CHAP. LXXIV GRASS-STEPPE. PRAIRIE 285 



closely allied to Drude's ' desert-steppes ', in which the ground is very 

 bare and shows itself on all sides. 



In Asia. 



On the Altai mountains there are herbaceous and grassy steppes 

 which, with their waving thyrsa-grasses and species of Gypsophila, are 

 similar to steppe on the ' black earth ' of southern Russia. 1 



Grass-steppe (Prairie) in North America. 



The North-American prairie is true steppe, and is evoked by the same 

 physical factors : Continental climate ; long severe winters with snow and 

 minimum temperatures ranging from 20 C. to 50 C. ; a summer, 

 hot and dry, often rainless from the middle of July onwards, but with 

 cold nights. The short vegetative season (May) is heralded by transitory 

 falls of rain. Low atmospheric humidity and the occurrence of dry 

 periods during the vegetative season cause the absence of forest and the 

 presence of steppe. Here too, at least in certain districts, two periods 

 of rest occur within the year ; and, here too, violent storms arise. 



The prairies are vast plains on whose horizon the curvature of the 

 earth is made manifest. The soil in the east appears to be like that 

 in southern Russia, namely a black loess, intermingled with sand, which 

 contains deep humus consisting of the remnants of countless preceding 

 vegetations, and thus including a boundless store of wealth for future 

 ages. Lesquereux suggested that the prairie is an ancient sea-bed which 

 has been slowly dried up the bed of the sea from which access of water 

 has been cut off by the upheaval of the Andes in the Tertiary period. 

 As regards supply of water, the prairies are more favourably situated 

 than are the Asiatic steppes ; they are more thoroughly permeated by 

 water, and are traversed by mighty rivers, with which arboreous vegeta- 

 tion is associated. With this exception prairies are treeless or show 

 open tree- vegetation, which, according to Sargent, forms at most 10-20 

 per cent, of the vegetation clothing the ground. 



Prairie is pure grassland, in which numerous Compositae (especially 

 Heliantheae and Astereae), Leguminosae (especially Galegeae), and other 

 perennial herbs, are intermingled with the grasses, which give to the 

 landscape its special appearance ; ' buffalo-grass ', according to Asa 

 Gray, consists of Munroa squarrosa, Buchloe (Bulbilis) dactyloides and 

 Bouteloua, also of many other genera, including Stipa, Andropogon, 

 Aristida, Panicum, Hordeum, and Elymus. This ' buffalo-grass ' land 

 is clothed with a low, velvet-like, grassy carpet, which, if it be not exactly 

 sward, is yet somewhat like it, and is green in early spring, but grey at 

 other times. Even in winter it supplies fodder. Here is, or rather was, 

 the home of the vast herds of bison and antelopes. 2 



In other places prairie is clothed with grasses, such as Spartina 

 cynosuroides, Panicum capillare, and P. virgatum, which nearly attain 

 man's height, as well as many Compositae, including Silphium and 

 Helianthus. 



1 Krassnoff, 1888 ; Martyanov, i88j- * Asa Gray, 1884. 



