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VESTAL— PHYTOGEOGRAPHY OF COLORADO 



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belt plains. The divide is also cooler than most parts of the 

 mountain-front and adjoining plains. The vegetation of the divide 

 is likewise transitional between that of rain belt and foothills, 

 with the more nearly mesophytic forms of grassland, and with 

 woody plants of the foothills extending many miles eastward from 

 the mountains. 



The effects upon the vegetation of the difference in distribution 

 of precipitation north and south of the divide are discussed in 

 the section on geographic relations, but may be summarized briefly 

 herewith. 



TABLE V 



Influence of seasonal distribution of rainfall on vegetation 



NORTHERN AREA 



The greatest rainfall is in April and 

 May. 



There is greater activity of vege- 

 tation, more luxuriant growth, and 

 greatest abundance of flowers in 

 spring. 



There are many spring-flowering 

 plants from the mountains of rather 

 mesophytic character, in mixture with 

 plains plants in the mountain-front 

 zone. 



Distribution of the bunch-grass 

 association and of the less xerophytic 

 plants, requiring a long season of con- 

 tinued moisture, is limited. 



The northern plains near the 

 mountain-front flower luxuriantly in 

 spring and early summer, but only 

 the more xerophytic composites, etc., 

 in late summer, in which respect the 

 plains are more Uke the driest plains 

 just east of them in late summer. 



SOUTHERN AREA 



The greatest rainfall is in July and 

 August. 



There is greater activity of vege- 

 tation and more luxuriant growth in 

 late summer. 



There is absence or scarcity of 

 spring-flowering mountain plants, and 

 greater prevalence of plains plants in 

 the mountain-front zone. 



Distribution of bunch-grass is less 

 restricted; there is a greater preva- 

 lence of late-flowering plants not 

 intensely xerophytic. as some of the 

 asters and goldenrods, etc. 



The southern plains near the moun- 

 tains contain fewer spring flowers, 

 but many long-season plants absent 

 from the dry plains and the northern 

 plains near the mountains are present, 

 as the annual sunflowers. In this 

 respect the plains are more like those 

 of the rain belt of eastern Colorado 

 in late summer. 



It is remembered that the southern area is in general drier and 

 warmer, with somewhat more xerophytic vegetation than the 

 northern area, and that differences in vegetation due to this cause 

 must be distinguished as well as possible from those due to different 

 distribution of rainfall. 



