WAX-SEAL METHOD FOR DETERMINING WILTING COEFFICIENT. 15 



during the night. The wilting of a plant in the hot part of the day is, 

 therefore, not conclusive evidence that moisture is not available. A 

 wilted condition in the early morning is considered the best proof that 

 the moisture content has been reduced to the true wilting point. 

 Check determinations were repeatedly made by placing the pots con- 

 taining wilted plants under a bell jar in nearly saturated air. These 

 plants were unable to recover their turgidity. Since the wilting point 

 is influenced to some extent by sudden changes in the temperature 

 and humidity of the air of the plant house, these conditions should 

 be determined and kept as uniform as possible during the growth of 

 the plants. 



The degree of accuracy which may reasonably be expected with the 

 wax-seal method when suitable precautions are taken is shown by the 

 series of determinations presented in Table I, which gives the wilting 

 coefficient for Kubanka wheat in each of several pots for three types 

 of soil. The plants were grown in an ordinary plant house, where the 

 average temperature was about 70° F. and the relative humidity 

 about 85 per cent. 



Table I. — Individual pot measurements of the wilting coefficient of three types of soil for 



Kubanka wheat. 





Wilting coefficient of— 



Plant. 



Fine 

 sand. 



Fine 

 sandy- 

 loam. 



Clay 

 loam. 



Kubanka wheat 



2.6 

 2.1 

 2.7 

 2.8 

 2.6 

 2.7 

 2.7 

 2.6 

 2.5 

 2.6 

 2.4 

 2.7 





16.9 

 16.8 

 16.4 

 16.2 

 15.5 

 15.7 

 17.3 

 16.7 

 15.6 

 16.0 

 16.0 

 16.2 



Do 





Do 





Do 



9.7 

 9.3 

 9.9 

 10.1 

 9.7 

 9.4 

 9.6 

 9.3 

 9.4 



Do 



Do 



Do •... 



Do 



Do 



Do 



Do 



Do 





Plant. 



Kubanka wheat 



Do 



Do 



Do 



Mean 



Probable er- 

 ror of mean. 



Probable er- 

 ror of sin- 

 gle observa- 

 tion „ 



Wilting coefficient of- 



Fine 

 sand. 



2.6 

 2.5 

 2.7 

 2.7 



2.59 



± .03 



± -11 



Fine 

 sandy 

 loam. 



9.66 



± .05 



± .18 



Clay 

 loam. 



16/3 

 16.3 

 16.7 

 16.3 



16.3 

 ± .09 



± .34 



Table I shows the individual determinations in each series, 

 together with the arithmetical mean, the probable error of the 

 mean, and the probable error of a single observation. The term 

 1 'probable error" is used in its usual mathematical sense, that is to 

 say, in the case of the series of determinations in sand the chances 

 are even that if the series were repeated the mean would lie between 

 the values 2.56 and 2.62; and if the experiment were repeated with 

 a single pot, the chances are even that the wilting coefficient for 

 this pot would fall between 2.48 and 2.70. The probable error of the 

 mean is about one two-hundredths, and that of a single observation 

 one-fiftieth of the actual wilting coefficient of the loam and clay 

 soils, while the corresponding probable errors in the case of sand are 

 about twice as great. It appears, therefore, that while there is 



230 



