a 
Relation of Climate to Vegetation. 121 
covering these areas, certain pronounced changes have 
been effected in the frequency of rainfall and in the con- 
stancy of supply of water, as marked by the flow of rivers 
and small streams. It has, therefore, been a somewhat 
common practice to refer such changes to effect upon the 
total rainfall, and to ascribe to the presence or absence of 
abundant vegetation of the arborescent form, a definite in- 
fluence upon climate. The question is of the greatest im- 
portance, as through its influence upon manufactures and 
water supply, as well as its effect upon tillage, it directly 
concerns some of the more important economic aspects of 
life. As at the present time, the changes referred to— 
de-forestation and re-forestation—are now taking place 
upon a large scale within the limits of the United States 
and Canada, we have a ‘convenient field of observation at 
hand, as a basis upon which to determine how far such 
opinions coincide with known facts. 
One of the most important functions of the plant is its 
power of transpiration, or its ability to liberate water from 
its structure in the form of aqueous vapor. Such transpira- 
tion is one of the important factors in determining the 
movement of water from the roots, where it has been 
absorbed from the soil, to the leaves, where it is utilized in 
the various chemical changes incident to growth. The ca- 
pacity of plants in respect to this function, or the amount 
of water they will thus liberate within a given time, is ex- 
tremely variable, though constant for any one species under 
uniform conditions of growth. Moreover, while many 
plants are structurally adapted to the freest possible 
transpiration, others are adapted to retardation of this 
function when the conditions of supply are limited, as must 
be the case in very hot and dry regions. In all cases, how- 
ever, transpiration is controlled by conditions of light and 
heat, as well as by the extent to which the surrounding at- 
mosphere is already charged with aqueous vapor. 
The general tendency of this function will in all cases be 
to establish a constant movement of water upward from the 
soil through the plant, until it is liberated from the leaves, 
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