24 THE UNIVERSITY SCIENCE BULLETIN. 



are also due Prof. W. C. Stevens for suggestions and criticisms 

 in the preparation of this paper. 



HISTORICAL. 



Although Sachs (7) recognized a wide range, from 1.5 per 

 cent in coarse sand to 12.3 per cent in a mixture of sand and 

 humus, in the moisture content of various soils when plants 

 wilted, he made his tests with a single plant species (the 

 tobacco), drew his conclusions, and then dropped this line of 

 investigation. Few have taken it up since. Hedgecock (4) 

 found that entire turgid plants of the same species had, at any 

 given age, approximately the same water content, regardless 

 of the differences in the soil or in the conditions under which 

 they were grown. On the contrary, the water content of plants 

 beginning to wilt varies with the soil, being always greater in 

 clay, loess, and saline soils than in loam, humus, or sand. He 

 also found that xerophytes could remove more water from the 

 soil than could mesophytes or hydrophytes; the former re- 

 moving all but 3 per cent, while the second named left in the 

 same soil under the same serial conditions at least 5 per cent. 

 Clements (3), independently, arrived at similar conclusions. 

 These were the chief contributions until Briggs and Shantz 

 (1) brought out their work on the "wilting coefficient." They 

 proposed the term and defined it as the percentage of water 

 (based upon the dry weight of the soil) remaining in this 

 when wilting had progressed to such an extent that recovery 

 by the plant was impossible even in an approximately satur- 

 ated atmosphere, without the previous addition of water to 

 the soil. In working out their results they maintained prac- 

 tically uniform conditions; their greenhouse had an average 

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

 maintained at 85 per cent. Such changes as did occur in these 

 factors were slight and gradual. A constant temperature for 

 the soils being examined was maintained by a specially-devised 

 water bath in which the containers were set. About twenty 

 different soils were examined, differing widely in all charac- 

 ters, and giving results ranging from 1 per cent in coarse dune 

 sand to over 30 per cent in the heaviest types of clay. For 

 plants, over a hundred^ species and varieties were tried out, so 

 selected as to give a range from extreme xerophytes to hydro- 

 phytes. In general, the amount of water remaining in any one 

 of these soils when the plants growing in it had fully wilted, 



