THE PLANT WORLD 237 



plants have been found in North America. Thus far at least, then, ex- 

 tended the refrigerating influence of the glacial period in the geological 

 past. If it reached beyond, no mountains sufficiently high have pre- 

 served a record to our days, in plants left stranded, as the ice-cap 

 retreated nortliAvard. 



These northern and eastern faces of San Jacinto are too steep, 

 rockj' and arid, too much exposed to the heat and glare of the desert, 

 to support other than the scanty shrubs and herbs that can cling in the 

 crevices. Their zonal distribution is, at least, obscure. But on the op- 

 posite side the case is different. There the mountain falls by easier de- 

 scents and sends out subsidiary ranges whose corrugated folds separate 

 it from the Pacific. Coniferous forests clothe its slopes, enfolding 

 meadows, Avith brooks and tiny lakelets. Here are conditions to encour- 

 age diversity of growths, and room for plant stratification to manifest 

 itself. Mr. H. M. Hall has carefully studied this interesting flora, and 

 in a recent paper" has traced out its disposition, and the causes which 

 determine it. 



In descending the mountain upon this side, one passes from the 

 Ai'ctic, successively through the Hudsonian, the Canadian and the Tran- 

 sectional life zones, to the Upper Sonoran ; or, to take the pines as an 

 index, leaves behind in turn the lumber-turg, the lodge-pole and the 

 yellow pine (Pinus fiexilis, P, Murrayana and P. ponderosa), before 

 reaching the great chaparral belt of the foothills. 



As in all these southern mountains, the various zones are somewhat 

 confused. In places they commingle, or they run promontories into one 

 another's territory, or intrude as "islands." The familiar causes of 

 such interpenetration are here supplemented by influences arising from 

 the proximity of the desert, influences by which the disposition of the 

 flora of the whole region bordering upon it are affected. It presents, 

 therefore, a complicated problem, much more difficult than that on the 

 desert side, where all the intermediate factors are practically eliminated. 



One of the most important services rendered by the plant-breeding 

 experts is to secure disease resistant, or immune, plants as the most 

 practical way to fight plant diseases. Striking success is reported in 

 this line. The development of cotton resistant to wilt diseaae is now 

 an assured fact, and a variety of cowpea has been discovered resistant 

 to wilt and to root knot. 



* A Botanical Survey of San Jacinto Mountain. By Harvey Monroe Hall, Univ 

 of Cal. Public. Bot. 1, pp. 1-140. June, 1902. 



