74 WILTING COEFFICIENT FOE DIFFERENT PLANTS. 



plants differ widely in this respect and that drought resistance is in 

 part due to the additional supply of water which is available to some 

 plants through the greater force which they exert upon the soil mois- 

 ture. The results of this investigation have led us to conclude that 

 the differences exhibited by plants in this respect are much less than 

 have heretofore been supposed and are so small as to be of little prac- 

 tical utility from the standpoint of drought resistance. As compared 

 with the great range in the wilting coefficient due to soil texture, the 

 small differences arising from the use of different species of plants in 

 determining the wilting coefficient become almost insignificant. 



The moisture content of the soil corresponding to the wilting of the 

 plant growing in a confined soil mass is usually spoken of as non- 

 available moisture. It has been found that plants are capable of 

 reducing the moisture content of the soil far below that correspond- 

 ing to the wilting point, and that a steady loss of moisture goes on 

 through the plant tissues even after the death of the plant. There- 

 fore the moisture content at the wilting point can not all be considered 

 as nonavailable, and the term wilting coefficient has been adopted 

 to indicate the moisture content of the soil at the time of the perma- 

 nent wilting of the plant. 



The wilting coefficient is then defined as the moisture content of 

 the soil (expressed as a percentage of the dry weight) at the time 

 when the leaves of the plant growing in that soil first undergo a per- 

 manent reduction in the moisture content as a result of a deficiency 

 in the soil-moisture supply. By a permanent reduction in the 

 moisture content of the leaves is meant a condition from winch 

 they can not recover their turgor in an approximately saturated 

 atmosphere without the addition of water to the soil. In the case 

 of most plants wilting accompanies the reduction of the water content 

 of the leaves and is the criterion used to determine the wilting 

 coefficient of a soil for that plant. The definition as stated is appli- 

 cable also to those plants which, owing to structural peculiarities, do 

 not give visible evidence of a reduction of the moisture content of 

 the leaves. The wilting coefficient for such plants is determined by 

 means of the balancing method. 



In making wilting-coefficient determinations the following pre- 

 cautions are necessary: 



(1) The soil used should be as uniform as possible. 



(2) The soil should be brought to a uniform water content before 

 being used. 



(3) All loss of water should be prevented except that due to the 

 transpiration of the plant. 



(4) All sudden fluctuations in temperature should be avoided. 



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