70 



A TEXTBOOK OP BOTANY 



[Oh. Ill, S 



in the concavitj' (Fig. 40), or corerlngs of liair.s or .scales 

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



escape of water without 



materially affecting the en- 

 trance of carlion dioxide : 

 and a rcrticiil positimi of 

 the green tissues, which 

 lessens the eva]:)orative ef- 

 fect of the noonday sun 

 without any effect upon 

 gas absorption. The collec- 

 tive I'esult 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 Erka. in croK.s 

 section; X 2s0. (From Keruer.) 





-Various forms of epidermal hairs and scales (trichomes) found 

 upon leaves ; much magnified. (From Korner.) 



microscope. Diverse functions have lieen ascribed to them, 

 in addition to their jjart in restricting transpiration, but 

 without convincing evidence. Perhaps they represent a kind 

 of play of growth forces rather than any adaptational devel- 

 opment. 



