44 PHYSIOLOGY [VEGETABLE. stance, though this is probably always the case at their first formation ; but the cell-wall may undergo consider able modification during the life of the cell. It may, for example, undergo lignification ; it then comes to consist largely of a substance termed " lignin," which is much richer in carbon than is cellulose ; this takes place typically in those cells which form woody or sclerenchymatous tissue. Or it may undergo cuticularization, when it conies to consist largely of a substance termed " suberin " or " cutin," which, like lignin, is richer in carbon than cellulose. Or, again, it may become gummy or mucilaginous. These chemical differences are accompanied by differences in the physical properties of the cell-wall. A cellulose cell-wall is extensible, capable of swelling from taking up water into itself by imbibition, and is readily traversed by water. A lignified or cuticularized cell-wall is more rigid and less capable of swelling by imbibition ; moreover, a cuticularized cell- wall is almost impermeable to water. A gummy or mucilaginous cell-wall is more extensible and more capable of swelling by imbibition. Struc- The structure of the plant -cell is not the same at all ture of periods of its life. When a cell is young the protoplasm occupies the whole of the cavity enclosed by the cell-wall. But in the course of growth the increase in bulk of the protoplasm is not nearly so great as the increase in sur face of the cell-wall, so that in the mature cell the proto plasmic contents form merely a rather thin layer known as the primordial utricle, which lies in close contact with the internal surface of the cell -wall at all points. There thus comes to be a relatively large cavity in the cell, the racuole, which is filled with a liquid, the cell-sap, consisting of water holding various substances, organic and inorganic, in solution. The structure of a mature living cell is then this : it consists of a cell-wall, lined Avith a layer of protoplasm, which encloses the vacuole, filled with cell-sap. The protoplasm of plants is endowed with all those f fundamental properties which are possessed by that of animals. When a plant is unicellular these properties are all exhibited, so far as they are necessary to the main tenance of the organism, by its protoplasm ; in other words, all the necessary vital functions are performed by the protoplasm of the single cell of which the plant con sists. The performance of all the necessary vital functions by the protoplasm of one cell obtains also in the case of not a few 7 multicellular plants, in those, namely, in which all the cells are similar to each other in structure and contents. In the great majority of multicellular plants, however, the functions are distributed to a greater or a less extent ; there is more or less complete physiological division of labour. In these plants the cells are not all similar in appearance, and their diversity is to be ascribed to their adaptation in different w r ays to the performance of parti cular functions. Further, the cells which have undergone modification in some particular direction for the perform ance of some particiilar function are grouped together in certain parts of the plant, and these parts are spoken of as "organs." Thus the roots of one of the higher plants are the organs for the absorption from the soil of water and substances in solution ; the leaves are the organs for the absorption of gases from the air, and, in virtue of the green colouring-matter chlorophyll, which their cells contain, they are also the organs in which certain im portant constructive processes are carried on. But the extent to w r hich physiological division of labour is car ried out in plants is not nearly so considerable as it is in animals, and accordingly the protoplasm of the dif ferent cells of plants exhibits only in a very slight degree that specialization of structure which is so conspicuous in animals. cells. hairs. Absorption. 1. Absorption of Water and Substances in Solution. The bodies of plants, unlike those of the great majority of animals, do not contain any internal cavity into which the food may be taken as a preliminary to its being absorbed by the tissues. The materials of the food of plants are therefore taken up directly from without into the cells of the absorbent organs. The cells which are especially con- Absorp- cerned in absorption are, in the higher and subaerial plants, tion by the root-hairs, thin -walled, unicellular, unbranched fila-? 1 ments which are developed from the epidermal cells some way behind the growing-point of the root ; in the lower plants, and even in those of the higher plants which lie submerged, all the cells of the plant may take part in absorption. Since the food is directly absorbed by the cells, and since the cells all possess a cell-wall, the materials of the food must be taken up in solution. Salts and other substances are, as a matter of fact, taken up by the ab sorbent cells in the form of watery solutions. Substances w r hich are soluble in water are dissolved in the water which is present in a greater or smaller proportion in all soils, and of those which are not soluble in w : ater many are brought into solution by the acid sap which saturates the walls of the root-hairs. The actual process of absorption is an in stance of diffusion through a membrane, that is, of osmosis. Osmosis Only such substances can be absorbed by a root-hair, for instance, as are capable of diffusing not only through the cell -wall but also through the protoplasmic primordial utricle. Further, only such substances can be absorbed by the root-hair as are present in larger proportion in the water to be absorbed than they are in the cell-sap of the root-hair ; this inequality between the proportion of any substance in solution in the liquid on the one side and in that of a membrane on the other is a necessary condition of osmosis. Hence, in order that the absorption of any particular substance by the root-hairs may be continuous, it is necessary that the substance in question should not accumulate in the cell-sap ; this accumulation is prevented either by the actual consumption (i.e., chemical decomposi tion) of the substance in the cell or by the withdrawal of it to supply the needs of adjacent cells. In fact, so far as the process of absorption is concerned, the cell-sap of the internal cells of the root stands in the same relation to the cell-sap of the root-hairs as the cell-sap of the root- hairs does to the external liquid ; and, as this relation exists between the successive internal layers of cells, there is set up a current of absorbed substances which travels from the surface towards the centre. It appears from the foregoing considerations that the Condi- amount of any particular salt absorbed in a given time tions ol depends upon (1) its diffusibility and (2) its consumption*? in the plant. Of these two conditions the second is the one which is of real physiological importance, and, if only the given time is sufficiently long, the first condition may be neglected. For instance, let us suppose that a plant is absorbing by its roots two salts the one (A) being very diffusible, the other (B} much less diffusible and that, whilst the former undergoes no change in the plant after absorption, the latter is at once decomposed. Now, if the time of observation is short, it may happen that the amount absorbed of the salt A will be found to be greater than that of the salt E ; but, if the time be extended, the amount absorbed of the salt E will certainly be found to be greater than that of the salt A. The explanation is that the salt A would at first be absorbed very rapidly, on account of its high diffusibility ; but the absorption of it would gradually diminish, in consequence of the accumu lation of it in the cell-sap of the plant, until it ceased alto
gether. The absorption of the salt Jj, on the other hand,