CHAPTER II. ELEMENTARY STRUCTURE OF PLANTS, OR VEGETABLE HISTOLOGY. SECTION 1.-OF THE CELL AS AN INDIVIDUAL. THE description of the elementary structure of plants is termed Vegetable Histology. All the lower kinds of plants are made up of one or more membranous closed sacs called cells; and all other plants, however complicated in their appearance and structure, are also made up of these simple bodies variously modified and combined together. The cell is therefore. the only elementary organ possessed by a plant; and hence necessarily requires our first and particular attention. I. Nature of the Cell and its Contents. In the very earliest stage of a plant's existence, the cell consists only of a naked mass of a semi-fluid substance to which the name of protoplasm has been given, in the midst of which is a nucleus. In a few cases the cell remains in this condition, and is then termed a primordial cell. But as a general rule this protoplasm very shortly surrounds itself on the outside with a thin transparent membrane of cellulose-the cell-wall-and in this condition three distinct parts can be observed in the cell (Fig. 8): 1, the cell-wall, a ; a FIG. 8.-A cell from con 6 NATURE OF THE CELL AND ITS CONTENTS. 2, the internal protoplasm above mentioned, b; and 3, the nucleus, c, which is a rounder, denser portion, lying in the midst of the protoplasm. At first the protoplasm completely fills the cavity, but, as the cell grows larger, drops (vacuoles) of a clear, watery fluid called the cell-sap (Fig. 9, s', s') make their appearance in it; and the nucleus, k', is then suspended in the cell and connected to the protoplasm lining its inner wall, by slender threads or bands of the same substance, p', p'. As the cell continues to enlarge, these vacuoles coalesce and form a single central sapcavity, s, s, and the protoplasm is then confined to a thin layer lining the interior of the cell-wall-the primordial utricle, p, with the nucleus, k, k, showing as a denser mass in an enlargement of the protoplasm on one side. In the fully developed cell, therefore, we distinguish-1, the cell-wall; 2, the protoplasm; 3, the nucleus; 4, the cellsap. FIG. 9.-Cells from the root of Fritillaria im perialis. h. Cell-wall. k'. Nucleus. k, k. Nucleus with nucleoli. Primordial utricle. pp. Protoplasmic threads. s, s. Cavity filled with cell-sap.. Vacuoles. (After Sachs.) Such is the nature of active vital cells, but after a time the protoplasm with its contained nucleus disappears, leaving the cell filled with air alone or water. Those cells only which contain protoplasm can grow, form chemical combinations, and produce new cells; while the old cells of the wood and bark are of use only in virtue of their physical properties, as, for example, giving firmness and acting as protecting envelopes to the living cells beneath, and in other ways. 1. The Cell-Wall (Figs. 8, a, and 9, h).-We have just seen that in a few cases the cell consists of nucleated protoplasm alone-or that, in other words, it has no cellwall. As a general rule, however, this condition of things CELL-WALL-PROTOPLASM. 7 very shortly disappears; for the protoplasm, having elaborated molecules of cellulose (CH1O), passes them to its outer surface, where they form a thin, colorless, transparent, continuous membrane, or cell-wall. As this membrane increases in age, it becomes thickened-as will be afterward explained by the intussusception of new molecules between the older ones, and eventually there are generally developed upon it either protuberances as in the case of some pollen-cells; or internal depressions, as may be seen in pitted and other kinds of cells (see page 17). Those cells which are isolated, or on the surface of the plant, have the various markings on their outer or free surface, while those that are united to form tissues have them on the internal surface of their cell-wall. The former is termed centrifugal thickening; the latter centripetal thickening. 2. The Protoplasm, as we have already seen, is the only part of the cell, and therefore of the whole plant, which is possessed of life; and the differences in the form, size, and nature of cells are due to the vital energy which this protoplasm is capable of exerting this energy is frequently spoken of as vital force. The appearance of protoplasm is as varied as is the form of the cells which it produces. It may be granular and opaque, or homogeneous and transparent; it may be almost fluid, or of the consistency of dough; or again it may be stiff, or even brittle: generally, however, it is of a light-gray color and more or less granular; but it is never a true fluid. In those cases where the protoplasm contains granules, it is formed of an outer thin layer called the ectoplasm, which is free from granules; and of an inner granular portion which is known as the endoplasm. According to Sachs, that matter only ought to be regarded as protoplasm which is perfectly transparent, and the granules where they occur are to be looked upon as "probably finely divided, assimilated food-material." 8 NUCLEUS-CELL-SAP. The Primordial Utricle (Fig. 9, p), as has already been observed, is the thin layer of protoplasm which lines the cell-wall and forms the boundary of the central cavity filled with cell-sap. It is frequently so thin and transparent that it can not be detected without the aid of reagents, which either color it or cause it to separate from the cellwall as mentioned above. By some authors the primordial utricle is differently characterized, and defined as the outer thin homogeneous layer or ectoplasm of the protoplasm. This is the sense in which it was essentially understood by Mohl. 3. The Nucleus, which exists in all the cells of the higher plants, and is absent from only a few of the lower forms, is differentiated from the surrounding protoplasm as a denser portion of the same substance. It usually presents a more or less rounded outline, and contains one (Fig. 8, c), two (Fig. 9, k', k'), or more, much smaller bodies, called nucleoli. It is always situated in, and more or less inclosed by, the protoplasm, as we have already seen, and never lie s loose in the cell cavity. It is the more vitally active part of the protoplasm in which it is contained. 4. The Cell-Sap is the watery fluid which is found in the interior of the cell; it contains dissolved or suspended in it all those food materials which are necessary for the life and growth of the cell. In the earlier stages of the cell's life, as we have already seen, the cell-sap as a substance distinct from the protoplasm does not occur, but is diffused generally through it and the cell-wall, and it is only as the cell enlarges that it first appears in the form of drops (vacuoles) in the protoplasm (Fig. 9, s', s'), and which by ultimately coalescing form a single cavity filled with sap, s, s. Besides containing substances which are necessary to the life of the cell, it contains also many things which have been thrown out from the protoplasm as no longer serviceable. Besides the fluid cell-sap, there are other important CHLOROPHYL GRANULES. 9 cell-contents, some of which, such as chlorophyl, starch, raphides, and aleurone grains, now require description. Chlorophyl and Chlorophyl Granules.-Chlorophyl is the coloring material which gives to leaves their green appearance. Its chemical composition, owing to the great difficulty there is of obtaining it pure, is not accurately known; but there seems much reason to believe that it is closely allied to wax. Chlorophyl does not commonly exist indiscriminately diffused throughout the interior of the cell, but is confined to special portions of the protoplasm which have been differentiated from the general mass. These portions of protoplasm are the so-called chlorophyl granules or chlorophyl grains (Figs. 18 and 39, chỉ), or as they are also termed, chlorophyl bodies and chlorophyl corpuscles; hence these structures are granules of protoplasm colored green by chlorophyl. If a plant is grown in the dark these granules remain pale colored; but if it be exposed to sunlight they speedily become colored green; hence light is necessary, with rare exceptions, for the formation of chlorophyl; and when so colored they have the power of breaking up the carbon dioxide of the air or the water in which the plant is growing, and, returning the oxygen to the air, retain the carbon, which they are able to mix with the elements of water in such proportions as to build up a molecule of starch, and some other carbohydrates. This process of building up starch and other allied substances out of the carbon dioxide of the air or water has been termed assimilation. It has just been said that chlorophyl is generally confined to the protoplasm forming the chlorophyl granules; this is true in all the higher plants, but there are some plants among the lower orders in which the green colored portions form plates or spiral bands; or the whole protoplasm, with the exception of the ectoplasm, may be capable of being colored green. |