Current Literature. 255 



the point at which a part of the water transpired will be supplied 

 from that stored in the leaf tissues and a loss of turgor will result. 

 The wilted condition of the leaves is regarded as permanent when 

 they cannot recover in an approximately saturated atmosphere 

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

 having aerial water storage tissues or thick, heavy leaves this 

 procedure cannot be followed, since they have no well defined 

 wilting point. The investigators, however, were able to de- 

 termine the wilting point by an ingenious device delicately balanc- 

 ing a potted plant in a horizontal position. As the plant loses 

 water through transpiration, water will move from the soil to 

 replace the water lost from the plant and the soil end of the 

 balanced structure will become lighter and rise in consequence. 

 When the soil is no longer able to supply the water to the plant at 

 a rate sufficient to meet the transpiration requirements, the plant 

 itself will begin to lose water from its storage tissues, will become 

 lighter, and hence the plant and the balanced structure will begin 

 to move upward. The moisture content of the soil at this point 

 is the wilting coefficient. The oscillations of the apparatus may 

 be recorded by a pointer and scale. 



The usual view presented by text books on plant physiology and 

 plant ecology is that some plants are capable of reducing the 

 moisture content of a given soil to a lower point than others, in 

 other words that the non-available moisture varies according to 

 the kind of plant used a . an indicator. As the result of over 400 

 experiments on this point the authors conclude that the differences 

 exhibited on the wilting coefficient by different species of plants in 

 the same kind of soil is very small, the maximum values being only 

 about 10 per cent, greater than the minimum values. On the 

 other hand when the same species is grown in different kinds of 

 soil, the values of .the wilting coefficients may vary by 3,000 per 

 cent, in the case of extreme soil types. The results of the ex- 

 periments indicate that to a very great extent the variations in 

 the wilting coefficients for different plants are dependent upon the 

 degree to which the roots of the plants penetrate every portion of 

 the soil mass. 



The wilting point according to the authors, does not mark the 

 minimum limit of moisture that is available to the plant, for it is 

 shown that water is steadily removed from the soil by dying and 



