796 PHYSIOLOGY. 



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

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

 same level in each, and then pnt 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 the more rapid removal in the 

 former case must be owing to absorption by the roots. In this 

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

 absorbed, v/hich will be found to be very considerable ; com- 

 monly in a few days, far exceeding in weight that of the plants 

 which are experimented upon. This imbibition of liquid by the 

 roots is independent of leaf-action. If the rootlets be healthy 

 and the tissues above them filled with fluid, it will always 

 occur. The great force of the action in stumps cut off a little 

 above the ground is well seen in such experiments as those of 

 Hales (see p. 799) and Hofmeister. Notwithstanding this, the 

 amount of fluid absorbed 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 propor- 

 tioned to transpiration 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, therefore all stimulants to transpira- 

 tion are at the same time exciters of absorption. When absorp- 

 tion and transpiration differ greatl}- 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 plants are grown in dark places, the fluids in 

 them are excessive in amount ; whilst if the atmosphere be too 

 dry, as is the case in plants grown in the sitting-rooms of our 

 dwelling-hoiises, transpiration is greater than absorption, and 

 hence they require to be frequently supplied with water. 



The mutual dependence of absorption upon transpiration should 

 also be borne in mind in the process of transplanting trees. 

 Transpiration is greatest at those seasons of the year when 

 plants are most abundantly covered with leaves, and when solar 

 light is most intense : we ought not therefore to transplant at 

 such periods, because, as it is almost impossible to do so without 

 some injury to the extremities of the roots (see p. 744), the 

 amount of fluid absorbed may be unable to compensate for the 

 loss by transpiration, and hence the plants will languish, or die, 

 according to circumstances. By transplanting in autumn or 

 spring, we do not expose the plants to such unfavourable con- 

 ditions, as the light is then less intense, and there are no leaves 

 upon them. (For further particulars on Absorption, see Absorp- 

 tion by the Boot, p. 744.) 



