314 allan hancock pacific expeditions vol. 13 



Desert Elements 



A considerable number of insular plant species have also been re- 

 ported from the California deserts. Table 8 lists fifteen of these, not 

 including many species of Mexican affinity which also occur in our 

 deserts. Several of these fifteen plants occur only on the islands or the 

 immediate coastal area and then again only in the desert. It may be 

 presumed that the relatively high alkalinity, and the high vv^inds of both 

 maritime and desert habitats produce an environment somewhat alike 

 in these respects. Also precipitation is relatively greater in interior 

 cismontane California than in either maritime or desert areas (U.S. 

 Weather Bureau, 1930) and these plants may not be able to compete 

 successfully with more mesic plants in the intervening area. Whether 

 the incursion of the ocean to the edge of the desert in the Cenozoic 

 (Reed, 1933) and subsequent retreat stranded some of these halic or 

 xeric plants in the desert or whether there was free migration along, 

 the margins of this ancient seaway is now impossible to deteiTnine. 

 Where so few species are involved no generalization can be established 

 since no satisfactory correlation can be obtained within the range of the 

 probability of error. 



Guadalupe relationships. The plants listed in table 8 as occurring 

 also on Guadalupe Island present a similar problem to that of the en- 

 demics (p. 303). Guadalupe Island lies nearly two hundred and fifty 

 miles south of San Clemente Island and fifty miles west of Cedros 

 Island and the coast of Baja California. Among the one hundred and 

 sixty-three species of plants reported from Guadalupe (Eastwood, 1929), 

 thirty-five are probably introduced, and ninty-one are to be found on 

 the Channel Islands. It is most remarkable that only two native, non- 

 Californian species are found on both Guadalupe and Cedros. Of the 

 twenty-seven species which occur on both islands ten are probably intro- 

 duced and fifteen occur on the Channel Islands. 



Genera are a better guide in studies of affinity than species alone, 

 especially when the present relationships may have roots in the geologic 

 past. Thus there are nine families and fifty-seven genera of the Guada- 

 lupe flora not reported from Cedros by Eastwood (1929), including 

 such outstanding genera as Polypodiujn, Eschscholtzia, Trifolium, 

 Ceanothus, Epilobiujn, Solanum, Castilleja, Plantago, and Stephano- 

 meria. On the other hand, eight families and sixty-six genera of the 

 Cedros flora, including such typical Mexican genera as Ephedra, Astra- 

 galus, Euphorbia, Echinocactus, Phaseolus, Viscainoa, Agave, Zizyphus, 

 Petalonyx, Teucrium, Acalypha, and Echinopepon, have not yet been 



