44 



PLANT RELATIONS. 



Fig. 



A branching hair from the leaf of common mullein. The illustration shows 

 the form, but not the many-celled structure of the hair. 



comparatively thick, although they may be very numerous 

 (see Figs. 4, 172). In this way each leaf exposes a small 



surface to the dry- 

 ing air and intense 

 sunlight. In our 

 southwestern dry 

 regions the cactus 

 abounds, plants 

 which have reduced 

 their leaves so much 

 that they are no 

 longer used for 

 chlorophyll work, 

 and arc not usually 

 recognized as leaves. 

 In their stead the 

 globular or cylin- 

 drical or flattened 

 stems are green and 



Pig. 37. A scale from the leaf of Shepherdia. These ° 



scales overlap and form a complete covering. do leal WOrK yr lgS. 



