54 WILTING COEFFICIENT FOE DIFFERENT PLANTS. 



Another cause of low determinations is found in allowing plants to 

 stand too long after they hare reached a wilted condition. As has 

 been shown, plants continue to extract moisture from the soil even 

 after they have reached a wilted state, and this loss continues even 

 after the death of the plant. It is evident, therefore, that if a plant 

 is allowed to stand for any considerable period after it has once 

 reached a permanently wilted condition the determination of soil 

 moisture will be below the true wilting coefficient. This is a source of 

 considerable error, especially with plants which do not wilt readily. 

 An example of such plants is the olive, which, if one were to rely 

 entirely upon the outward signs, would reduce the moisture in the 

 soil far below the wilting coefficient. It is very easy to demonstrate 

 by means of the balancing method that in such cases the plant has 

 long since passed the point corresponding to the wilting coefficient, 

 i. e.. the point of permanent decrease of the water content of the 

 leaf cells. The determination at the time the leaves curl or drop 

 is not a true measure of the wilting coefficient, but of the moisture 

 content at some point between the wilting coefficient and the hygro- 

 scopic coefficient of the soil. It is therefore essential in making these 

 determinations that considerable care be exercised in avoiding the 

 excessive wilting of the plants used. 



A third source of error results from the distillation of water from 

 one portion of the soil mass to another. The pot standing in the open 

 becomes warm during the day. Dining the early evening, when the 

 air cools below the temperature of the soil, rapid distillation of water 

 takes place and the water is driven out of the warmer soil and con- 

 denses on the sides of the pot. which are cooler than the soil. This 

 distillation can be observed even in a comparatively dry soil, and plants 

 which are allowed to stand in pots exposed to this temperature 

 fluctuation develop an extensive surface root system. This distilla- 

 tion taking place in the ordinary greenhouse once or twice a day is 

 sufficient to maintain the plants in fairly good condition long after 

 the soil has been reduced below the wilting coefficient. In early 

 experiments this distillation was found to be a serious source of error. 

 In the later experiments this error has been avoided by means of the 

 constant-temperature bath. 



With the precautions which have been employed the errors due to 

 obtaining too low a wilting-coefncient determination are compara- 

 tively slight, and it has been possible to eliminate practically all of 

 the errors under this head. It seems likely, however, that the early 

 investigators who concluded that there were comparatively great 

 differences in the ability of different plants to remove moisture from 

 the soil were largely misled as a result of the errors which have been 

 noted above 



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