198 



ELEMENTARY STUDIES IN BOTANY 



be guarded against, as a means by which the working cells 

 are kept at a working temperature. It is found that the 

 heat from the sunlight would raise the temperature of a 

 leaf dangerously if it were not tempered by the cooling 

 effect of transpiration. Experiments show that if tran- 

 spiration is stopped, leaves perish quickly. Transpiration, 

 therefore, is a protection of very great importance ; while 



FIG. 162. Leaves of geranium shifting position according to the direction of the 

 light : A, the plant exposed to vertical rays of light ; B, the same plant exposed to 

 oblique rays of light. 



/ 



the danger of transpiration is simply that the supply of 

 water may not be equal to the necessary and beneficial loss. 

 Another very important advantage of transpiration is 

 that the continual loss of water from the leaves results in 

 a continual movement of water into the leaves. This mass 

 movement of water in the plant towards the regions of loss 

 (the regions of transpiration) is often called the transpira- 

 tion current. This does not explain how the water moves 



