LIFE OF THE WHOLE PLANT. 779 



ever, already been referred to in treating of the Special Physi- 

 ology of the Elementary Tissues, and of the Koot, Stem, and 

 Leaf ; so that it now remains only for us in this place to give 

 a general recapitulation of the functions of the plant, and to 

 consider them as working together for the common benefit of 

 the whole. 



1. Absorption. — The root, as already noticed (see p. 730), is 

 the main organ by which food is taken up in a state of solution, 

 for the uses of the plant. No solid matter can be absorbed in an 

 undissolved condition ; and this absorptive power is owing to 

 the superior density of the contents of the cells of the young 

 extremities of the roots over the fluid matters surrounding them 

 in the soil leading to the production of endosmotic action (see 

 page 723, and^^'. 1098) between them. 



That the roots do thus absorb fluid matters may be proved 

 by a very simple experiment. Thus, if we take two glasses of 

 the same capacity, and pour water into them until it is at the 

 same level in each, and then put the roots of a vigorous growing 

 plant in the one, and expose both in other respects to the same 

 influences of light, heat, and air, it will be noticed, that the 

 water will gradually disappear from the glasses, but from that 

 in which the roots are placed far more rapidly than from the 

 other without them, and hence the more rapid removal in the 

 former case must be owing to the absorption of the roots. In 

 this way we can also estimate, in some degree at least, the 

 amount absorbed, which will be found to be very considerable, 

 commonly in a few days, far exceeding in weight that of the 

 plants which are experimented upon. The amount of fluid ab- 

 sorbed by the roots is directly dependent upon the activity with 

 which the other processes of vegetation are carried on, and more 

 especially by the quantity of fluid matters transpired by the 

 leaves ; indeed absorption is directly proportioned to transpi- 

 ration in a healthy plant ; for as fluid is given off" by the leaves, 

 it is absorbed by the roots to make up for the deficiency thus 

 produced, and hence, therefore, all stimulants to transpiration 

 are at the same time exciters of absorption. "When absorption 

 and transpiration vary much in amount, the plants in which 

 such a want of correspondence takes place become unhealthy ; 

 thus when transpiration is checked from deficiency of light, as 

 when they are grown in dark places, the fluids in them are ex- 

 cessive in amount ; whilst if the atmosphere be too dry, as is the 

 case in plants grown in the sitting-rooms of our dwelling- 

 houses, transpiration is greater than absorption, and hence re- 

 quire to be frequently supplied with water, or otherwise they 

 will suffer from a deficiency of fluid. 



The mutual dependence of absorption upon transpiration should 

 also be borne in mind in the process of transplanting trees, &c. 

 Thus, as transpiration is greatest at those seasons of the year 



