Plant Associations at the Desert Laboratory. 41 



It appears, then, that the flood-plain is the natural habitat 

 of a number of species, many of which are incapable of successful 

 growth elsewhere, while a few grow fairly well, but not at their 

 best, under different conditions beyond these limits. Taken as 

 a whole, the plants of the flood-plain find there their real home, 

 and they exhibit — such of them, at least, as have been carefully 

 observed with reference to this — a striking conformity of the 

 root system to the peculiarities of the soil in which they are 

 growing. This, as already stated, is of fine texture, retentive of 

 moisture, and of great depth, with the water-table varying in 

 level, but apparently never beyond the reach of the long tap- 

 roots of the mesquite and Acacia. 



The root-system of these plants consists of a tap-root which 

 grows rapidly downward, and when developed is always within 

 reach of a permanent, deep water-supply, and a system of widely- 

 spreading lateral roots which are in relation to more superficial 

 layers of the soil. Thus the plant is admirably fitted to absorb 

 water largely from the upper layers when these are moist, and 

 at the same time, and also in times of drought, without any inter- 

 val of precarious supply, to draw on the deeper sources below. 

 The contrast between this and the shallow root-systems of many 

 of the great trees of eastern mesophytic forests, familiar to 

 everyone who has seen them up-rooted by heavy winds, is highly 

 instructive. There is little wonder that the mesquite and 

 Acacia constricta have tenaciously held their places through all 

 vicissitudes and promise to be dominant in their habitat until 

 actually rooted out. 



There are evident movements of vegetation now taking 

 place within this association. Bigelowia Hartwegii, a native 

 weed, has spread with wonderful rapidity under the mesquite 

 within the last three years of favorable seasons; and other species, 

 both weeds and useful plants, have been brought in at various 

 times and by difi"erent agencies to such an extent as to give to 

 the vegetation, in places, a distinct change of character. It 

 should be added that Bigelowia (and presumably other low- 

 growing plants of the flood-plain) has a far less extended root- 

 system than ■; the mesquite, obtaining water from relatively near 

 the surface. If the root-system does not reach to the water- 

 table, and this can hardly occur, it would seem that this plant, 



