116 



PLANT SOCIOLOGY 



(Fig. 63). The Bad Lands of South Dakota have a rainfall of some 40 

 to 50 cm. About the Mediterranean and in the Western Alps naked, 

 scoured, marly slopes play a far too prominent role in the landscape 

 (Fig. 64). Even in a forest climate newly exposed subsoil presents 

 insuperable difficulties for the establishment of woody plants on 

 account of its physicochemical character. The blue-gray marls of 

 the Aptien of the southwestern Alps and northern Africa and the 

 Miocene, Eocene, and Permian marls of Languedoc are notorious in 



Fig. 63.- 



-An erosional edaphic desert; the "Bad Lands" of South Dakota, d'notc 

 by Hayivard.) 



this respect. The study of these rain-erosion phenomena and the 

 efforts to halt their destructive effects lead to a recognition and classi- 

 fication of the plant species of dynamic genetic importance. These 

 plants offer a decided resistance to erosion and in addition act as first 

 pioneers leading to a revegetation of the areas (Fig. 65), 



Dew. — Dew provides a water supply for cryptogams, especially 

 in regions of scanty rainfall. 



Plantefol (1927) experimented on the absorption of dew by cushions 

 of Hylocomium. triqucirum. He found that in August and September, 

 after rainless nights, the water content of the moss cushions had 

 increased to 100 per cent of the dry weight, whereas on the previous 

 day it was 20 to 30 per cent. At subalpine levels at Lus-la-Croix- 

 Haute, the water content rose to 233 per cent of the dry weight after 



