Page:Encyclopædia Britannica, Ninth Edition, v. 19.djvu/64

Rh 54 PHYSIOLOGY [VEGETABLE. ferment and one or more organic acids. The use of the nectar is to attract insects, and thus to ensure cross- fertilization. The use of the excretions of the carnivorous plants is to dissolve the organic matter (usually insects) which has been deposited on the leaves, so as to bring it into a form in which it can be absorbed. Waste- Many of the waste-products are not excreted, but remain products i n the plant. Thus the terpenes are indeed excreted by the cells which line the resin-ducts, but these ducts have no aperture on the surface of the plant. Similarly the caoutchouc and gutta-percha which are contained in the laticiferous tissue of certain plants have no means of egress. This is also true of the tannic acid, of the calcium carbonate (usually) and oxalate, of the alkaloids, and of silica. These substances are usually deposited in the cells. Calcium carbonate and oxalate are deposited in the form of crystals either in the cell -wall or in the cell -cavity, and silica in the cell-wall. In some cases calcium carbonate is deposited on cellulose processes which extend into the cell from its wall, the whole body being termed a &quot; cystolith.&quot; 3. Plastic Products. The principal non- nitrogenous plastic products are the carbohydrates and the fats (gly- cerides); to these, as suggested above, some of the organic acids are perhaps to be added. Starch. Of the carbohydrates the substance which most demands attention is starch. Starch makes its appearance in the form of minute granules in chlorophyll-corpuscles in which constructive metabolism is being actively carried on. It was thought that it was the immediate product of the decomposition and recombination of carbon dioxide and of water in the corpuscle under the influence of light, the process being represented by some such equation as the following 6C0 2 + 5H 2 = C 6 H 10 5 + 60. 2 . Increased knowledge of the conditions under which starch makes its appearance in plants has, however, made it evident that this is not the case. Starch may be regarded as the first visible product of the constructive metabolism going on in a chlorophyll-corpuscle, but it is not the first non-nitrogenous substance formed. That, as has been pointed out, is probably a body allied to formic aldehyde. The starch is formed as one of the products of the decom position of the protoplasm of the corpuscle. That this is so is made evident when the formation of starch in parts of plants which are not green and which are not exposed to light is considered. In the cells of these parts there are certain small colourless protoplasmic bodies which are termed &quot;starch-forming corpuscles&quot; or &quot;leukoplasts,&quot;and it is by these that the starch-granules are formed. It appears that the starch-granule is formed in the first instance by the decomposition of a portion of the protoplasm of the corpuscle, successive layers of starch being deposited upon the primitive granule by the decomposition of successive layers of protoplasm. It is in this way that the stratified structure of these starch-granules is produced. There is no reason for assuming that the process of starch-formation in chlorophyll-corpuscles is essentially different from that in leukoplasts ; on the contrary, in view of the close re lationship of these bodies, the one being convertible into the other, there is every reason for believing it to be the same. There is, however, this functional difference between chlorophyll-corpuscle and leukoplast, that in the former the synthetic processes, i.e., the construction of protoplasm, begin with such simple substances as carbon dioxide, water, and mineral salts, whereas in the latter they begin (see below) with tolerably complex substances, such, for example, as glucose and asparagin. Starch, then, is the immediate product, not of constructive, but of destructive metabolism. Glucose. Various kinds of sugar, notably glucose and cane-sugar, are also commonly to be found in plants. Glucose may be regarded as having been derived by the action of an unorganized ferment from one or other of the other carbo hydrates, except in certain plants, the Onion for example, in which it appears to be formed in the chlorophyll-cor puscles in the first instance. Nothing is known at present Cane- as to the mode of origin of cane-sugar, which exists in su gar. such large quantities in certain plants, as the Beet and the Sugar-cane. With regard to the fats, it is commonly assumed that Fats, they are formed directly from the carbohydrates, because in oily seeds, for example, as the starch which they contain when young diminishes in quantity it is replaced by fats. There is, however, sufficient evidence to prove that the fats are the products of the decomposition of protoplasm. The disappearance of the starch in ripening oily seeds is due to its being used up in the construction of protoplasm, as the protoplasm undergoes decomposition in connexion with the formation of fat. The nitrogenous plastic products are proteids and Proteid amides. There can be little doubt that the proteids may be derived from protoplasm. If the molecule of living protoplasm be regarded as an extremely complex one, there is no difficulty in inferring that proteid may be one of the products of the decomposition of the proto plasm-molecule. The amides may also be products of the Amide; decomposition of protoplasm, or they may be formed from proteids by the fermentative action of living protoplasm or by the action of some as yet undiscovered unorganized ferment, as pointed out above. It has been stated that the plastic products are so called because they are substances which can be used in the con structive metabolism of plants. But it must not be inferred that they are so used immediately. The very fact that it is possible to detect their presence in considerable quantity is a proof that this is not the case. They are largely stored up either for the use of the plant itself at some future time, or for the benefit of the progeny of the plant. In a perennial plant, for example, plastic products are stored in the persistent parts for the use of the plant when it recommences its active growth ; they are also stored up in seeds and spores to be used by the young plant during the early stages of germination. Plastic products thus stored up are termed &quot;reserve materials,&quot; and the organs in which they are deposited are termed &quot;depositories for reserve materials.&quot; Tho non-nitrogenous reserve materials are Non- stored up in the form of carbohydrates or of fats. The nitro- starch which is formed in the green parts of the plant ge (which is, be it observed, a temporary reserve material) ma t e. is converted into a soluble substance, probably glucose, rials, and is conveyed in solution to the depository ; and from it, directly or indirectly, the non -nitrogenous reserve materials are formed. The reserve carbohydrates are stored up either in the insoluble or the soluble form. In the former case they are deposited as starch-granules, or as cellulose (as in the endosperm of the Date) in thick cell- walls ; in the latter they exist as various forms of sugar in solution in the cell-sap. The starch-granules are formed in these depositories by the leukoplasts. From the soluble non-nitrogenous substance, probably glucose, together with nitrogenous substances conveyed to the cells, the leuko plasts construct protoplasm ; and it is as the result of the decomposition of this protoplasm in a certain way that starch is formed. This is true also of the reserve cellu lose. From the plastic materials, both nitrogenous and non -nitrogenous, which are supplied to the cells proto plasm is constructed, and the external layers of proto plasm undergo decomposition in such a way that cellulose is formed and deposited in successive layers upon the internal surface of the cell -wall. The various kinds of