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 ANATOMY

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Boyle Lecture, Oxford, 1894; see also Monster {Ency. Brit. vol. xvi. p. 762) and Bateson, Materials for the Study of Variation, 1895. History of Anatomy.—To the detailed history given in vol. i. pp. 799 et seqq., it is only necessary to add that recent researches on the early history of anatomy will be found in the several articles entitled “ Archieologia Anatomica ” in the Journal of Anatomy for 1898, 1899, and 1900. (a. Ma.) Anatomy of Plants. The term “ Anatomy,” originally employed in biological science to denote a description of the facts of structure revealed on cutting up an organism, whether with or without the aid of lenses for the purpose of magnification, is restricted in the present article, in accordance with a common modern use, to those facts of internal structure not concerned with the constitution of the individual cell, the structural unit of which the plant is composed. For a description of the cell itself, the article Cytology must be consulted. In all but the very simplest forms the plant-body is built up of a number of these cells, associated in more or less definite ways. In the higher (more complicated) forms the cells differ very much among themselves, and the body is composed of definite systems of these units, each system with its own characteristic structure, depending partly on the characters of the component cells and partly on the method of association. Such a system is called a tissuesystem, the word tissue being employed for any collection of cells with common structural, developmental, or functional characters to which it may be conveniently applied. The word is derived from the general resemblance in the texture of plant substance to that of a textile fabric, and dates from a period when the fundamental constitution of this substance from individual cells was not yet discovered. The study of tissues is known as Histology, and it is with this that the present article is necessarily mainly concerned. The method of treatment will be evolutionary; that is to say, an endeavour will be made to trace the gradual steps through which the more complicated tissue-systems of the various groups of plants have been developed from simple cell-collections, during that descent of the more complicated from the simpler plants which constitutes the fundamental postulate of the comparative study of form in modern biology. The terms morphology and morphological wib be used throughout wherever such comparative consideration with this evolutionary implication is involved.^ It must be understood, however, that the evolution of form is invariably governed by the vital needs of the organism, so that a consideration of the vital relations of the plant to its surroundings, and of the actions and functional relations of its organs and tissues, is an indispensable condition of the construction of a real and intelligible morphology. _ In this way our outline of morphological anatomy will involve an outline of physiological anatomy. It will easily be realized that since the plant-body consists entirely of tissues, through which all its functions are performed, an account of the evolution of plant-tissues is nearly equivalent to an account of the evolution of plants. Our task will be simplified, however, by the omission of direct reference to the tissues of the specialized reproductive organs, for which the reader is referred to the articles on the different groups of plants. Even so it must be apparent that in the present article nothing but the barest sketch of the subject, in which much is left out and raariy topics are treated dogmatically, is at all possible. The articles Botany (Ency. Brit. vol. iv. pp. 85-93, and 99-108), and Histology, Vegetable (vol. xii.), give good general accounts of the nature of vegetable tissues, particularly in the flowering plants, and in the former are many figures, for the most part admirable, illustrating histological

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structure, which the reader may consult with advantage. Much of the terminology and some of the descriptions are, it is true, out of date, a result that must always soon occur in a very rapidly advancing branch of science, as plant anatomy was twenty years ago. The morphological point of view, as sketched above, has been fully applied to planttissues only in very recent years, so that in an article of earlier date it is only natural that the provisional nature of descriptive terminology should become very apparent after the lapse of a few years. An account of the structure of plants naturally begins with the cell, which is the proximate unit of organic structure. The cell is essentially an individualized mass of protoplasm containing a differentiated protoplasmic body, called a nucleus. But all cells which are permanent tissue-elements of the plant-body possess, in addition, a more or less rigid limiting membrane or cell-wall, consisting primarily of cellulose or some allied substance. It is the cell-walls which connect the different cells of a tissue, and it is upon their characters (thickness, sculpture, and constitution) that the qualities of the tissue largely depend. In many cases, indeed, after the completion of the cell-wall (which is secreted by the living cell-body) the protoplasm dies, and a tissue in which this has occurred consists^ solely of the dead framework of cell-walls, enclosing in the cavities or lumina originally occupied by the protoplasm simply water or air. In such cases the characters of the adult tissue clearly depend solely upon the characters of the cell-walls, and it is usual in plant anatomy to speak of the wall with its enclosed cavity as “the cell,” and the contained protoplasm or other substances, if present, as cell-contents. This is in accordance with the original use of the term “ cell,” which was applied in the 1 (th century to the cavities of plant-tissues on the analogy of the cells of honeycomb. The use of the term to mean the individualized nucleated mass of living protoplasm, which, whether with or without a limiting membrane, forms primitively the proximate histological element of the body of every organism, dates from the second quarter of the 19th century. We proceed to a systematic account of the anatomy of the different groups of plants, beginning with the simplest, and passing to the more complicated forms. Thallophyta. The simplest members of both the Algae and the Fungi (q.v.), the two divisions of the Thallophyta, the lowest of the four great groups into which the plant - kingdom is divided, have their bodies each composed of a single cell. In the Algae such a cell consists essentially of—(1) a mass of protoplasm provided with (2) a nucleus; and (3) an assimilating apparatus consisting of a coloured protoplasmic body, called a chromatophore, the pigment of which in the pure green forms is chlorophyll, and which may then be called a chloroplast. The whole is covered by the cellmembrane (Fig. 1 A). It is from such a living and assimilating cell, performing as it does all the vital functions of a green plant, that, according to current theory, all the different cell-forms of a higher plant have been differentiated in the course of descent. Among the Green Algfe the differentiation of cells is comparatively slight. Many forms, even when multicellular, have all their cells identical in structure and function, and are often CelI aa(j spoken of as “ physiologically unicellular. ” The cells Ussue dif. are commonly joined end to end in simple or branched fereatjati0a filaments. Such differentiation as exists in the higher ja types mainly takes two directions. In the fixed forms, the cell or cells which attach the plant to the substratum otten have a peculiar form, containing less chlorophyll, and constituting a rudimentary fixing organ or rhizoid (Fig. 1 C). In certain types living on damp soil, therhizoids penetrate the substratum, and m addition to fixing the plant absorb food substances (dissolve salts) from the substratum (QUdocladium, Fig. 1 D).