CHAPTER II. SPECIAL PHYSIOLOGY OF THE NUTRITIVE FUNCTIONS. 693 



specially adapted leaves. Submerged aquatic plants absorb their 

 food entirely or mainly from the water in which they live. 



As already mentioned (p. 668), the food of plants is always 

 absorbed in the fluid form ; either as a liquid or as a gas. The 

 liquid food, consisting of a watery solution of various substances, 

 is absorbed from the soil most commonly by the roots, or, in the 

 absence of roots, by other members (shoots, leaves) which have 

 become specially adapted for the performance of this function ; 

 the gaseous food (C0 2 ) is absorbed from the air by the green 

 parts (shoot) of plants, and, in the more highly differentiated 

 forms, more especially by the leaves. 



Absorption of Liquids. When an organ which is adapted for the 

 absorption of liquids is performing its function, two processes are 

 in operation, namely, the absorption of the water, and the absorp- 

 tion of the substances which the water holds in solution : it is 

 important to clearly distinguish between these two processes, for 

 though they are necessarily carried on simultaneously, yet the 

 water and the dissolved substances are not absorbed in any con- 

 stant proportion. The two processes, in fact, depend upon some- 

 what different conditions. The cells of the absorbent organ take 

 up the water in consequence of the presence in solution in the 

 cell-sap of osmotically active substances, such as organic acids and 

 acid salts, which attract water into the cells. The substances 

 in solution in the water are absorbed in virtue, first, of their 

 diffusibility, that is their power of passing through membranes, 

 and secondly, of the fact that the cell-sap in the interior of the 

 absorbing cells contains less of the substances in question than 

 does the water outside. Hence any indiffusible substance which 

 may be dissolved in the water will not be absorbed by the cells, 

 neither will any substance of which there is already an equivalent 

 quantity in solution in the sap of the cells, although the absorp- 

 tion of water may be actively taking place. These facts find 

 their general expression in the following statement which is 

 known as the Law of Absorption: for the watery solution of any 

 substance capable of being absorbed, there is a certain degree of 

 concentration at which the proportion of the amount of the sub- 

 stance absorbed to that of the water absorbed is the same as that 

 of the solution ; if the solution be more concentrated, the pro- 

 portion of water absorbed will be greater, and that of the sub- 

 stance less ; if the solution be more dilute, the proportion of the 

 substance absorbed will be greater, and that of the water less. 



