258 



THE FORMATION 



covering on their faces, while the forces which produce disintegration oper- 

 ate from above or below. 



317. Succession by binding aeolian soils. Dunes (thinia) are classic ex- 

 amples of the reaction of pioneer vegetation upon habitats of wind-borme 

 sand. The initial formations in such places consist exclusively of santi- 

 binders, plants with masses of fibrous roots, and usually also with strorig 

 rootstalks, long, erect leaves, and a vigorous apical growth. They are al- 

 most exclusively perennial grasses and sedges, possessing the unique pro^i- 



Fig. G6. Thicket formation (Qucrcus-Holodiscus-driodium) , stage V 

 of the talus succession. 



erty of pushing up rapidly through a covering of sand. They react by 

 fixing the sand with their roots, thus preventing its blowing about, and also 

 by catching the shifting particles among their culms and leaves, forming a 

 tiny area of stabilization, in which the next generation can establish a foot- 

 hold. The gradual accumulation of vegetable detritus serves also to enrich 

 the soil, and makes possible the advent of species requiring better nourish- 

 ment. Blov/outs (oneinio) arc almost exact duplicates of dunes in so 

 far as the steps of revegetation are concerned ; while one is a hollow, and 

 the other a hill, in both the reaction operates upon a wind-swept slope. 

 Sand-hills (amathia) and deserts (ereutia) show similar though less marked 



