various functions. But, in actual operation, these two processes are simultaneous; every member is developed as an organ for the performance of some special function. Factors in Evolution.—Evolution in the race involves progressive differentiation in the individual; hence the causes of evolution and of differentiation must be the same. The evolution of higher from lower plants, it is generally assumed, has proceeded by variation. With regard to the causation of variation Darwin says (Origin of Species, ch. v.) : " In all cases there are two factors, the nature of the organism, which is much the most important of the two, and the nature of the conditions. The direct action of changed conditions leads to definite or indefinite results. In the latter case the organization seems to become plastic, and we have much fluctuating variability. In the former case the nature of the organism is such that it yields readily, when subjected to certain conditions, and all or nearly all the individuals become modified in the same way." In spite of the statement that the " nature of the organism " is the most important factor in variation, the tendency amongst evolutionists has been to take much more account of the influence of external conditions. Exceptions to this attitude are Lamarck, who speaks with regard to animals (but not to plants!) of f` la composition croissante de l'organisation " (Philosophic zoologique, t. i.), and Nageli, who attributes variation to causes inherent in the " idioplasm," and has elaborately worked out the view in his Abstammungslehre. The position assumed in this article is in agreement with the views of Lamarck and of Nageli. All but the lowest plants visibly tend towards or actually achieve in various degrees the differentiation of the body, whether sporophyte or gametophyte, into stem, leaf, root, &c., that is, the differentiation of parts not previously present. It is inconceivable that external conditions can impart to an organism the capacity to develop something that it does not already possess: can impart to it, that is, the capacity for variation in the direction of higher complexity. The alternative, which is here accepted, is that differentiation is essentially the expression of a developmental tendency inherent in the protoplasm of plants. Just as every crystallizable chemical substance assumes a definite and constant crystalline form which cannot be accounted for otherwise than by regarding it as one of the properties of the substance, so every living organism assumes a characteristic form which is the outcome of the properties of its protoplasm. But whereas the crystalline form of a chemical substance is stable and fixed, the organized form of a living organism is unstable and subject to change. Influence of External Conditions.—This position does not, however, exclude the influence of external conditions; that influence is undeniable. Darwin's expression " the nature of the organism " has been interpreted in the preceding paragraph to mean an inherent tendency towards higher organization; that interpretation may now be completed by adding that the organism is susceptible to, and can respond to, the action of external conditions. There is every reason to believe that plants are as " irritable " to varying external conditions as they are to light or to gravity. A change in its external conditions may act as a " stimulus," evoking in the organism a response of the nature of a change in its form. As Darwin has pointed out, this response may be direct or indirect. In illustration of the indirect response, the evolution of the Bryophyta and of more highly organized plants may be briefly considered. It is generally admitted that life originated in water, and that the earliest plants were Algae. The study of existing Algae, that is of plants that have continued to live in water, shows that under these conditions no high degree of organization has been reached, though some of them have attained gigantic dimensions. The primitive water-plants were succeeded by land-plants, a land-flora being gradually established. With the transition from water to land came the progressive development of the sporophyte which is the characteristic feature of the morphology of the Bryophyta and of all plants above them in the scale of life (see Bower, Origin of a Land-Flora). This evolution of the sporophyte is no doubt to be correlated with the great change in the external conditions of life. There is no conclusive ground for regarding the action of this change as having been direct, it is more reasonable to regard it as indirect, having acted as a general stimulus to which the ever-increasing complexity of the sporophyte was the response. Adaptation.—The morphological and physiological differentiation of the plant-body has, so far, been attributed to (I) " the nature of the organism," that is to its inherent tendency towards higher organization, and (2) to the " indefinite results " of the external conditions acting as a stimulus which excites the organism to variation, but does not direct the course of variation. The " definite results " of the action of external conditions have still to be considered. It is a familiar observation that climatic and edaphic (nature of soil) conditions exert an influence upon the form and structure of plants (see PLANTS : Ecology of). For instance, some xerophytes are dry and hard in structure, whilst others are succulent and fleshy. This so-called direct effect of external conditions upon the form and structure of the body differs from the indirect effect in that the resulting variations bear a relation, of the nature of adaptation, to those conditions; the effect of the conditions is not only to cause variation, but to cause variation in a particular direction. Thus all existing hygrophytes (excepting the Algae) are considered to have been derived from land-plants which have adapted themselves to a watery habitat. The effect can also be demonstrated experimentally: thus it has been observed that a xerophyte grown in moist air will lose its characteristic adaptive features, and may even assume those of a hygrophyte. Climatic and edaphic conditions are not, however, the only ones to affect the structure and composition of the body or its parts; other conditions are of importance, particularly the relations of the plant to animals and to other plants. For instance, the " animal traps " of carnivorous plants (Drosera, Nepenthes, &c.) did not, presumably, originate as such; they began as organs of quite another kind which became adapted to their present function in consequence of animals having been accidentally caught. It is also probable that the various forms of the angiospermous flower, with its many specialized mechanisms for pollination, may be the result of insect-visits, the flowers becoming adapted to certain kinds of insects, and the insects having undergone corresponding modification. Parasites, again, were derived from normal autotrophic plants, which, as the parasitic habit became more pronounced, acquired the corresponding characteristics of form and structure; there is, in fact, the group of hemi-parasites, plants which still retain autotrophic characters though they are root-parasites. Though adaptation to the environment seems sometimes to be considered, especially by neo-Lamarckians, as equivalent to, or at least as involving, the evolution of higher forms from lower, there does not appear to be any evidence that this is thecase. The effect of external conditions is confined to the modification in various directions of members or organs already 'existing, and cne very common direction is that of reduction or entire disappearance of parts: for instance, the foliage-leaves of certain xerophytes (e.g. Cactaceae, Euphorbiaceae), of parasites, and of saprophytes. Moreover, had the evolution of plants proceeded along the line of adaptation, the vegetable kingdom could not be subdivided, as it is, into the morphological groups Thallophyta, Bryophyta, Pteridophyta, Phanerogamia, but only into physiological groups, Xerophyta, Hygrophyta, Tropophyta, &c. In endeavouring to trace the causation of adaptation, it is obvious that it must be due quite as much to properties inherent in the plant as to the action of external conditions; the plant must possess adaptive capacity. In other words, the plant must be irritable to the stimulus exerted from without, and be capable of responding to it by changes of form and structure. Thus there is no essential difference between the " direct " and the " indirect " action of external conditions, the difference is one of degree only.. In the one case the stimulus induces indefinite variation, in the other definite; but no hard-and-fast line can be drawn between them. Adaptive characters are often hereditary, for instance, the seed of a parasite will produce a parasite, and the same is true of a carnivorous plant. On the other hand, adaptations, especially those evoked by climatic or edaphic conditions, may only be shown by the seedling if grown under the appropriate external conditions; here what is hereditary is not the actual adaptation, but the capacity for responding in a particular way to a certain set of external conditions. Summary.—The general theory of differentiation propounded in this article is an attempt at an analysis of the factors termed by Darwin " the nature of the organism " and " the nature of the conditions." It is assumed, as an inevitable conclusion from the facts of evolution, that plant-protoplasm possesses (r) an inherent tendency towards higher organization, and (2) that it is irritable to external conditions, or to changes in them, and can respond to them by changes of form which may be either indefinite or definite (adaptive). Thus it is that the variations are produced upon which natural selection has to work. Material Cause of Differentiation.—It may be inquired, in conclusion, if there are any facts which throw light upon the internal mechanism of differentiation, whether spontaneous or induced; if it is possible to refer it to any material cause. It may be replied that there are such facts, and though they are but few as yet, they suffice to suggest an hypothesis that may eventually prove to be a law. Sachs was the first to formulate the theory that morphological differences are the expression of differences in material composition. He considered, for instance, that stems, leaves, roots and flowers differ as they do because the plastic substances entering into their structure are diverse. This view he subsequently modified to this—that a relatively small proportion of diverse substance in each of these parts would suffice to account for their morphological differences. This modification is important, because it transfers the formative influence from the plastic substances to the protoplasm, suggesting that, the diverse constituents are produced (whether spontaneously or as the result of stimulation) as secretions by the protoplasm. It is an obvious inference that if a small quantity of a substance can affect the development of an entire organ it probably acts after the manner of an enzyme. Beyerinck has, in fact, gone so far as to speak of " formative enzymes." It is not possible to go into all the facts that might be adduced in support of this view: one case, perhaps the most pregnant, must suffice. Beyerinck was led to take up the decided position just mentioned by his researches into the conditions determining the formation of plant-galls as the result of injury by insects. He found that the development of a gall is due to a temporary modification of the part affected, not, as is generally thought, in consequence of the deposition of an egg by the insect, but of the injection of a poisonous substance which has the effect of stimulating the protoplasm to develop a gall instead of normal structure. If this be so,it may justifiably be inferred that both normal and abnormal morphological features may be due to the presence of enzymatic substances secreted by the protoplasm that determine the course of development. At any rate this hypothesis suggests an explanation of many hitherto inexplicable facts. For instance, it has been pointed out in the article on the reproduction of plants that the effect of the fertilization of the female cell in the ovule of a phanerogam is not confined to the female cell, but extends more or less widely outside it, inducing growth and tissue-change. The ovule develops into the seed; and the gynaeceum and even more remote parts of the flower, develop into the fruit. The facts are familiar, but there is no means of explaining them. In the light of Sachs's theory the interpretation is this, that the act of fertilization causes the formation in the female cell of substances which are transmitted to adjacent structures and stimulate them to further development.
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