Ch. Ill, 6] TRANSPIRATION FROM PLANTS 



47 



ing the question as to the 

 relation between the two 

 processes. While closely 

 related, they are not iden- 

 tical, as shown by the 

 modern studies on rela- 

 tive TRANSPIRATION, that 



is, the ratio between tran- 

 spiration and the contem- 

 poraneous evaporation, as 

 determined by suitable in- 

 struments. In brief, tran- 

 spiration is evaporation 

 affected considerably by 

 the structure and physi- 

 ology of the leaf. 



The profound effect of 

 external conditions upon 

 transpiration has many 

 important consequences. 

 Thus, a conjunction in 

 high degree of light, heat, 

 dryness, and winds, as 

 happens at times in our 

 gardens, can cause wilting 

 in some plants even when 

 they have ample soil 

 water, because the roots 

 cannot absorb, or the 

 :;tems conduct, water as 

 fast as transpiration re- 

 moves it. In such cases 

 a check in the transpira- 

 tion, by the coming of 

 night or a spraying by the 

 gardener, is promptly fol- 



