220 



PLANT RELATIONS. 



the tumbleweed habit is prominently developed. Certain 

 low and densely branching plants are lightly rooted in 

 the soil, so that at the close of their growing period they 

 are easily uprooted by the wind, and are rolled to great 



FIG. 189. Tree-like yuccas from the arid regions of Africa, showing the very numer- 

 ous thick and pointed, sword-like leaves. 



