REVIEWS 113 



example, the moisture content of the soil varies from 4.65 per cent 

 of absolute weight in series 4, to 6.66 per cent in series 1, and each 

 series has different percentages of soil moisture. At no point is the 

 exact wilting coefficient, 19.1 per cent given. This difficulty is, 

 however, of minor importance, and is unavoidable; for in moistening 

 a soil it is impossible to bring the moisture content to the exact point 

 desired. The inconvenience in using the figures can readily be over- 

 come by smoothing off the values with a curve. Of more interest is 

 the degree of correspondence between the values for the intake of 

 water by the seeds at the same moisture content of the soil. As 

 might be expected, the values do not exactly correspond. For ex- 

 ample, in series 1, a soil moisture of 19.80 per cent shows an intake 

 by the seeds of 46.54 per cent of their air-dry weight, while in series 

 2, with a slightly lower soil moisture, 19.34, the seeds take in 49.31 

 per cent of their air-dry weight. This divergence does not, however, 

 affect the validity of the conclusions. 



The tests prove beyond dispute that the intake of water by Xan- 

 thium seeds is approximately uniform for a given moisture content in 

 a given soil. Since the internal force of Xanthium seeds can be deter- 

 mined at practically any moisture content, this means that the water- 

 absorbing power of Xanthium seeds can be used as a measure of the 

 force exerted by soil of any moisture content in withholding water 

 from the roots of the plant. 



The results for the sand are just as interesting and striking as the 

 results for the heavy soil. Shull found from preliminary tests that 

 this sand had no "back-pull" until the moisture fell below 2 per cent. 

 This was to be expected since the moisture equivalent was 2.41 per 

 cent, and the wilting coefficient 1.3 per cent. The range between air-dry 

 and the wilting coef^cient was therefore restricted to less than one- 

 tenth of the range for the heavier soil. This necessitated the use of 

 very much liner divisions in the soil moisture content. For example, 

 a difference of only .016 per cent of soil moisture makes a difference 

 of 3.5 per cent in the water intake of the Xanthium seeds, and a differ- 

 ence of only .237 per cent of moisture in the soil makes a difference 

 of 12.17 per cent in the seed. This must have made the experimenta- 

 tion more difficult. It shows strikingly the difference which ex- 

 tremely small amounts of moisture make in the water-withholding 

 force of light soils below the wilting coefificient. The "back-pull" of 

 a light soil between its air-dry condition and its wilting coefficient 

 covers the same range of force as that exerted by a heavy soil, only 



