44 



PLANT RELATIONS. 



A branching hair from the leaf of common mullein. The whole plant has a 



felt-like covering composed of snch hairs. 



l>r 



comparatively thick, altliougli they may be very immerf>ns 

 (see Figs, -i, 167). 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 

 chloropliyll work, 

 and are not usuallv 

 recognized as leaves. 

 In their stead the 

 gl< )l)ular or cylin- 

 drical or fla.ttened 

 stems are green and 

 do leaf work (Pigs. 



Fia. 37. A scale from the leaf of S'/i/ji/ien/ia. These 

 scales overlap and form a complete covering. 



