Pure water or weak saline solutions, such as are generated in the soil under certain circumstances, pass readily through membrane—that is, the molecules of the one shift and change places with those of the other—while those of gummy or albuminous substances like protoplasm do not. After a time, if there is no outlet for the water absorbed, or if it is not utilized within the plant in some way, absorption and diffusion cease, the cell becomes saturated with water, and until something happens to disarrange the balance, no more is absorbed. But, even in the case where the cell is saturated with water, it may still take up other liquids, because the diffusive power of those other liquids, in relation to the cell-wall and to the protoplasm, is different from that of water, and this absorption may go on in its way till saturation point is reached for each one of them, just as in the case of water. On the other hand, it may happen that the plant may be saturated with other substances, and incapable of taking up more of them, while at the same time pure water may be freely taken up. Just so much and no more of each particular substance is absorbed, the exact quantity of each being regulated in all cases by the condition and requirements of the cells, their membranous walls, and their contents. Thus it happens that some particular substances may be found by the chemist to exist in large relative proportions in the plant, while the quantity in any given sample of the soil from which it must be derived is sometimes so small as to elude detection. The plant in this case, or some part of it, is so greedy, if we may so say, for this particular substance, that it absorbs all within its reach, and stores it up in its tissues or uses it in some way, the demand ensuring supply. On the other hand, the soil may contain a large quantity of some particular ingredient which is incapable of being absorbed, or which the plant does not or can not make use of, and, in consequence, none is found within the plant. The supply is present, but there is no demand. The different physical requirements of the plant supply also the explanation of the fact that different plants, grown in the same soil, supplied with the same food, yet vary so greatly in chemical composition. Thus, when wheat and clover are grown together, and afterwards analyzed, it is found that while lime is abundant in the clover, it is relatively in small quantity in the wheat; and silica, which is abundant in the wheat, is absent from the clover. Poisonous substances even may be absorbed, if they are of such a nature as to be capable of absorption; and so the plant may be killed by its own action—by suicide, as it were. The entrance of water into the plant and the entrance of those soluble materials which a plant derives from the soil are therefore illustrations of the process of osmosis, and are subjected to all the conditions under which osmosis becomes possible, or under which it ceases to act.