1906] SHANTZ—VEGETATION OF THE MESA 27 
and the surface of the soil rises to a temperature much higher than 
that of the air. 
To understand and explain the appearance of the vernal and 
prevernal flora it is necessary to take into account the winter 
condition. The water content is low at the time of the appearance 
of the first flowers. The north slopes have a higher percentage of 
water than the other exposures, and although the temperature is 
lower it is here that the greatest number of spring flowers are pro- 
duced. Only very resistant species seem to be able to survive the 
long dry winter and produce flowers in the spring on the south 
slopes. The high temperature which the soil of this exposure reaches 
during the winter would surely start growth at a period when 
such development would be disastrous to the life of the plant. 
This probably explains the abundance of the north slope vernal 
flora and the paucity of the south slope flora during the same 
period. 
Before the spring rains have begun, at the end of the long dry 
period, this prevernal flora makes its appearance. These plants 
are never very abundant on the Mesa proper, but appear in great 
numbers on the hillsides. In each case the flowers appear either 
without any foliage or with very little. 
The most prominent society of this period is the Pulsatilla soci- 
ety, characterized by P. hirsutissima. Its distribution within the 
region studied is limited to the north slopes and it is never found on 
the south slopes. The plants are often very numerous and consti- 
tute almost the only growing vegetation of this period; forming a 
zone along the north and east slopes of the Mesa which is only inter- 
rupted at places of south or southern exposure. 
Leucocrinum montanum forms a society which is less exclusive 
than’the former and is at the same time less distinct. It reaches 
its maximum development on north crests, but may occur in almost 
any situation except the south exposure. Over the greater part 
of the Mesa L. montanum is mingled with Townsendia exscapa, 
and these two plants constitute the only vegetation of the Mesa top 
at this period. 
Cymopterus acaulis and Phellopterus montanus seldom form 
communities. In their distribution they show a marked alternation, 
