XEROPHYTE ASSOCIATIONS. 201 



reduction of the exposed surface may be accurately regu- 

 lated to suit the need (see §38). 



150. Reduced leaves. — In regions that 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 ia 



Fig. 173. Leaves from the common basswood (Tilia). showing the effect of environ- 

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

 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 done tree are strikingly smaller, 

 much thicker, and more compact. — After Cowles. 



evident from the fact that the same plant often produces 

 smaller leaves in xerophytic conditions than in moist con- 

 ditions. One of the most striking features of an arid 

 region is the absence of large, showy leaves (see Fig. 172). 

 These reduced leaves are of various forms, such as the 

 needle leaves of pines, or the thread-like leaves of certain 

 sedges and grasses, or the narrow leaves with inrolled 

 margins such as is common in many heath plants. The 



