RESEARCH METHODS IN STUDY OF FOREST ENVIRONMENT. 71 
Problems and Some Definitions. 
Such being the general situation, it is evident that the ecologist 
has a number of related problems to solve before the measurement 
of moisture has much meaning. In the following paragraphs cer- 
tain terms have been introduced which will have a rather definite 
usage in the later discussion. 
1. The total moisture must be obtained as the basis for all ex- 
pressions of current conditions in the soil, unless they are measured 
directly in terms which will give osmotic pressure. 
2. The nonavailable moisture must be measured with reference to 
the plant or plants concerned, either directly by wilting tests, or 
through some established relationship between the wilting coefficient 
on the one hand, and the antiosmotic pressure (P') the capillary mois- 
ture, the moisture equivalent, or the hygroscopic coefficient, on the 
other. 
3. The available moisture may be expressed as the difference be- 
tween the total and the nonavailable moisture. Such an expression 
may have some direct ecological significance in indicating the prob- 
able duration of the moisture supply and the life tenure of the plants. 
4. The availability of the moisture is seen in the general relation- 
ship between the available moisture and the total moisture, and may 
be expressed by a ratio such as 3 : 4, or by a decimal such as 0.75, on 
an unattainable scale of unity. 
5. The coefficient of availability is a more exact expression of the 
relation between the osmotic pressure of the plant (P) and the anti- 
osmotic pressure of the soil water (P') 5 and is a measure of the possi- 
ble rate of intake. Thus, if the soil has a freezing-point depression 
of 0.5°. and the plant of 1.5°, the respective osmotic pressures are 
P'z=6.025 and P= 18.04 atmospheres, and the possible rate of intake 
is indicated by the difference, which is approximately 12.02 atmos- 
pheres. It is perfectly evident, however, that the osmotic pressure 
in the root tips may be very little greater than the osmotic pressure 
of the soil, while there may be a very great increase in passing from 
the roots to the leaf tips where water is being lost most rapidly. As 
this is also the most convenient point for measuring the osmotic pres- 
sure in the plant, and such can here be accomplished without seri- 
ously disturbing the plant, it is suggested that the osmotic pressure 
at the leaf tips should be the basis for expressing the plant condition. 
In this event, the actual availability of the water is obviously affected 
by distance, or the mean osmotic gradient from the soil to the leaf 
tips. The coefficient of availability (A A) must therefore be ex- 
P—P' 
pressed by — j — ; , in which L is the distance in centimeters from the 
root tip, or point of measuring the soil condition, to the leaf tip. 
