Chap. IX] 



DESERTS 



641 



globose shape, as in the Sahara, and considerably higher on moister soil 

 than in the driest spots. Of the two commonest shrubs, Larrea tridentata 

 is one to one and a half meters, and Franseria dumosa only a third of a meter 

 high, the dimensions of the other species varying between these extremes. 

 Reduced size of leaf-blades and dense coatings of hair, which give to the 

 vegetation a grey tint, appear to be the most usual protective devices against 

 transpiration in shrubs. Thorny plants are common (Fig. 371). 



The vegetation associated with subterranean supplies of water is mainly 

 shrubby ; perennial herbs are relatively rare, and only very few of these are 

 perennial in their subterranean parts alone (Cucurbita palmata, C. foetidis- 

 sima, Rumex hymenosepalus). The remaining herbaceous perennials are 

 woody at their base, and may therefore rather be described as under- 

 shrubs. 



Fig. 379. North American desert in the San Francisco Mountains, Arizona. Agave applanata, 

 var. Parryi. From a photograph by Parry, reproduced by Mulford, op. cit. 



As in the Sahara, the spring rains educe a short-lived rain- flora 

 relatively rich in species which are all annuals, with the exception of the 

 three above-mentioned herbs, whose perennial parts are entirely subter- 

 ranean. In accordance with the very unequal amount of the rainfall, this 

 flora, which appears in February and dies in April, varies greatly in 

 luxuriance in different years. Thus, according to Coville, C. R. Orcutt 

 found in the Colorado desert, a few weeks after the great storm in February 

 1891, stems of an Amaranthus ten feet high; a year later, when the 

 rainfall was very small, he collected in the same place some fruiting 

 specimens of the same species that were only a decimeter high. 



Coville, without specially searching for small specimens, collected in 

 Surprise Canon (Panamint Mountains) some fruiting specimens of Lepidium 

 lasiocarpum, 11 mm. high; of Mimulus rubellus, 17 mm.; of Draba 



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