26 BOTANICAL GAZETTE [JULY a 
sky occur most frequently during the afternoon, which on this account 
also receives much less sun than does the forenoon. The record 
for sunshine for June and July 1904 makes this clear (fig. 6). 
The difference in the light received by the east and west slopes, — 
together with the resulting differences in temperature, relative 
humidity, and water content, is of importance in explaining the | 
vegetation of these exposures. 
The Bouteloua (grama grass) formation. 
This formation occupies practically the whole Mesa region and — 
the low land surrounding it. It extends for many miles north, — 
south, and east, but no attempt has been made to determine its © 
limits. It seems to be the formation most typical of the high plains 
and extends eastward far into western Kansas and Nebraska. 
The season of growth may be divided into two periods: the ; 
vernal, or in this case the spring and early summer period; and — 
the aestival, or late summer and autumn period. Each of these 
grand periods may be divided into two periods: the first into the 
prevernal and vernal, the second into the aestival and autumnal. 
PREVERNAL ASPECT. 
Habitat—Physical and available water content very low. Rain- — 
fall o.48-2.43°™ (April 1904, 4.68°™); ay. daily, 0.008°™. Rela-— 
tive humidity: mean, 44 per cent.; range 1-100 per cent. Wind — 
6.09'™ per hour. Temperature: mean 7°2 C.; mean max. 14°4 C.} 
mean min. o° C.; range —11°1 to 25° C.; soil, 13°C. Light dura 
tion 66 per cent., exposure varied, cover open, soil clay to. gravel. 
Duration April. 
PRINCIPAL SPECIES: Leucrocrinum montanum Nutt., Townsendia exscap4 
(Richards) Porter, Pulsatilla hirsutissima (Pursh) Britton. ; 
SECONDARY SPECIES: Cymopterus acaulis (Pursh) Rydb., Phellopterus : 
montanus Nutt. 
The winter conditions are xerophytic, there being very little 
rain and seldom a snow cover. The mean relative humidity is — 
low—40 to 50 per cent., and sometimes falling as low as 1 per cent. — 
The temperature in winter rarely falls below —18° C. and often rises ; 
above 16° C. The records show 56 per cent. of possible sunshine — 
