SOIL CONDITIONS AND WATER SUPPLY 55 



The plant draws water from the soil after it has wilted, and 

 indeed after it has died as the result of desiccation. A series 

 of determinations on wheat by Briggs and Shantz (1912a) 

 showed that the amount of water in the soil on the death of 

 the plant was 15 per cent, less than at wilting, and that four 

 months later the dead plant had reduced the soil moisture 

 by a further 20 per cent. A vivid illustration of the fact that 

 wilting follows inadequacy rather than failure of supply is 

 given by soft-leaved garden plants, which sometimes flag on 

 a hot summer day and recover their turgor in the cooler 

 and more humid night hours, although no addition has been 

 made to the reserve of water in the soil. For these reasons 

 terms like " available " or " non-available " water are not 

 happy. The term " growth " water is better, for when 

 wilting sets in the turgor essential for active growth is lost, 

 and, while we know little of the relation of loss of turgor 

 to other cell activities, we know that photosynthesis is 

 stopped. Wilting is therefore an important physiological 

 condition. It has been claimed, further, that permanent 

 wilting is accompanied by plasmolysis of the root hairs, 

 and by a break in the column of water in the conducting 

 vessels (Bakke, 1918) ; but this is not quite certain. The 

 term " wilting coefficient " introduced by Briggs, and now 

 in general use to denote the percentage of moisture in the 

 soil when wilting takes place, is the most expressive and 

 accurate yet proposed. 



Wilting is a result of a failure of water income to cover 

 water outgo. If we analyse this statement we see that the 

 processes involved are : (a) the rate of transpiration, 

 depending on various atmospheric conditions, and also on 

 the state of the plant ; (b) the rate of conduction, depending 

 on the structural features of the species ; (c) the rate of 

 absorption, depending on the suction force of the root hairs 

 and the supply from the soil, which in its turn depends on 

 (d) the amount of moisture present ; (e) the forces with 

 which it is held; and (/) the rate at which it moves. 

 The last two conditions are at least partly functions of (d). 

 Wilting is therefore a resultant of several factors. 



