22 BOTANICAL GAZETTE : [JANUARY 
described in a previous paper.? The plants are grown in small 
glass pots, evaporation from the soil surface being prevented by 
means of a wax seal. The conditions are maintained as nearly 
uniform as possible until the plants wilt permanently. Special 
care is taken to secure uniformity in the texture and water content 
of the soil mass before filling the pots. Sudden fluctuations in 
soil temperature are avoided by keeping the pots in a water bath 
during the growth of the plants. When these precautions are 
observed, the physiological measurement of the wilting coefficient 
is as accurate as the physical methods of measuring the moisture 
retentiveness of a soil. It is shown in the paper already referred 
to that the probable error of the mean wilting coefficient for 13 or 
more determinations is only about 0.005 of the actual determina- 
tions in the case of loam and clay soils. For single determinations 
the probable error is about 0.02 of the mean value. In the case 
of sands, the corresponding probable error is about twice as great 
as in the loam and clay soils. 
INDIRECT DETERMINATION OF THE WILTING COEFFICIENT 
In all plant investigations in which the water supply may become 
a limiting factor, it is necessary to determine from time to time the 
amount of moisture in the soil available for plant growth. If we 
make the specific assumption that growth cannot take place unless 
the water content of the soil is equal to or exceeds the wilting 
coefficient, then the percentage of soil moisture available for growth 
at any time is represented by the actual moisture content minus 
the wilting coefficient. If the actual water content is less than the 
wilting coefficient, then the percentage of available water is nega- 
tive, that is, water to this amount must be added to the soil before 
any growth can take place. 
The percentage of moisture in the soil at the wilting point 
varies greatly in different types of soil. This appears to have 
been established first by Sacus,3 and has been further investigated 
2 Briccs, L. J., and SHANtz, H. L., A wax seal seh for determining the lower 
limit of available soil moisture. Bor. Gaz. 51:2 g- 1911; also The wilting 
coefficient for different and its indirect Seta iia U.S. Dept. Agric., 
Bur. Pl. Ind., Bull. 230, 1911 
3 Sacus, J.,{ Bericht iiber die physiologische Thitigkeit an der Versuchstation in 
Tharandt. Landw. Versuchs-Stat. 1:235. 18 
