196 PLANT STUDIES 
reduction of the exposed surface may be accurately regu- 
lated to suit the need (see 38). 
139. 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 is 
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 dune 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 
