296 PLANT STUDIES 



reduction of the exposed surface may be accurately regu- 

 lated to suit the need (see §38). 



139. Reduced leaves. — In regions tliat are rather per- 

 manently dry, it is observed that the plants in general pro- 

 duce smaller leaves than in other regions (see Fig. 173). 

 That this holds a direct relation to the dry conditions is 



Yiv. 173. Leaves from t lie common bass^vood (7^(7/(0. fallowing the effect of environ- 

 ment ; those at the right being from a tree growing in a river bottom (meaophyte 

 conditions) ; those at the left being from a tree growing upon a dune, where it is 

 exposed to intense light, heat, cold, and wind. Not only are the former larger, 

 but they are much thinner. The leaves from the dune tree are strikingly smaller, 

 much thicker, and more compact. — After Cowles. 



evident from the fact that the same i>lant often jiroduces 

 smaller leaves in xerophytic conditions than in moist con- 

 ditions. One of tlie most striking features of an arid 

 region is the abseiK'O of large, showy leaves (see Fig. 172). 

 These reduced loaves are (jf various forms, such as the 

 needle leaves of pines, or tlie tlircad-likc^ leaves of certain 

 sedges and grasses, or the nari'nw leaves "witli inrolled 

 margins such as is common in many heath plants. The 



