TliE FREE WATEU OP THE SOIT.. 203 



weight of water in 24 liours of hot and dry summer 

 weather. 



Tlie water exhaled from tlie 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, ai-e nearly equal in 

 quantity and niutually dependent during the healthy ex- 

 istence of vegetation. 



Circumstances that Influence Transpiration.— ^ Tlie 

 structure of the leaf, including the character of the epi- 

 dermis, and the number of stomata as they aflfect exhala- 

 tion, has been considered in " How Crops Grow," (pp. 

 286-8). 



h. 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 Avith moisture, and va- 

 ries through a wide range according to the conditions just 

 named. 



0. The oxidations that are constantly going on, within 

 the plant may, under certain conditions, acquire suflicient 

 intensity to develop a jierceptible 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 

 tem.perature. These interior changes can have no apprecia- 

 ble influence upon transpiration as compared Avith those 

 that depend upon external causes. Sachs found in some 

 of his experiments (p. 3G) that exhalation took place from 

 plants confined in a limited space over water, Sachs be- 



