CH. XII] TRANSPIRATION 123 



readily proved by the foregoing experiments : the leaves 

 transpire more water per hour in a dry than in a moist 

 atmosphere, other conditions being equal, and more at 

 higher than at lower temperatures. Under ordinary cir- 

 cumstances, however, one of the most potent factors in 

 promoting transpiration is light : the leaves may transpire 

 more than twice as much water per unit of time in full 

 daylight as in darkness, and this phenomenon is of 

 immense importance to the living plant. Naturally the 

 leaves lose more water in a dry wind than in still air, 

 other conditions being equal, but experiments also show 

 that this is not as simple as it appears at first sight : the 

 mechanical shaking of the leaves, by wind or otherwise, 

 accelerates the transpiration owing to some action other 

 than the merely more rapid removal of the moisture. 



Transpiration, moreover, must not be regarded as the 

 mere evaporation of water from the surface of the lamina : 

 experiments show that, on the one hand, a dead leaf loses 

 water more rapidly than a living one of the same size and 

 kind, and that, on the other, the transpiration varies with 

 the age of the leaf. As a rule the transpiration is most 

 active in young leaves which have just finished their princi- 

 pal growth : leaves which are still growing in surface and 

 thickness transpire less, and so do older leaves in which 

 the cell-walls are becoming thicker and more indurated, 

 and no doubt Hohnel is right in regarding the period of 

 maximum transpiration as correlated with the perfection 

 of the stomata, before the cuticle and cell- walls have 

 become less permeable. 



The intensity of transpiration, under like conditions, 

 varies also in different plants. The rule is that herbaceous 

 plants, and especially grasses, transpire more actively than 

 trees and woody plants generally, while the process is re- 

 duced to a minimum in evergreens with thick coriaceous 



