288 D. T. MACDOUGAL 



to maintain themselves on such elevated slopes with but little adjust- 

 ment. Similar survivals might ensue along the lower drainage lines, 

 where the underflow in streamw^ays and washes might support a 

 moisture-loving vegetation as it does in southwestern Africa and 

 southwestern America. So much for survival by localization. A 

 second manifestation would be shown by restriction of seasonal activi- 

 ties. The rate of evaporation on the lower levels might be lessened 

 by lower temperatures during the winter season and at this time 

 rapidly acting annual plants might carry through their cycle of activity, 

 remaining dormant in the form of heavily coated seeds during the 

 warmer, dryer period of the year. Perennials with deciduous leaves 

 might display a coincident activity. This survival of moisture- 

 loving plants in a region of pronounced desert character is most 

 marked, however, in places where the precipitation occurs within 

 definite moist or rainy seasons, such as the great Sonoran desert in 

 which two maxima of precipitation occur, separated by periods of 

 extreme drought. Both the winter and the summer rainy seasons 

 are characterized by the luxuriant growth of broad-leaved annuals, 

 which might not be distinguished from those of any moist region. 

 Some species are active during the summer season, and others 

 during the winter, while a smaller number perfect seeds during both 

 seasons. A number of perennials parallel this activity of the annuals 

 with the result that in the most arid parts of Arizona, according to 

 the unpublished researches of V. M. Spalding, half of the native 

 species are in no sense desert plants, requiring as much moisture 

 for their development as do those of Maryland, Michigan, or Florida. 

 The desiccation of a region is seen therefore not to result directly in 

 the extermination of moisture-loving types, but rather to the reduc- 

 tion of their relative or numerical importance and a limitation of their 

 activities to hmited periods, or moist seasons. 



Two types of vegetation may be definitely connected with arid 

 conditions, representing as they do the morphogenic action of 

 water which has been a predominant one in the development 

 of the seed-plants. In one form the chief operation has been one 

 of reduction and protection of surfaces. Leaves have been reduced 

 to linear vestiges representing various parts of the foliar organ, 

 branches to spines or short rudiments as in certain Fouquieriaceae, 



