668 PART IV. — THE PHYSIOLOGY OF PLANTS. 



a conversion of kinetic into potential or latent energy, and the 

 catabolism, by a conversion of potential into kinetic energy. 



These various points will now be severally considered. 



1. Absorption. The main idea connected with this function is 

 the taking up of water and other substances into the plant from 

 without ; but it must not be overlooked that, in a multicellular 

 plant, the cells absorb from each other. 



In any case, the function of absorption depends upon the 

 physical process of diffusion through membrane of substances in 

 solution, or osmosis. For instance, supposing two adjacent cells; 

 one of which has its cell- sap charged with sugar, whereas that 

 of the other has none; the sugar will diffuse through the inter- 

 vening cell-wall until the sap in both cells holds the same pro- 

 portion in solution. This being the mode of absorption, it is 

 clear that the substances can only be absorbed in the fluid form, 

 either as liquids or gases. 



So far the function of absorption would appear to be a simply 

 physical process. It must, however, be borne in mind that the 

 cell-wall is lined by living protoplasm, which modifies the purely 

 physical diffusion through the cell- wall, both as regards the nature 

 and relative quantity of the substances which pass into or out of 

 the cell ; so that the physical laws of osmosis, as determined by 

 experiments with dead membrane, are not directly applicable to 

 the osmotic phenomena of a living cell. 



I There is one manifestation of the osmotic properties of living 

 plant-cells which is of such fundamental importance, particularly 

 in connexion with movement, that it requires special mention. 

 It is this, that the cells tend to take up such large quantities of 

 water, that a considerable pressure is set up in the cell between 

 the cell-sap, on the one hand, and the elastic cell- wall on the 

 other. This state of tension is known as turgidity or turgescence, 

 and a cell in this state is said to be turgid. The conditions upon 

 which turgidity depends are three : first, the presence of osmoti- 

 cally active substances in the cell-sap to attract water ; second, 

 the presence of a layer of protoplasm lining the cell- wall ; third, 

 the presence of an elastic cell- wall. With regard to the first of 

 these conditions, the necessity for it is obvious. It appears that the 

 osmotically active substances in question are especially the organic 

 acids or acid salts, which are abundantly produced in the meta- 

 bolism of plants. The significance of the second condition is, 

 that the layer of protoplasm prevents, at least within certain 



