Recent History of the Rocky Mountain Flora. 243 
arboreal vegetation and of rainfall, the former being in great measure dependent 
on the latter. Its plains are treeless except along the water-courses; the 
mountains bear trees along sheltered ravines and on the middle and higher 
slopes upon which there is a considerable condensation of moisture; "but, 
whenever they rise to a certain height (about 11,000 feet) in latitude 37° to 
41°, they are woodless from cold and other conditions attending elevation 
although the higher mountains enjoy an abundant condensation of moisture 
mostly in the form of snow. Three belts may, therefore, be distinguished 
in the vegetation of the Rocky mountains. ı. An arid and woodless district, 
which occupies a large part of the area. 2. A forest belt adorning the mountain 
slopes. 3. An alpine unwooded belt above the timber-line. 
1. The woodless region may be divided into three well-marked districts: 
ı. the lower Rocky Mountain slopes (parks and higher foothills); 2. the desert 
interior, which will be considered separately under the caption, Great Basin 
and 3. the eastern lower foothills and mesas that merge into the eastern 
woodless plains’). The mesas are long, gently sloping, flat topped ridges ex- 
tending from the bases of the foothills out towards the plains in an easterly 
direction, The vegetation of the north slopes of the mesas differs from that 
of the south slopes. The varying amount of moisture and sunshine are the 
important factors determining this difference. The east and west ends of the 
mesas are also different in their plants. The west end, being closer to the 
foothills, is sheltered from the afternoon sun, while the east is not thus pro- 
tected. In the Spring small streams from de melting snow on the hills supply 
the west end of the mesas. Pine trees and Populus tremuloides grow there, 
while at the east, there are no trees at all. A gradual transition is noticed 
from the xerophytic formations of cactuses, Yucca, Astragalus, Artemisia and 
. desert grasses on the east to the pine groves with their desert species of 
Pulsatilla and Delphinium on the west. Such shrubs as Cercocarpus, Ribes, 
and Raus trilobata grow among the pines or east of them for a distance. 
Sometimes a mesa will have certain shrubs growing near the top on the south 
slope, but none farther down on the same slope. This is accounted for by 
the same fact that the spring snow lasts for a long time on the mesa, and as 
it slowly melts, it trickles down the south slopes to be quickly absorbed by 
the soil not far from the crest, while on the other hand, there is sufficient 
moisture on the north slope to permit the growth. of shrubs for some distance 
from the foothills. Finally, it is important to note that the flora of the mesas 
is, a complex of mountain and plain species. At one season of the year, the 
vegetation of the mesas has the character of the mountain region. At another 
season, the prairie plants are more abundant, The slight change in the angle 
of the sun’s rays causes the replacement of the Zewcocrinum by a Pentstemon 
of higher altitudes. Later the prairie Yucca comes into blossom, and then 
1) RAMALEY, Francis: Remarks on the Distribution of Plants in Colorado east of the Divide. 
Postelsia 1901: 32 
16* 
