62 BOTANICAL GAZETTE [july 



regions with fringing mud banks derived from the river deposits. 

 The greater part of the vegetation of the coastal slopes and plains, 

 however, is made up of species which also extend southward along 

 the shores of the Gulf, and are found but sparingly to the northward. 



The vegetation of the Salton basin is subjected to the action of 

 extreme aridity and also of a high concentration of soil salts, com- 

 prising types of the most pronounced character, both of halophytes 

 and xcrophytes. The elevations included in the delta are dry moun- 

 tain slopes and support a desert vegetation. 



The mesas adjoining the northern part of the Gulf of California 

 appear to offer the most extreme desert conditions in North America. 

 The rainfall at Yuma at the northern extremity of the delta was less 

 than 25 mm during 1903, and years have been noted at points farther 

 south in which no precipitation occurred. The entire normal pre- 

 cipitation in the desert in Baja California is probably no greater than 

 the amount of water condensed as dew in eastern United States. 

 The desert apparently extends to the slopes of the central elevation 

 of the peninsula to the westward, which reaches an elevation of over 

 3000™. This mountain wall probably acts as a barrier which shuts 

 off moisture-laden winds from the Pacific and causes the aridity of 

 the region. The southern and eastern limits of the extreme desert 

 cannot be defined with the information now at hand. 



The vegetation of the desert areas in the regions of greatest aridity 

 consists chiefly of types devoid of massive storage organs, and of 

 perennials with laticiferous sap, while a large number of forms secrete 

 volatile oils or exude resinous gums. The determination of the 

 causal relations of these adaptations cannot be made safely by a 

 general interpretation of the aspects of vegetation, but it is to be 

 seen that in a region in which surplus rainfall or ground water never 

 occurs, storage organs would be manifestly useless, and indeed their 

 formation impossible. Species with spinose branches and minute 

 leaves which are quickly discarded during unfavorable periods are 

 abundant. Several species of Ephedra with functionally useless 

 leaves and chlorophyllose stems are included, while Lycium and 

 Frankenia with small succulent easily detachable leaves are highly 

 characteristic of some localities; the last-named species are about 



