140 BULLETIN 1059, U. S. DEPARTMENT OF AGRICULTURE. 



should be understood that these are arbitrary terms, and that they 

 only express relatively the conditions under which a given plant in a 

 given soil may be obtaining its water from time to time, perhaps a 

 little more clearly than these conditions can be expressed through 

 the percentage of available water. 



12. Finally, the plan of expressing the relation between the plant 

 and its moisture supply currently and accurately through the osmotic 

 pressure of each has been conceived. The difference in osmotic pres- 

 sure in favor of the plant expresses the degree of control of the 

 plant over its water supply; but, since the highest osmotic pressure 

 in the plant is likely to be attained at that point which is farthest 

 from the roots, where also the danger is greatest, it is evident that 

 in considering the availability of water to this point there must be 

 considered the distance through which the differential pressure must 

 operate; or, in other words, the osmotic gradient, say, per centimeter 

 of stem tissue, etc. This gradient will also be affected by gravity. 

 As the coefficient of availability, therefore, a term has been proposed 

 which brings these elements into their proper relations with definite 

 values. Thus, 



AA= p - p :- a 



h 

 a formula which promises to be especially enlightening in studying 

 the phenomena of growth in older trees, as they compete with one 

 another and reach their limits of height for a given site. The value 

 of AA is seen to fluctuate with each change caused by water loss or 

 accretion in the region where P is determined, as well as with gradual 

 changes in the conditions of the soil moisture. 



13. The discussion has included other aspects of the soil, which, 

 with the possible exception of nutrition, it is believed should be 

 considered only as indicator aspects ; that is, these aspects will only 

 serve to explain the phenomena of soil moisture. They include al- 

 kalinity or acidity, humus content, composition (as indicated by the 

 sizes of the soil particles), and the capillary transporting power of 

 the soil. 



14. In the study of seedlings during their most critical periods of 

 establishment— this being the period when the character of the plant 

 society is most largely determined — it is believed that the percentage 

 of available moisture within' reach of the usually short roots is of 

 primary importance. To determine this with any accuracy will be 

 found difficult on account of the usually heterogeneous character of 

 the soil layer that will be involved. The proper study requires : 



(a) Determination of root depth at each examination. It is 

 through this determination that a distinction between species may 

 be made. 



