LEAVES 



27 



clear; and doubtless they may serve other purposes also, 

 or sometimes may even be of no use whatever to the 

 plant. It has been 

 suggested that in re- 

 gions of intense light a 

 covering of hairs is an 

 effective sun - screen. 

 The explanation is 

 that being dead struc- 

 tures, containing air, 

 they reflect the light, 

 thus diminishing the 

 amount that reaches 

 the working cells. As 

 is well known, hairs 

 are by no means re- 

 stricted to leaves, but 

 occur on all parts of 

 plants. 



(5) Small leaves are 

 characteristic of dry 



regions, in this way each leaf exposing a small surface to 

 the drying air and intense light. That this reduction in 



size holds a direct relation to 

 the dry conditions is evident 

 from the fact that the same 

 plant often produces small 

 leaves in a dry region and 

 larger ones in moist condi- 

 tions. In the case of the 

 cactus, a large group in the 

 dry regions of the Southwest, 

 the leaves have become so 

 colorless water storage tissue <tc), mu ch reduced that they are 



and the central cells containing . 



chioropiasts (c). no longer used in photosyn- 



FIG. 23. Scale from the leaf of Shepherdia ; such 

 scales overlap and form a complete covering. 



e 



