THE LEAF AND ITS FUNCTIONS 75 



stream of water is thus passing through the plant body, entering 

 at the root-hairs and leaving through the stomata. The total 

 quantity of this water often amounts to several hundred times as 

 much as the final dry weight of the plant itself (Fig. 39). 



Fig. 38. — Tran.spiration from the leaf-blade. Cress-section of a blade 

 including a vein. Arrows indicate passage of water from the vein, through 

 the mesophyll cells, into the air-spaces, and out through the stomata. Solid 

 arrows, liquid water; dotted arrows, water-vapor. 



The Rate of Transpiration. — The rate of water-loss varies 

 greatly according to the kind of plant, the soil conditions, the 

 season of the year, the time of day, and various environmental 

 factors. As a general rule, we find that the rate tends to increase 

 under conditions which favor increased evaporation, such as high 

 temperature, bright light, rapid air movement and low humidit}"; 

 and to decrease under environments of the opposite character. 



Transpiration is by no means controlled entirely by factors 

 which influence evaporation alone. The rate of water-loss from 

 a given leaf-surface and from an equal area of free water do not 

 rise and fall exactly together, the transpiration from the living 

 leaf sometimes being relatively higher and sometimes relatively 

 lower. There must, therefore, be factors in the leaf itself (as 

 opposed to those in the external environment) which tend to 

 accelerate or to retard transpiration. The most important of 

 these is doubtless the opening and closing of the stomata, which 

 we have already discussed. Changes in the concentration of the 

 sap in the mesophyll cells also probably determine to some extent 

 the rate at which water evaporates from their surfaces. 



The actual amount of water transpired during the growing 

 season may be large or small, depending on the size of the plant, 



