LOSS OF WATER 253 



Below are wilting coefficients for various plants in sandy loam: 



Plant Wilting coefficient 



Barley 6.3 



Corn 6.5 



Oats 5.9 



Peas 6.9 



Rye 5.9 



Sorghum 5.9 



Tomatoes 6.9 



Vetch 6.1 



Wheat 6.3 



As stated above, the wilting coefficient is always greater than 

 the hygroscopic coefficient. In fact, the results of Briggs and 

 Shantz have shown that the two coefficients are always related to 

 each other as 100 :68, that is, the wilting coefficient is 



hygroscopic coefficient 

 068 



The moisture holding capacity of the soil is the percentage re- 

 tained against gravity when free drainage is possible. The wilting 

 coefficient equals 



the moisture holding capacity — 21 



2J9 



Before wilting, plants may draw excessive amounts of water 

 from the fruits, resulting in the shriveling of the fruit (apples, 

 grapes, and Citrus) or causing the physiological disease known as 

 " drought spot " and " bitter pit." The fruit also may be shed early 

 and in an immature condition (fleshy fruits and cotton). 



Internal Factors. — By internal factors we mean those con- 

 nected with the plant and which do not lie outside it. One such 

 factor is the age. When the leaves and plant parts are young, 

 transpiration is greatest. Later, bs the cutin and suberin form, 

 the epidermal cells are not so permeable, which state connected 

 with the decrease in soil water as the season advances, results in a 

 gradual diminution in transpiration from spring to fall. This 

 may also be explained in part by the accumulation in the leaves of 

 salts, which tends to check transpiration. 



Transpiration may be retarded also by various regulatory de- 

 vices, the most important of which are the stomata. The stomatal 

 apparatus consists of epidermal cells called guard cells arranged 



