BOTANICAL GAZETTE 



[JULY 



For a comparative study of the moisture relations of seeds and 

 soils at the wilting coefficient of the soil, a series of soil samples 

 was obtained from Washington, D.C. The necessary data regard- 

 ing these soils are given in table II. 



* The hygroscopic moisture was determined at the time of use in an or dinary dry oven. The other 

 figures were [urnished by Dr. Lyman J. Briggs, of Washington, D.C. 



Methods. — While the internal forces of Xanthium seeds have 

 been approximated by osmotic means, many seeds lack semi- 

 permeable coats. For such seeds a vapor pressure method has 

 been used which gives results which are in a way comparable to 

 the osmotic measurements. It consists essentially in measuring the 

 vapor pressure equilibrium of the air-dry seeds over sulphuric acid 

 of varying strength, and calculating the internal pressure of the 

 seed from the vapor pressure of the solution over which it was 

 found to be in equilibrium. Owing to our slight knowledge of the 

 concentrated solutions and of the exact relations of colloids to 

 water vapor, the calculations can give only a rough estimate of 

 the internal forces of the seeds, but they are near enough to the 

 osmotic determinations to be of great interest. 



The sulphuric acid series was chosen with some reference to 

 the Landolt-Bornstein tables to facilitate calculation. Begin- 

 ning with water, the series included 16, 26.5, 35, 39, 50, 54, 57.5, 

 66, 73, 84.5, and 96-99 per cent H 2 S0 4 . These fluids were placed 

 in tightly sealed, small, wide-mouthed bottles. The seeds to be 



