Chap. IX] 



DESERTS 



6 43 



Desert-like stretches (Fig. 385) appear also on the plateau at the eastern 

 foot of the Rocky Mountains, wherever the soil possesses a permeable 

 character, whereas soils richer in clay bear steppe-formations that are 

 distinctly xerophilous even though they may be closed. These dry steppes 

 (Fig. 386), whose prevailing grasses are Buchloe dactyloides and Bouteloua 

 oligostachya, support, often in very great num- 

 ber, a low species of Opuntia, O. polyacantha 

 (Fig. 384), with its members inserted on the 

 angles of the stems. From the beginning of 

 July onwards, after the grasses have dried up 

 and disappeared, the epigeous living vegetation 

 is almost entirely composed of the Opuntia ; it 

 forms green patches between the straw-yellow 

 ribs of 'pepper-grass' (Lepidium interme- 

 dium), which frequently takes the foremost 

 place in the composition of such formations 

 — ' Pepper-grass-cactus formation.' 



The ' under-shrub formation ' marks the 

 purely desert character, in which deep-rooted, 

 small-leaved, very hairy species of sage-brush 

 (Artemisia), in particular A. tridentata (Figs. 

 376, 377), form the open vegetation, which 

 is interrupted by extensive bare intervening 

 spaces. In places other Compositae of no 

 less marked xerophilous appearance predomi- 

 nate in the formation, for instance, Eurotia 

 lanata(Fig. 383, 7), Bigelowia graveolens (Fig. 

 ^H^, 1), Gutierrezia Sarothrae, and others. 

 All these under-shrubs attain a height of one 

 to one and a half meters. Artemisia filifolia 

 characterizes the sandy tracts ; in contrast with 

 its allies mentioned above, it forms pure green, 

 very tall bushes. 



THE DESERT AND SEMI-DESERT 

 IN MEXICO. 



Fig. 382. North American desert- 

 flora. Atriplex canescens. 

 Natural size. 



The Mexican plateau on the whole pos- 

 sesses a dry climate, but one usually much 



richer in rain than that of the North American desert. In published 

 meteorological tables one nowhere finds a desert climate, but rainfalls 

 of 500 mm. and more, which, even at high temperatures, must involve 

 a less meagre vegetation except on very permeable soil. Mexico how- 

 ever possesses a climate that changes with extraordinary rapidity, both in 



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