Many perennials benefited by spring rains. 
Piaces rare where there is absolutely no vegetation in deserts. 
Qases populated by rank growth of trees and herbs. 
T'‘lants grouped into two ecological categories: 
1. Those dependent on nain. 
2. Those dependent on ground water. 
Kamball imithe Ne Asdesert. 
Climate in the N. A. desert. 
“Dry lakes”, swamps, soil alkaline. 
Devoid of vegetation except around edges,—chenopods, 
sedges, grasses, etc., arranged in zones. On sand mes- 
quit, chenopods, etc., other zones. 
On elevations,—pines and junipers. Shrubs,—sage brush, etc. 
XVII. ARCTIC AND ALPINE PLANT SOCIETIES. 
tT. Arctic Plant Socteties: 
Arctic zone of plant life. 
“Cold waste”, sometimes compared with desert. 
Ground frozen to great depth. 
Character of tree growth, at polar limit. 
I. stunted, often table like. 
2. Dead tops,—often at quite regular distances. 
3. Under deep snow bent and horizontal. 
4. Growth in length slow in proportion to growth in 
diameter, but here very siow also. 
Trees 83 mm. diameter (3 1-3 in.) 544 annual rings. 
Polar tundra, related to heath or peat moors. 
1. Heaths, saxifrages, dwarf willow, etc. 
2, Lichen tundra ( Cladomia, Alectonalsetes): 
3. Moss tundra (Polytrichum). 
Conditions of environment. 
Ist, “Temperature, long cold winter, low summer tempera- 
ingie: | 
and. Light, long winter mght, continued daylight in sum- 
mete 
ard, Co'd ground water in summer. 
4th. High winds. 
sth. Very dry air of long winter. 
6th. Lessened precipitation, snow usually not deep. 
44 
