46 THE WORK OF THE ROOTS [chap. 



have been expected from the continual loss of water by 

 transpiration from the leaves. By weighing a bottle and 

 its growing plant on a sensitive balance, and reweighing 

 after a given time, the loss of water by transpira- 

 tion under different conditions of temperature, illumina- 

 tion, eta, can be readily determined, thus checking 

 the conclusions reached by the previous experiments 

 upon the leaf. It is only by the root that water enters 

 a plant : when a gardener syringes the leaves or waters 

 the paths and stages of a greenhouse in which plants are 

 flagging, he does not thereby add water to the leaf; by 

 saturating the air with moisture he checks transpiration, 

 and thus enables the intake by the roots to make up for 

 the evaporation at the leaf surface. Anything that 

 checks the development of root, or for a time deprives 

 the plant of its proper amount of root, renders the plant 

 more liable to die from lack of water; thus plants after 

 repotting should be kept for a short time in a close 

 atmosphere in which little transpiration can take place 

 until fresh roots have developed, and a transplanted tree 

 requires special attention in a dry season to maintain a 

 moist soil round the mutilated root system. Again, crops 

 on a well-drained soil, even though it is light and little 

 retentive of moisture, will withstand a period of drought 

 longer than on a heavy undrained clay, because in the 

 former case the root system extends deeply into the soil, 

 whereas in the latter it is cut short near the surface by 

 the stagnant, airless water. A plant begins to wilt and 

 will eventually die when the transpiration from the 

 leaves is greater than the supply brought in by the root 

 from the soil, and it will be found that roots are not able 

 to extract the whole of the water present in the soil. 

 If plants are grown in a pot, and as soon as they begin 

 to wilt the soil is turned out and a sample weighed and 

 put to dry, even a sandy soil will be found to contain 



