138 FORREST SHREVE 



elevation, and they have important groups of both moisture 

 and temperature factors which cause a pronounced vertical 

 differentiation of the vegetation. 



Mountains which lie outside the equatorial regions of the 

 earth almost invariably exhibit differences of vegetation on their 

 north-facing and south-facing slopes. These differences, due 

 superficially to slope exposure, are underlaid by complex as- 

 semblages of factors which are by no means the same in all of 

 the diverse climatic provinces in which such difTerences may be 

 observed. The constant direction of the trade wind at Jamaica 

 causes the marked dissimilarity of the north and south slopes 

 which has been mentioned, and thereby masks any effect of slope 

 exposure that would be possible at the low latitude of that 

 island. In the Santa Catalinas, on the other hand, there is a 

 well-marked difTerence of vegetation on opposed slopes at all 

 elevations. The difference is very conspicuous at certain alti- 

 tudes, and is commonly about as great as the difference between 

 situations of the same slope exposure which are 1000 vertical 

 feet apart. 



One of the most familiar features of the tropical rain-forest 

 is the ''stratification" of its various plant types, by virtue of 

 which the dominant trees shelter smaller trees, these form a 

 canopy for tree-ferns or shrubs, these in turn shade large herba- 

 ceous plants, ferns or small shrubs, while on the ground itself 

 are the smallest and most hygrophilous of herbaceous plants. In 

 the somewhat stunted forest of the Blue Mountains there is 

 not such a pronounced stratification of the vegetation as may 

 be seen in the richer forests of the lowlands, but it is neverthe- 

 less a very noticeable feature of the ravines and more gentle 

 slopes, particularly of the windward side of the range. In the 

 Santa Catalina mountains the lowest, or Desert slopes, and the 

 lightly forested Encinal region are naturally without any strati- 

 fication of the vegetation, and the pine forests of the higher ele- 

 vations are as nearly devoid of it as are the pine forests of the 

 southeastern United States. In the heavier fir forests of the 

 highest summits of the Santa Catalinas there is the usual scat- 

 tering accompaniment of lax and slender deciduous trees and a 

 ground cover of small shrubs and herbaceous plants. 



