48 



WILTING COEFFICIENT FOR DIFFERENT PLANTS. 



a rate sufficient to meet the transpiration requirements. The plant 

 itself will then begin to lose water from its storage tissues and the 

 plant side of the system will consequently become lighter, with the 

 result that the plant will eventually move upward or in an opposite 

 direction to its former motion. The moisture content of the soil at 

 this point is the wilting coefficient. 



Between these two extreme conditions there occurs a brief period 

 during which the soil is able to supply some water to the plant, but 

 not in sufficient quantity to offset the amoimt lost when the trans- 

 piration is greatest. During this time the balanced system will 



Fig. 2. — Apparatus for determining the wilting coefficient for plants with water-storage tissues. The 

 glass pot containing the cactus is clamped into a brass ring which is supported on knife-edges and pro- 

 vided with suitable weights for adjusting the sensibility and restoring the balance. 



appear sluggish and uncertain in its movement, as if no changes in 

 the moisture content were taking place, although weighings made 

 upon the whole system will show that water is being lost. 1 



One form of apparatus which has been used in making these 

 determinations is shown in figure 2. It consists essentially of a 

 brass ring suspended by steel knife-edges which are so inserted that 

 the edges lie in the same straight line. The glass pot is held within 

 this ring and is further supported on two rods extending horizontally 



1 It is possible with a balanced system of this kind to distinguish between absorption and transpiration 

 by determining the loss in weight and the weight which must be added to the soil (or plant) centroid to 

 restore the balance of the system. 

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