THE RELATION OF EVAPORATION TO THE WATER 



CONTENT OF THE SOIL AT THE 



TIME OF WILTING 



WILLIAM H. BROWN 

 Bureau of Science, Manila, P.I. 



It is well known that after a rooted plant has wilted and died 

 as a result of drought, the soil still contains an appreciable quan- 

 tity of water. This so-called ' 'non-available" water content is 

 known to vary with different soils and has been thought to vary 

 also with different species of plants, but has usually been treated 

 as constant for a given soil and species. 



The wilting of a plant, as the soil becomes dryer, does not 

 mean that the plant has ceased to obtain water from the soil 

 but simply that the foliage is not obtaining it fast enough to 

 replace that lost through transpiration. As the soil dries the 

 films around its component particles decrease in thickness and 

 the attraction of the particles for water apparently increases. 

 Consequently as the soil becomes dryer it is increasingly more 

 difficult for the plant to obtain water, and the latter passes into 

 the plant more and more slowly. This continues until the plant 

 cannot obtain moisture fast enough to replace that lost through 

 transpiration, after which it wilts and finally dies. If, when a 

 plant is about to wilt owing to the dryness of the soil, the rate 

 of w r ater loss were to be greatly checked, the plant should con- 

 tinue to be turgid until the soil has become still dryer, in other 

 words, the lower limit of "available" water in the soil should 

 vary with different rates of water loss through transpiration. 

 Since transpiration is due to the ability of the air to remove 

 water from the leaves, the soil moisture content at which wilt- 

 ing occurs should depend on the evaporating power of the air. 



The effect of stomatal closure at the time of wilting, in reduc- 

 ing the transpiration rate, should npt affect this conclusion. 



121 



THE PLANT WORLD, VOL. 15, NO. 6, JUNE, 1912 



