THE FREE WATER OF THE SOIL. 203 
weight of water in 24 hours of hot and dry ‘summer 
weather. 
The water exhaled from the leaves must be constantly 
supplied by absorption at the roots, else the foliage soon 
becomes flabby or wilts, and finally dies. Except so far 
as water is actually formed or fixed within the plant, its 
absorption at the roots, its passage through the tissues, 
and its exhalation from the foliage, are nearly equal in 
quantity and mutually dependent during the healthy: ex- 
istence of vegetation. 
Circumstances that Influence Transpiration.—a. The 
structure of the leaf, including the character of the epi- 
dermis, and the number of stomata as they affect exhala- 
tion, has been considered in “How Crops Grow,” (pp. 
286-8). 
6. The physical conditions which facilitate evapora- 
tion increase the amount of water that passes through 
the plant. Exhalation of water-vapor proceeds most: 
rapidly in a hot, dry, windy summer day. It is nearly 
checked when the air is saturated with moisture, and va- 
ries through a wide. range according to the conditions just 
named, 
ce. The oxidations that are constanily going on within 
the plant may, under certain conditions, acquire sufficient 
intensity to develop a perceptible amount of heat and 
cause the vaporization of water: It has been repeatedly 
noticed that the process of flowering is accompanied by 
considerable elevation of temperature, (p. 24). In general, 
however, the opposite process of deoxidation preponder- 
ates with the plant, and this must occasion a reduction of 
temperature. These interior changes can have no apprecia- 
ble influence upon transpiration as compared with those 
that depend upon external causes. Sachs found in some 
of his experiments (p. 36) that exhalation took place from 
plants confined in a limited space over water. Sachs be- 
