140 BITLLETIX 1059, U. S. DEPARTMENT OE 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 
jDressure 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 ;- G 
h 
a formula which promises to be especially enlightening in studying 
the j)henomena 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. 
