PLANT ECOLOGY AND FLOEISTICS OP SALTON SINK. 99 



not only need moisture sufficient to induce the seeds to germinate, but that it be con- 

 tinued long enough to enable the young plant to establish itself. Such conditions can 

 occur on the desert but exceptionally, perhaps only at intervals of 100 years or more; or, 

 it may be that an investigation of the ages of the desert trees would afford indications 

 of secular changes in the desert climate. 



Only one of the desert trees forms an exception to the absence of adolescent individuals. 

 These may be seen of various ages among the groups of Parosela spinosa, a tree which 

 grows almost exclusively along the dry water-courses of the desert; it is, indeed, as certain 

 an indication of a "dry wash" as is a line of willows of a living stream in a well-watered 

 district. Soaked by the occasional torrents which pour down these washes, their soil 

 affords an exceptional seed bed to the bordering Paroselas. 



ASSOCIATIONS OF THE XEROPHYTIC FORMATION. 



While the xerophytic flora has many points in common throughout the whole Sink, 

 it is differentiated into three associations, dependent upon the nature of the several soils. 

 They may be distinguished as the associations of the detrital slopes, of the Imperial clays, 

 and of the drifted mounds. 



THE DETRITAL ASSOCIATION. 



The plants of the first of these associations occupy the detrital soils along the north- 

 eastern margin of the Sink, extending from the high contour line either to the alkaline 

 flats at the upper end, or, at the lower, as at Durmid, to the present shore of Salton Sea. 

 Eventually, as the Sea evaporates and disappears, the whole lower border will be along 

 the lacustrine flats and marshes of the central depression. A less well-marked area of 

 this association is situated on the opposite side of the Sink, at its upper end. As has been 

 already explained, these areas of washes and detrital fans have soils more permeable to 

 water and to the roots of plants than the other arid soils of the Sink. They are also more 

 subject to torrential floods and have, perhaps, a slightly greater annual precipitation. 

 Consequently the plant population is comparatively denser, although still open, and there 

 is a greater diversity of species. Some of the leading species, as Atriplex canescens, are 

 equally prominent elsewhere, but others, which are here frequent, are seldom or never 

 present in the other associations. This is especially true of those which grow along the 

 upper margin, where a few of the canon plants come down into the Sink flora. 



In passing upwards to this margin there is a certain succession of plants encountered, 

 indicating a zonalization, which depends partially upon soil and, probably, partially 

 upon deep-seated water tables. For, arid as these mountains are, to a certain extent 

 they are reservoirs of water, as is demonstrated by the occasional springs found in the 

 canons. It is evident that as the drainage slowly percolates through the detrital cones 

 the parts nearest the base of the mountain will contain the greatest and most reliable 

 amount of water, and this can not but have its effect upon the plant covering. 



This zonal distribution is readily observable in passing upward from Mecca to the 

 mouth of Red Canon, in the Chuckawalla Mountains. Leaving the alkaline flats, which 

 have been elsewhere described, a narrow zone of dry lacustrine soil is encountered. It is 

 covered with an open growth of low bushes of Atriplex canescens, the open intervals quite 

 abundantly grown up with dwarfed Plantago fastigiata, Cryptanthe sp., and other annual 

 herbs. Beyond this begins the detrital soil, where the Atriplex is still abundant, but is 

 mixed with much Petalonyx thurberi and Franseria dumosa, and a few Larreas. A little 

 higher leguminous trees appear, Prosopis pubescens, P. glandulosa, Cercidium torreyanum, 

 Olneya tesota, and Parosela spinosa, all but the first two being confined to this association. 

 These grow along and near the washes, where the soil is most open and where there is 

 probably more deep-seated water. The principal herbs here are Coldenia plicata, Psathy- 

 rotes ramosissima, and Chamcesyce parishii. Towards the upper limits of the Sink Parosela 



