Page:Encyclopædia Britannica, Ninth Edition, v. 16.djvu/873

Rh MORPHOLOGY 841 plants, as well as of at least the higher animals, have been studied with much and ever-increasing accuracy of detail. (See ANA TOMY, HISTOLOGY, EMBRYOLOGY.) Both vegetable and animal tissues have been simply classified both according to their adult forms and according to the embryonic layers from which they respectively arise. This scrutiny of plant and animal structure over and above the special generalizations of the botanist and the /oologist has afforded much result to general histology. The improvement of technical methods has of late years largely aided the progress of discovery. A return from the study of the cell- aggregate to that of the cell has commenced, and the question of i -ell -structure may bo said to be again paramount in histology. The process of transverse division has of late been much elucidated, and, although its complex details cannot here be entered upon, the result has been to establish a minute and thorough correspond ence in cases so widely dissimilar as pollen -grains from a flower-bud, the epidermis of a tadpole, or the cells of a tumour a result which obviously enhances the morphological completeness of the cell theory. Minor modes of cell-multiplication also are not without their morphological interest. Gemmation, familiar in the yeast plant, occurs in other low and simple organisms, and may probably lie identified with the formation of polar vesicles in ova as a modi fication of transverse division. Schleiden had supposed all new cells to originate within pre-existing cells, and this process, known as free -cell-formation, may really be observed in various plant and animal tissues. The protoplasm groups itself round new nuclei, the new cells being in fact formed much as Schwann had in his turn supposed ; but these nuclei have repeatedly been shown to arise from segmentation of the original nucleus, and thus this pro cess too seems a mere modification of the general one of transverse division. Conjugation, too that coalescence of two similar cells which may be observed in many Algae, Fungi, and Protozoa is to be considered as the undifferentiated form of that fertilization which occurs in higher animals and plants, the two apparently similar masses having become respectively differentiated into ovum and spermatozoon, or into egg-cell and antherozoid. An indefinite number of amoeboid cells sometimes flow together into a single mass, a phenomenon regarded by some as multiple-conjugation, or perhaps more probably as an almost mechanical coalescence of exhausted cells, from which conjugation proper and finally fertili zation may indeed have originated. The amoeboid cells of higher animals similarly unite when drawn, and this formation of plas- inodia, as these are termed, seems to be a deep-seated property of the aniceboid cell. Similarly, too, the process of rejuvenescence which occurs in many of the lowest plants and animals, such as Protococcus and Amoeba, where the protoplasm passes from a rest ing and encysted to a naked and mobile stage, has many analogues not only among the Protista but even in the tissues of higher animals, while the phases which the lowest organisms more or less exhibit the encysted, the ciliated, the amoeboid, and the plasmo- dial may be regarded as the fundamental forms of a &quot;life-cycle,&quot; fully represented indeed only in such extremely low organisms as Protomyxa and Myxomycetes, yet nowhere completely suppressed. The very highest plants and animals may thus be considered as aggregates of more or less differentiated and variously arranged encysted, amoeboid, and ciliated cells, while their development and subservient changes, their variations normal and pathological, in reality exhibit phases more or less distinct of the ancestral life- cycle. The examination of the precise modes of cell -division, particularly in the hands of botanists (see BIOLOGY, and summary in Sachs s l^orlcsuncjen ubcr Pflanzcn Physiologic, 1883), are also constantly throwing the most interesting light upon the structure of the adult organism. Thus then, in our own day as in those of Bichat or Schwann, the labours of the histologist, when inspired by higher aims than that of the mere multiplication of descriptive detail, are of supreme morphological importance, and result in the demonstra tion of a unity of organic structure deeper even than any which we owe to Linnaeus or Cuvier, Goethe or Geoffrey. 4. Individuality. Probably no subject in the whole range of biology has been more extensively discussed than that of the nature of organic individuality. The history of the controversy is of interest, since besides leading up to solid results it serves, perhaps better than any other case, to illustrate the slow emergence of the natural sciences from the influence of scholastic thought. Starting from the obvious unity and indivisibleness of Man and other higher animals, and adopting some definition such as that of Mirbel (exceptionally unmetaphysical, however), &quot;Tout etre organise, complet dans ses parties, distinct et separe des autres etres, est un individu,&quot; it was attempted times without number to discover the same conception elsewhere in nature, or rather to impose it upon all other beings, plants and animals alike. The results of different inquirers were of course utterly discrepant. It seemed easy and natural to identify a tree or herb corresponding to the individual animal, yet difficulties at once arose. Many apparently distinct plants may arise from a common root, or a single plant may be decomposed into branches, twigs, shoots, buds, or even leaves, all often capable of separate existence. These, again, arc decompos able into tissues and cells, the cells into nucleus, &c., and ultimately into protoplasmic molecules, these finally into atoms, the inquiry thus passing outside organic nature altogether and meeting the old dispute as to the ultimate divisibility of matter. In short, as Haeckel remarks, scarcely any part of the plant can be named which has not been taken by some one for the individual. It is necessary, therefore, briefly to notice some of the principal works on the subject, and these may conveniently be taken in descending order. While Cassini practically agreed with Mirbel in attempting to regard separate plants as individuals, the widest interpretation of the individual is that of Gallesio (1816), who proposed to regard as an individual the entire product of a single seed, alike whether this developed into a uni-axial plant extended continuously like a Banyan, or multiplied asexually by natural or artificial means like the Weeping- willow or the Canadian Pond weed, of each of which, on this view, there is only a single individual in Britain, happily discontinuous. At once the oldest and most frequently maintained view is that which regards the bud or shoot consisting of a single axis with appendages as the plant-individual, of which the tree represents a colony, like a branched hydroid Polyp. This conception, often attributed to Aristotle, but apparently without foundation, appears distincuy in the writings of Hippocrates and Theophrastus, the latter saying, The bud grows on the tree like a plant in the ground. &quot; The aphorism of Linnaeus, Gemmae totidem herbae, &quot; is well known ; and in this view C. F. Wolff and Humboldt concurred, while Erasmus Darwin supported it by an appeal to the facts of anatomy and development. The most influential advocate of the bud theory during the first half of the present century was, however, Du Petit- Thouars, who, although starting much as usual with a &quot;principe unique d existence,&quot; supported his theory on extensive though largely incorrect observations on stem structure and growth. For him the tree is a colony oiphytons, each being a bud with its axillant leaf and fraction of the stem and root. Passing over numerous minor authors, we dome to the central work of Alex. Braun (1853), in which, as Sachs has clearly pointed out, the illegitimate com bination of Naturphilosophie with inductive morphology reaches its extreme. He reviews, however, all preceding theories, admits the difficulty of fixing upon any as final, since the plant, physio logically considered, is rather a dividuum than an individuum, and proposes as a compromise, or indeed as a partial cutting of the knot, the adoption of the shoot as the morphological individual, comparable to an animal, especially because, unlike the cell, leaf, &c., it includes all the representative characters of the species. Darwin and Spencer on the whole also accept the bud or shoot as at any rate the most definite individual. The theory of metamorphosis naturally led Goethe, Oken, and others to regard the leaf as the individual, while Johannes Miiller, Steenstrup, and others adopted the same view on various physio logical grounds. Gaudichaud elaborated a theory intermediate between this view and that of Du Petit-Thouars, according to which the plant was built up of individuals, each consisting of a leaf with its subjacent internode of stem, which was regarded as the leaf-base, and this was supported by Edward Forbes and others, while the nominally converse view that of the leaf as a mere outward ex pansion of the stem-segment was proposed by Hochstetter. Though sundry attempts at identifying various tissues, such as the fibro-vascular bundles, as the constituent individuals may be passed over, those associated with the cell theory are of great importance. Schwann decided in favour of the cell and regarded the plant as a cell-community, in which the separate elements were like the bees of a swarm, a view virtually concurred in in all essential respects by Schleiden, Virchow, and other founders of the cell theory. Yet, although the structure and functions of the plant are ultimately and exclusively cellular, it is impossible to ignore the fact that, save in the very lowest organisms, these are subordi nated and differentiated into larger aggregates, and form virtually but the bricks of a building, and hence the later theories outlined above. Of attempts to find the individual in the nucleus or the protoplasm granules it is of course unnecessary to speak further. So far the theories of absolute individuality. The conception of relative individuality is well traced by Fisch upwards from the more or less vague suggestions in the writings of Goethe, Roeper, and the elder De Candolle to its clear expression in Alphonse do Candolle and Schleiden, both of whom take the cell, the shoot, and the multi-axial plant as forming three successive and subordinated categories. Nageli too recognized not only the necessity of establish ing such a series (cell, organ, bud, leafy axis, multi-axial plant) but the distinction between morphological and physiological in dividualities afterwards enunciated by Haeckel. Passing over the difficulties which arise even among the Protozoa (see FORAMINIFERA), we find that a similar controversy (fully chronicled in Haeckel s Kalkschwdmmc] has raged over the in dividuality of Sponges. While the older observers were content to regard each sponge-mass as an individual, a view in which Lieberkuhn XVI. 1 06