ORIGINATION OF SELF-GENERATING MATTER 613 
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 streamways 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the unpublished researches of V. M. Spalding, half of the native 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 limited 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, 
