70 



A TEXTBOOK OF BOTANY 



[Ch. Ill, 8 



in the concavity (Fig. 40), or coverings of hairs or scales 



(Fig. 41), all of which arrangements tend to delay the 



escape of water without 

 materially affecting the en- 

 trance of carbon dioxide: 

 and a vertical position of 

 the green tissues, which 

 lessens the evaporative ef- 

 fect of the noonday sun 

 without any effect upon 

 gas absorption. The collec- 

 tive result of these features 

 is to give the characteristic 



grayish condensed aspect to the vegetation of dry places. 

 The trichomes of plants are indeed remarkable in their 



variety, and often in their beauty when viewed through the 



Fig. 40. — Leaf of Erica, in cross 

 section ; X 280. (From Kerner.) 



Fig. 41. 



Various forms of epidermal hairs and scales (trichomes) found 

 upon leaves ; much magnified. (From Kerner.) 



microscope. Diverse functions have been ascribed to them, 

 in addition to their part in restricting transpiration, but 

 without convincing evidence. Perhaps they represent a kind 

 of play of growth forces rather than any adaptational devel- 

 opment. 



