WAX-SEAL METHOD FOR DETERMINING WILTING COEFFICIENT. 23 



than the same plants grown in a dry atmosphere. There are, then, 

 three ways in which a plant may respond to a dry atmosphere, i. e., 

 by increasing root area, by decreasing leaf area, and by decreasing 

 transpiration capacity per unit area of leaf surface. In addition 

 to the responses noted above, we have found a decided reduction 

 in transpiration immediately preceding and during the period of 

 wilting. These modifications combine to minimize the effect of a 

 high saturation deficit on the wilting coefficient of a soil when plants 

 are grown from the outset under the conditions under which they 

 finally wilt. Although the assumption has been made x that the 

 wilting point will be proportionately higher in an arid than in a 

 humid region, our results both in the laboratory and in the field (see 

 p. 63) indicate that this is not the case. 



Table VIII. — Comparison of the wilting coefficient for Kubanka wheat in a dry green- 

 house and in a glass damp chamber. 



In dry greenhouse. 



In damp chamber. 



Observation. 



Coeffi- 

 cient. 



Observation. 



Coeffi- 

 cient. 



Observation. 



Coeffi- 

 cient. 



Observation. 



Coeffi- 

 cient. 



1 



8.9 



9.0 



10.0 



10.7 



100 



10.2 



10.2 



13.0 



10.0 



9.1 



8.2 



9.0 



9.2 



9.1 



9.8 



9.0 



10.5 



9.7 



9.8 



9.6 



11.1 



8.9 



23 



9.7 

 9.5 

 9.4 

 9.6 

 9.7 

 9.6 

 8.9 

 9.6 

 9.4 

 9.9 



10.7 



10.2 

 8.8 

 8.1 

 8.4 

 9.3 



10.1 

 9.0 

 9.3 

 9.2 

 9.6 



11.4 



45 



9.0 

 9.4 

 11.4 

 11.7 

 10.2 

 8.8 

 9.3 

 9.1 



1 



9.6 



2 . 



24 



46. 



2 



9.1 



3 



25 



47. . 



3 



9.2 



4 



26 



48. '. 



4 



9.6 



5 



27 



49 



5 



11.0 



6 



2S 



50 



6 . 



10 7 



7 



29 



51 



7 



8.4 



8 



30 



52 



8 



11.1 



9 



31 





9 



9.9 



10 



32 



10 



11.0 



U 



33 



11 



8 5 



12 



34 



12 



7.8 



13 



35 



13 



9.9 



14 



36 



14 



9.0 



15 



37 



15 



8.6 



16 



38 



16 



9.8 



17 



39... 







18 . 



40 



9.6 



± .072 



± .52 



9.6 



19 



41 



Probable error 

 of mean 



Probable error 

 of single ob- 

 servation 



Probable error 

 of mean 



Probable error 

 of single ob- 

 servation 





20 



42 



± .17 



21 



43 





22 



44 









± .68 



LIGHT INTENSITY. 



The observations given in Table IX were made to determine the 

 effect of light intensity on the wilting coefficient. The shade used 

 was a single thickness of cheesecloth, which reduced the intensity of 

 direct sunlight to about one-half normal. It will be seen from this 

 table that the two series of observations are in substantial agreement, 

 the mean difference of the two series being only 0.2 ±0.11 per cent. 

 These results indicate that a considerable change in the light intensity 

 has no marked effect upon the value of the wilting coefficient. 



i Free, E. E. Studies in Soil Physics. Plant World, vol. 14, 1911, pp. 116-118. 



Livingston, B. E. Relation of Soil Moisture to Desert Vegetation. Botanical Gazette, vol. 50, 1910, 

 p. 251. 



230 



