134 



THE DESERT 



Absence of 

 large leaves. 



Exhaust of 

 moisture. 



minimum surface for a given bulk of tissue.* 

 There is no waste, no unnecessary exposure of 

 surface. Then there are some members of the 

 family like the " old man " cactus, that have 

 thick coatings of spines and long hairy growths 

 that prevent the evaporation of moisture by 

 keeping off the wind. Then again the cacti 

 have no leaves to tempt the sun. Many of the 

 desert growths are so constructed. Even such 

 a tree as the lluvia d'oro has needles rather than 

 leaves, though it does put forth a row of tiny 

 leaves near the end of the needle ; and when we 

 come to examine the ordinary trees such as the 

 mesquite, the depua, the palo breya, the palo 

 verde, and all the acacia family, we find they 

 have very narrow leaves that have a fashion of 

 hanging diagonally to the sun and thus avoid- 

 ing the direct rays. Nature is determined that 

 there shall be no unnecessary exhaust of moist- 

 ure through foliage. The large-leafed bush or 

 tree does not exist. The best shade to be found 

 on the desert is under the mesquite, and unless 

 it is very large, the sun falls through it easily 

 enough. 



* I am indebted to Professor Forbes of the University 

 of Arizona for this and several other statements in con- 

 nection with desert vegetation. 



