80 BULLETIN 1059, U. S. DEPARTMENT OE AGRICULTURE. 
adapted to radically different habitats, there may be, at least under 
certain conditions of wilting, radical differences in the coefficients. 
Another important phase of the matter is that certain soils may 
have a peculiar reaction on one species and not on others; as, for 
example, a highly acid or strongly limey soil. It is therefore the 
part of wisdom to test the nonavailable moisture of any soil by the 
use of at least the predominating or type species found on the 
soil, and of as many other species as possible. 
DIRECT DETERMINATION OF THE WILTING COEFFICIENT. 
The writers cited above have given such thorough consideration 
to this and the succeeding subjects that a complete discussion here 
appears almost useless. The treatment of forest soils and forest 
species, however, has brought out a number of new problems, so that 
it is almost impossible to overlook any phase of the question in 
this discussion. Constant comparison will be made with the treat- 
ment found desirable for field crops and related plants. 
It is well to bear in mind from the outset the point brought out 
by Briggs and Shantz that the wilting coefficient represents merely 
the moisture point at which wilting first occurs to such an extent that 
the plant does not recover if placed in a saturated atmosphere. The 
plant may actually draw considerable water from the soil after 
this, and might be theoretically conceived to pass moisture to the 
atmosphere until the soil and atmosphere were in vapor-pressure 
equilibrium. The wilting coefficient is. however, the practical ex- 
pression for nonavailable moisture. 
The fact must also be strongly emphasized that the point at Avhich 
wilting occurs must depend in a very large measure on the rate at 
which the plant is transpiring; or. in other words, on atmospheric 
conditions and sunlight. Therefore, as the soils approach dryness, 
the conditions should be maintained at a fairly definite standard. 
It will usually be feasible to prevent the occurrence of temperatures 
in excess of 70° F.. as well as sudden changes in temperature, and 
to exclude direct sunlight. It would also be desirable to control 
atmospheric moisture, though this is a very difficult thing to do in 
ordinary rooms. 
In the tests with seedlings of coniferous trees it has been found 
exceedingly difficult to determine when permanent wilting occurs. 
There is no doubt that seedlings of this kind have developed a power 
of resistance, or recovery, far in excess of that of most plants. This 
probably consists in an extremely low rate of transpiration when 
the moisture becomes deficient; but the difficulty of observation may 
be mainly ascribed to the fact that the stems, and to a lesser extent 
the leaves, become stiff and woody at a very early age, so that 
shriveling rather than collapse is the phenomenon that evidences 
