220 



PLAICT EELATlONS. 



the tumbleweecl 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 



^IQ. 189. Tree-like yuccas from the arid regions of Africa, sliowing the very numer- 

 ous thioli; and pointed, sword-liJie leaves. 



