200 BOTANICAL GAZETTE [September 



cockleburs are the dominant species. If one looks closely, many 

 small annuals and numerous specimens of Riccia jiuitans are seen ; 

 but one misses the bulrushes and sedges he would find in similar 

 places in the east. The usual fate of such young growth is to be 

 washed away upon the return of high water, but should this fail 

 to happen for a year or so, the young cottonwoods may be large 

 enough to hold the soil, and a forest develops. Other character- 

 istic plants of this association are Baccharis Wrightii, Helianthus 

 annum, Dysodia papposa, Onagra Jamesii, Amorpha fruticosa, 

 and Rumex Berlandieri. In more sandy places one meets Aster 

 spinosus, Maurandia Wislizenii, Sesuvium sessile, and Cycloloma 

 atriplicifolia. 



Much of the valley is under ditch and as a consequence does 

 not show the characteristic vegetation, but along the ditches a 

 dense thicket usually develops, composed of Cassia, willows, sun- 

 flowers, Solidago canadensis var. arizonica, and others. 



3. Bigelovia association 



On higher ground, where the water level is deeper, there is found 

 a variety of edaphic plant associations due chiefly to differences 

 in slope and soil and the consequent ability to hold water. But on 

 much of this area the dominant plant is Chrysothamnus (Bigelovia) 

 Bigelovii, a low shrubby perennial, almost leafless, but the green 

 shoots retain their color throughout the year, so that in winter, 

 when the prevailing color of the landscape is brown, this formation 

 may be detected ten miles away. It covers most of the higher 

 gravel beds of the valley and the dissected border of the mesa, but 

 stops abruptly and completely at the edge of the more level mesa. 

 With the exception of the rock surfaces of the mountains, this is 

 the most xerophytic of all our situations; the steep clay hills 

 quickly shed what little water falls on them. In sandy places 

 Yucca glauca is fully as abundant as the Bigelovia, and in places 

 where a foot or two of sand covers a stratum of adobe, the Yucca 

 becomes the dominant plant. In places where the sand is deep 

 and extensive, such as the wider valleys or arroyos, a society, of 

 which Parosela (Dalea) scoparia is the abundant plant, takes pos- 

 session of the soil, often to the entire exclusion of Bigelovia, but not 



