72 PLANT GEOGRAPHY 



of one side of a mountain-chain to rain-bearing winds, 

 as in the coast-mountains of Brazil, the Eastern Andes, 

 the western slopes of the Western Ghats, and the south 

 side of the Himalayas; and even the steepness of 

 slope. 



CLOUD. The cloud-girdle, or zone, on a mountain 

 where the air becomes super-saturated, which shifts its 

 position upwards and downwards daily, has a marked 

 effect. Its upper margin has night rainfall and sunshine 

 by day; the lower margin both rainfall and shade by 

 day. In the tropics it" is the zone characterised by 

 Bamboos. 



DIRECTION OF MOUNTAIN-CHAINS. The direction of 

 mountain-chains is a factor of paramount significance, 

 not merely because of its influence upon wind and rain, 

 but also as affording possible pathways of migration. As 

 the latitudinal zones of climate are already the greatest 

 barriers to the northward or southward extension of 

 the area inhabited by a species, mountain-chains running 

 mainly east and west, such as the Himalayas and Central 

 European ranges, may seem to bound floras, but will 

 do little to assist in blending them. Ranges funning 

 mainly in meridional directions, however, such as those 

 of America, East Africa, and Australia, extend a bridge 

 across differing climatic zones which plants may traverse 

 at varying altitudes with but little change in the climatic 

 conditions to which they are accustomed. Thus it is 

 suggested that the genus Fagus, originating in northern 

 temperate latitudes, has made its way across the 

 Equatorial Zone to form various species in the extreme 

 south of America and Australasia. 



SLOPE. The angle of inclination or slope of the 

 surface is, perhaps, mainly instrumental in determining 

 the drainage and the rapidity with which humus, or 

 other surface soil, is liable to be removed. 



EXPOSURE. Of greater import, perhaps, is aspect or 

 exposure, whether a sloping surface faces to windward 

 or to leeward, whether towards that point of the compass 

 whence the sun shines, or towards that which is in 

 shadow. The indirect effects of exposure, as, for 

 example, through rainfall, can hardly be exaggerated. 

 Mahabaleshwar, on the windward side of the Western 

 Ghats, has 240 inches of rain annually, while Poona, on 

 the leeward, has but 24 inches. The effect upon duration 



