CHAP. LXXIV GRASS-STEPPE. PAMPA 287 



somewhat undulating, uniform treeless surface is clothed with perennial 

 grasses and herbs, like ' a shoreless sea of grasses on whose horizon the 

 eye finds no resting point, save where the sun rises and sinks '- 1 The 

 genera represented are Melica, Stipa, Aristida, Andropogon, Pappophorum, 

 Panicum and Paspalum. Between the grasses grow numbers of herbs 

 belonging to many families : these include Verbena, Portulaca, Apocy- 

 naceae, Compositae, Eryngium, and others. Curiously enough, there are 

 very numerous European species, which have succeeded in exterminating 

 the inland vegetation for miles, and include not only such thistle-like 

 Compositae as Cynara Cardunculus, Silybum Marianum, and Lappa, 

 but also Lolium perenne, Hordeum murinum, H. secalinum, Medicago 

 denticulata, and Foeniculum capillaceum. In the Flora of Buenos Ayres, 

 according to Otto Kunze, at least three-quarters of the species are intro- 

 duced, and largely Mediterranean in source. Floristic differences present 

 themselves in the pampas, sometimes in the form of associations ; accord- 

 ing to F. Kurtz, 2 different dominant species give rise to 



Verbena-pampa, 



Junquillo-pampa (with Sporobolus arundinaceus), 



Tupa-pampa (with Panicum patagonicum), 



Zamba-pampa, 



Chinata-pampa, and so forth. 



West of the Parana, consequently in more continental districts, the 

 likeness to Russian steppe is obviously greatest, as the grasses are taller 

 and more stiff-leaved, and, as in the latter steppe, grow in tufts between 

 which the soil is bare. 



The soil is mostly a sand loess, which in many places is saline. The 

 climate is like that of steppe and prairie. Rain may be withheld for 

 long periods; whereupon the soil becomes a dry mass impervious to 

 water, and downpours of rain are thus useless, as they flow away over it. 

 Storms blow over the plains without hindrance. Yet the climate shows 

 distinctive features ; there is neither a severe winter nor any long-lasting 

 covering of snow ; moreover, dew is abundant. Consequently the grassy 

 vegetation remains green for a long time, even throughout winter in 

 certain districts. 



Growth-forms. The oecology of the constituent species has not 

 been thoroughly investigated. The number of annual species is very 

 small. 



Bulbous plants are represented in small numbers. 

 Tree-growth is not excluded, and in this respect an additional resem- 

 blance to grass-steppe reveals itself ; trees may be successfully planted 

 even where there is no flowing water. C. Darwin 3 accordingly sought 

 for some geological cause of the absence of trees. Koppen 4 was the first 

 to solve the problem. He pointed out that here it is not the absolute 

 amount, but the frequency, of rainfall which is the determinant factor. 

 The rainfall in the pampas is not small (being 40-100 centimetres annually), 

 but days with rain are very few in number, so that each month experiences 

 periods of drought. The rainiest month seldom includes more than 5-10 

 days of rain on the average. 



1 See Grisebach, 1872. * F. Kurtz, 1893. 



8 C. Darwin, 1845. * KQppen, 1900. 



