230 BOTANICAL GAZETTE [MARCH 
considered to be an important factor in drought resistance, the 
additional supply of water thus made available to some plants 
being supposed to be sufficient to carry them through a dry period 
when other plants would succumb to drought. With this point of 
view in mind, the present writers have made an extensive series of 
determinations with a number-of plants, including native plants 
from semi-arid and arid regions, to determine the variation exhibited 
in their ability to reduce the moisture content of the soil before 
permanent wilting takes place. The results of these investigations 
have led us to conclude that the variation exhibited by different 
plants is much less than has heretofore been supposed, and that it 
is insignificant compared with the range in moisture retentiveness 
exhibited by different soils. 
The observations given in the following table embrace about 
1300 observations made with plants growing in 20 different types 
of soil. The actual wilting coefficients as observed range from less 
than 1 per cent in the case of the dune sands to 17 per cent for the 
clay loam. In order to reduce the results of these observations to 
a comparable basis, the relative wilting coefficient has been deter- 
mined by taking the ratio of each individual determination to the 
mean of all the determinations made with that soil. If the wilting 
coefficient for a particular plant is higher than the average, the 
ratio will be greater than unity. If, on the other hand, a plant is 
capable of reducing the moisture content below the point attained 
by other varieties before wilting occurs, then the ratio will be less 
than unity. In this way it is possible to combine all the observa- 
tions made upon different soils and to determine the extent to 
which any particular plant is able to reduce the soil moisture 
content below the point reached by other plants before wilting 
occurs. 
It will be seen from table I that the extreme values of the 
relative wilting coefficient for all plants upon which six or more 
determinations are made are represented by 0.92 for Japan rice 
and 1.13 for Colocasia. The latter plant has an extremely coarse 
root system, the fine fibrous roots so characteristic of the grasses 
being wholly absent. A plant with a root system of this kind does 
not reduce the water content of the soil uniformly. At the time 
