204 BOTANICAL GAZETTE [September 



fall rains and the occasional snow flurries during the winter afford 

 them sufficient water for growth in favorable situations, and they 

 are ready to blossom with the spring rains. The 



most 



ous examples in this class are rhacetia corrugata, some of the loco 

 weeds (Astragulus sp.), Draba, Gilia, and sometimes Gaura coc- 

 cinea, Sideranthus spinulosus, and many of the plants that are 

 ordinarily summer annuals may occasionally develop during the 

 winter and blossom with the first shower of spring or summer. 

 Indeed, the one feature of the vegetation of this region that attracts 

 the attention of one accustomed to more humid regions is the 



most 



many of the shrubs. W 



can one speak of spring, summer, and autumn flowers here. They 

 grow and blossom when the rains come, be that March or August. 

 During 1909 and 1910 the rains came July 20 and July 23 respec- 

 tively, and the result was that the mesa was brown and lifeless 

 until then, but by August 1 it was a garden, nearly covered by a 

 mat of vegetation, made up of grasses, Abronia, Allionia, Town- 

 sendia strigosa y Houstonia humifusa, Plantago Purshii, Asclepias 

 br achy Stephana, Wedelia incarnata, Russian thistle, and Solanum 

 elaeagnifolium. By September 10 all was over and the mesa had 



* 



assumed its usual brown hue. Thus in six weeks the annuals and 

 the underground perennials had grown, flowered, and matured their 

 seeds. The exceptions to this rule are those plants in the first 

 class, the larger shrubs, the cacti, yuccas, and other plants having 



stems 



With M 



come the blossoms of the stemless evening primroses (Oenothera 



chimaja (Cymopt 



When J 



arrives, we have the flowers of the cacti, yuccas, and the desert 

 willow; while September brings out the blossoms of Bigelovia and 

 October. the Gutierrezia, if there has been rain. This formation 

 and the next two are classed as Upper Sonoran by Merriam and 

 his followers. 



middle of the mesa. 1 z miles 



ping of a layer of sandstone causes a succession of springs to appear, 

 and about these springs are cottonwoods, Juncus, Houttuynia y and 

 other plants of the valley. In other words, a spring changes Upper 



