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 nonav ail able 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') , 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'=6.025 and /*= 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 

 tli is is also the most convenient point for measuring the osmotic pres- 

 Biire in fche 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. Tin- coefficient of a/voMability (AA) must therefore be ex- 



I'—P' 

 pressed by r . m which I-* is the distance in centimeters from the 



root lip. or points of measuring the soil condition, to the leaf tip. 



