FACTORS AFFECTING PLANT DISTRIBUTION 19 



than in an interior region. In the second place, elevation above sea 

 level produces a local climatic effect in some ways like the climate 

 of the central portions of continents. The sunshine intensity is 

 great, and the water loss rapid. On the windward side of the 

 mountain range, however, the precipitation is heavy, since the air 

 containing moisture deposits its load as it rises and cools in pass- 

 ing over the summits. The influence of the mountains in catching 

 water is felt in all the territory on the windward side between 

 them and the ocean, even in essentially dry regions, since they con- 

 tribute to stream flow. 



More important as reservoirs of water than the oceans even 

 are the land masses. It is not difficult to see that if an air current 

 loaded with moisture from an ocean reservoir reached a land area 

 and deposited some of this water, the air current would become 

 rapidly dry as it passed over the land. Very little moisture would, 

 under these conditions, ever be transported inland for any great 

 distance. An examination, however, of the continents of the earth 

 shows that a good deal of water falls on the land at some distance 

 from oceans, leading up to the obvious conclusion that a current of 

 air is picking up water vapor and precipitating water all the time 

 as it passes over a continent. The amount of water evaporated 

 from oceans and evaporated from the land has been compared. 

 Brueckner has calculated that the land supplies from its own area 

 seven-ninths of the precipitation which falls upon it. This means 

 that the land is a much more abundant source of the moisture car- 

 ried by winds than the oceans are. A current of air moving over 

 the land and precipitating moisture would be continually having 

 more moisture added to it, a re-enforcement of the supply so to 

 speak, from the evaporation of water from the land surface. 



Zon has constructed a balance sheet showing the circulation of 

 water on the earth's surface. The study of Zon's figures lead to 

 the following conclusions: (a) that the bulk of the water evapor- 

 ated from the oceans is reprecipitated into the oceans, (b) the bulk 

 of the water evoparated from the land is reprecipitated on land, 

 that the precipitation falling upon land is nearly all (four-fifths) 

 furnished by the land area. The examination of Table 1 explains 

 these statements in statistical form. 



