96 ILLINOIS ACADEMY OF SCIENCE 



laljoratory in sealed vessels and dried at 104 degrees, C. The 

 percentage of water was calculated in terms of the dry weight 

 of the soil. 



Since the mere knowledge of the water content of the 

 soil is of little value toward the understanding of the real 

 water conditions to which the plant is being subjected, it be- 

 comes necessary, to relate such facts to the plant directly in 

 some manner that they may be at all significant. On the 

 whole, the most satisfactory methods yet devised for getting 

 at this difficulty are probably those described by Briggs and 

 Shantz, (6) by which they were able to show a definite relation 

 between the plant and the water content of any given soil. 

 Their method of determining the wilting coefficient directly, 

 by the wax seal method, and indirectly by means of the centri- 

 fuge, etc., are now well known. HoAvever. it may not be 

 amiss to recall that in one of their indirect determinations of 

 the wilting coefficient they employed the following formula 

 which they found to hold, apparently, for all types of soils 

 investigated. (*) 



AMlting coefficient equals moisture equivalent divided by 

 1.84. The moisture equivalent here being that amount of 

 water which the soil has the power of holding against a given 

 high centrifugal force. The number 1.84 is a constant. 



Tn the autumn of 1912 a centrifuge of the type used by 

 the government workers in the soil investigations was in- 

 stalled in the Botanical laboratory. By means o^ this appar- 

 atus the moisture equivalents of the various soils brought in 

 from Chicaeo Lawn were determined. The force used was 

 always 1.000 gravities, acting during 30 minutes. 



By the above method the soils of the two stations were 

 found to have the following wilting coefficients: station 1, the, 

 7.5 cm. depth. 28 per cent and the 2.^ cm. depth. 20 per cent; 

 station 2. the 7.S cm. depth. 23 per cent and the 25 cm. depth, 

 19 per cent. 



Fig. 3 gives the graphs of the water content of the soils 

 from station 1 for the entire season, plotted with percentage 

 of water as ordinates and weekly intervals as abscissae. The 

 soil at the 7 .S cm. depth represented by graph a, and at 25 

 cm. by graph b. The horizontal dotted lines, c and d, mark 

 the wilting coefficients of these soils respectively, as calcul- 

 ated by the method described. The water content and the 

 wilting coefficients of the two soils of station 2 are represented 

 in fig. 4. exactly in the manner as are those of sta. 1, in Tig. 3. 



(*) For important contrary data see Caldwell, J. S. The re 

 lation of environmental conditions to the phenemonon of permanent 

 wilting in plants. Physiol. Researches 1:1-56, 1913. 



