48 PHYSIOLOGY VEGETABLE. such as Dicotyledons and Conifers, which have the fibro- vascular bundles arranged in a ring in the stem. When a ring of tissue, extending inwards as far as the cambium- layer, is removed from the stem of a dicotyledonous plant the following facts are to be observed : (1) that the leaves which are borne on branches arising from the stem above the level at which the ring of tissue has been removed will not exhibit any signs of withering ; (2) that the part of the stem below the incision will not increase in thickness to nearly the same extent as the part above the incision. From these facts it is clear (1) that the operation in question has not materially affected the conduction of water and food- materials in solution upwards to the leaves, and, since the wood is the only unimpaired tissue, it is obviously in the wood that the upward current travels ; and (2) that the operation has materially affected the conduction of organic nutrient substances to the parts below the incision, the diminished growth of these parts being the result of in adequate nutrition ; this effect of the operation is to be ascribed, principally at least, to the destruction of the con tinuity of the bast-tissue. Laticifer- In various families of vascular plants, and in some Dustissue cellular plants also (certain Fungi), there are to be found vessels ce ^ s f rmm g what is known as " laticif erous tissue," which probably assist in distributing both food-materials and organic nutrient substances throughout the plant. In some plants (Euphorbiacese, Asdepiadacex, Morese, ifec.) the cells are quite distinct from each other, and extend from one end of the plant to the other, growing with its growth, so that they attain a very considerable size, and are much branched ; these are spoken of as " laticiferous cells." In other plants (Cichoriacese, Papaveracex, &c.) the cells are comparatively small, and fuse together to form an intri cate network; these are spoken of as " laticiferous vessels." The cells of the laticiferous tissue contain a milky liquid, termed " latex," which consists of water holding inorganic salts, sugar, gum, extractives and proteids, in solution, and holding in suspension resinous and fatty bodies. The cells contain protoplasm in addition, and not uncommonly starch-granules. Food of Plant*. Food of A rough idea of the nature of its food can be obtained plants, by analysing a plant. It is found that, in the process of incineration, a considerable weight of its dry solid is burned up and given off in the form of gas ; this represents the combustible or organic portion of the plant. The in combustible residue, the ash, is found to be of a mineral or inorganic nature. The gases given off are carbon dioxide, watery vapour, and nitrogen, showing that the combustible portion of the plant contained the elements carbon, hydrogen, and nitrogen. In the ash occurs a num ber of elements, of which the principal are sulphur, phos phorus, potassium, calcium, magnesium, iron, sodium, chlorine, and silicon. But it does not necessarily follow that, because any given chemical element can be detected in a plant, that element is to be regarded as part of the food of the plant, for, as has been already pointed out, plants may absorb substances which in no way contribute to their nutrition, or are even injurious. When an element enters into the chemical composition of the substances of which the organized structure of the plant consists (as C, H, O in starch and cellulose, C, H, O, N, S, P in pro teids), then it is clear that this element must form part of the food; but, when, as in the case of the rest of the elements mentioned above, an element does not thus con tribute to the building up of the organized substance of the plant, its admission to the rank of a food -material must be the subject of direct experiment. It has been ascertained that many of the elements enumerated above, though, so far as is known, they are not essential constitu ents of the organized structure of the plant, are neverthe less essential to the maintenance of its life ; they may not, indeed, go to build up the plant-substance, but in some way or other they promote the metabolic processes. The method which has afforded the most valuable results Wate: bearing upon the relative physiological importance of cu tui various food-materials is that which is known as " water- culture." It consists in growing plants with their roots immersed in water holding certain salts in known quantities in solution. The mixture of salts can, of course, be varied at pleasure, and the effect upon the plant of the absence of certain elements, as of their presence in smaller or larger quantities, can be observed. Further, by an analysis of that portion of the solution which remains unabsorbed at the close of the experiment, the proportion in which the various salts have been absorbed can be ascertained. The elements of the food of plants may be conveniently classified into two groups, the first consisting of those which enter into the composition of organized plant-sub stance, the second consisting of those which, without ac tually entering into the structure of the plant, are essential to the proper performance of the metabolic processes. To the first group belong the elements C, H, O, N, S, P ; to the second, K, Ca, Mg, Fe, Cl (?). We will now briefly discuss the form in which the various chemical elements are absorbed, and their use in the economy of the plant, beginning with those which enter into the composition of organized plant-substance. A few words will also be said about those elements, such as sodium and silicon, which, though always present in the ash of plants, appear to have no real physiological signifi cance as far as nutrition is concerned. Carbon. This element constitutes a large percentage of the total Food dry weight of plants. It enters into the composition of all the elem organic substances, such as starch, cellulose, and other carbohydrates, fats and other hydrocarbons, proteids, organic acids, alkaloids, &c., which may be present in plants. The form in which carbon is absorbed depends upon the nature of the plant. It may be broadly stated that all those which contain chlorophyll absorb their carbon in the form of carbon dioxide, whereas those which do not contain chlorophyll absorb their carbon in the form of more complex carbon compounds which contain C, H, and O, and in which the C is directly combined with H. Moreover, in green plants it is only those cells which contain chlorophyll that can absorb carbon dioxide, and this only under the influence of light. It must not be assumed, however, that plants containing chlorophyll are in capable of absorbing complex carbon compounds. It is known from the researches of Darwin and others that the "insectivorous " plants absorb such compounds by their modified leaves, and it is known also that a number of green plants, such as the Mistletoe, the Rattle, and others, live parasitically on other plants. It has indeed been proved by direct experiment that green plants can ab sorb substances such as urea, glycocoll, asparagin, leucin, tyrosin, which are all highly complex carbon compounds. The physio logical distinction to be drawn between plants which do and those which do not contain chlorophyll is really that the former arc capable of assimilating carbon in a simple compound, such as C0 2 , whilst the latter are incapable of doing this, and require, there fore, compounds of more complex constitution. Plants which do not contain chlorophyll are either parasites (that is, they live upon other living organisms) or saprophytes (that is, they live upon the products of the waste and decay of other living organisms). The plants which do not contain chlorophyll are the Fungi and a few Phanerogams, Epipogium Gmclini, Cuscuta, Monotropa, Latkriea, Corallorhiza. Of these the Fungi include both parasites and sapro phytes ; Epipogium Gmelini is a saprophyte, Ciscuta a parasite, and Monotropa, may apparently be either the one or the other. The Orobanchcse, which are parasitic, and Ncottia, which is saprophytic, have not a green colour, but small quantities of chlorophyll have nevertheless been detected in them. Hydrogen. This element is absorbed by all plants in the form of water and of ammonia and its compounds ; it may also be absorbed in the form of organic compounds. Oxygen. Oxygen is taken up either in the free state, or in com bination in the form of water or of salts ; it may also be absorbed in the form of organic compounds. The free oxygen absorbed is especially concerned in the processes of destructive metabolism, the
combined oxygen in those of constructive metabolism.