Plant Associations at the Desert Laboratory. 61 



are suited only to distinctively xerophytic plants, such as grow 

 where peculiarly trying conditions prevail. Under these con- 

 ditions as might have been anticipated, nearly all of the per- 

 ennial species of the flood-plain abruptly cease; in fact, the 

 creosote bush is the single one that is capable of successfully 

 holding the ground on the worst places, and even this is dwarfeJ 

 in the struggle to maintain existence where the water supply is 

 never abundant for any length of time, and through a large part 

 of the year is necessarily meager in the extreme. Yet, in spite 

 of these untoward conditions, the Larrea claims this zone as its 

 own, and has produced upon it an almost pure growth of a single 

 species. 



It is impossible, as yet to enumerate completely the pecu- 

 liarities of this plant, structural and physiological, which have 

 enabled it, more successfully than any other, to cope with the 

 worst of desert conditions as they exist in the Southwest. Some 

 of these, however, are obvious. It is provided with a root- 

 system which both penetrates deeply and also branches widely 

 near the surface, thus being in a position to avail itself of water 

 that accumulates at either level; it is capable, as few other species 

 are, of absorbing appreciable quantities of water thraugh its 

 leaves, and these are admirably protected by varnish and in 

 other ways against excessive transpiration. With the coming on 

 of drought a large percentage of its leaves are thrown off and a 

 still further reduction of surface is accomplished by the leaves 

 becoming checked in their development long before they have 

 attained their full size, as seen in individuals of the slopes com- 

 pared with those of the flood-plain and washes. Its mode of 

 branching and reproduction from the root is also advantageous. 

 Whole systems of branches die, and thus permit their share of 

 water from the soil to go to others, and, if under extraordinary 

 stress, the whole top perishes, fresh shoots from the root come up 

 upon the return of rain, and vigorous growth again takes place. 

 Fitted in so many ways, and probably in others less obvious, to 

 cope with the vicissitudes of its habitat, the Larrea holds complete 

 sway in this zone, and from Texas to California the broad belt 

 of creosote bush, covering the long slopes that form the approach 

 to the mountains at different altitudes, presents a most striking 

 and characteristic feature of the landscape. 



