288 HOW CROPS GROW. 



May 22d and September 4th, no less than 36 times ita 

 weight of water. 



Exhalation is not a regular or uniform process, but varies 

 with a number of circumstances and conditions. It de 

 pends largely upon the dryness and temperature of the 

 air. When the air is in the state most favorable to 

 evaporation, thr loss from the plant is rapid and large. 

 When the air is saturated with moisture, as during dewy 

 nights or rainy weather, then exhalation is nearly or 

 totally checked. 



The temperature of the soil, and even its chemical com 

 position, the condition of the le:if as to its age, texture, 

 and number of stomata, likewise affect the rate of ex 

 halation. 



Exhalation is a process not necessary to the life of the 

 plant, since it may be suppressed or be reduced to a 

 minimum, as in a Wardian case or fernery, without evident 

 influence on growth. Neither is it detrimental, unless the 

 loss is greater than the supply. If water escapes from the 

 leaves faster than it enters the roots, the plant wilts ; and 

 if this disturbance goes on too far, it dies. 



Exhalation ordinarily proceeds to a large extent from 

 the surface of the epidermal cells. Although the cavities 

 of these cells are chiefly occupied with air, their thickened 

 walls transmit outward the water which is supplied to 

 the interior of the leaf through the carnbial ducts. Other 

 wise the escape of vapor occurs through the stomata. These 

 pores appear to have the function of regulating the exhala 

 lion, to a great extent, by their property of closing, when 

 the air, from its dryness, favors rapid evaporation. They 

 Are, in fact, self-acting valves which protect the plant from 

 tco sudden and rapid loss of water. 



Access of Air to the Interior of the Plant, Not only 

 does the leaf allow the escape of vapor of water, but it 

 admits of the entrance and exit of gaseous bodies. 



