252 PLANT PHYSIOLOGY 



with photosynthesis. Light tends to cause the stomata to open 

 so that food may be manufactured (in a fashion described below), 

 and when the stomata are in this open position more water escapes 

 from the leaf. In addition to this direct light effect, there is an 

 equally important heat effect, because the heat energy required 

 to evaporate the water comes very largely from the light. Brown 

 and Escombe estimated (1900) that a sunflower leaf on a bright 

 day transpired 275 c. c. per square meter of leaf surface per hour, 

 which requires 166,800 g. cal. of heat. Since the leaf area receives 

 during this time a total of about 600,000 cal., it follows that 

 nearly 30% of the total amount of energy received by the leaf 

 is used to evaporate the water from it. Transeau made some calcu- 

 lations on corn and concluded that here about 45% of the total 

 light energy was used up in transpiration. 



The Wilting Coefficient.— The amount of water in the soil is, 

 of course, an external factor of prime importance in determining 

 transpiration. Maximum transpiration occurs neither when the 

 soil is too dry nor when it is too moist. The wilting coefficient is 

 the percentage of water in the soil (based upon dry weight) when 

 permanent wilting takes place, i. e., when the plant is not revived 

 by placing in a saturated atmosphere. When the wilting point is 

 reached, the plant can no longer grow, although the water in the 

 soil may be used to a certain extent to keep the plant from dying 

 by completely drying out. The wilting coefficient is greater than 

 the hygroscopic coefficient, i. e., than the percentage of hygroscopic 

 water in the soil, which means that a plant can never absorb all the 

 capillary water of the soil. 



The wilting coefficient has been found by Briggs and Shantz 

 (1912) to vary with the soil rather than with the plant, i. e., the 

 wilting coefficient of a soil is approximately the same no matter 

 what kind of plant may be growing on it, although this applies 

 actually to plants of the same general water needs. Corn cannot 

 draw more water from a soil, in other words, than barley or peas 

 as the accompanying table shows, but if mesophytes (corn) and 

 xerophytes (cacti) are compared, a different story might be told. 



Wilting coefficient 

 Type of soil ( per ccnt ) 



Coarse sand 0.9 



Fine sand 2.6 



Sandy loam 8.3 



Loam 11-0 



Clay loam 13.6 



