892 
Journal of Agricultural Research 
Vol. XXVIII, No. 9 
pits. However, it is evident that non-nitrogenous materials are most commonly 
stored in the form of starch and frequently this is partially or wholly replaced 
during the winter by sugar or fats. A sudden increase in temperature during the 
winter or early spring causes a reformation of starch. Under favorable condi¬ 
tions this may occur more than once in the course of a single winter. 24 There is 
furthermore, a distinct advantage in the economy of the plant in producing, 
from the same nutrient material, substances of higher or lower osmotic concen¬ 
tration according to the character which metabolism assumes. 25 
The supply of reserve food is an important factor in the inception of early 
spring growth. The reserve food materials stored up in the autumn are probably 
largely utilized in leaf and also in blossom formation, when the latter precedes 
leaf formation. Growth does not begin simultaneously in the evergreen and 
deciduous trees of the Intermountain Region. It would appear reasonable to 
assume that diameter growth proper, as distinguished from any preliminary 
swelling of the tissues, may be delayed in the deciduous trees until the new 
leaves have developed and have become sufficiently active photosynthetically 
to supply the requirements of rapid cell formation. Evergreen conifers, on the 
other hand, have an adequate amount of living leaf tissue to supply the requisite 
materials for growth as soon as growing temperatures are reached in the spring. 
But, even in conifers, the stored food materials are principally, if not wholly, 
consumed in the incipient stages of the current growth while growth during 
the main part of the growing season is largely influenced by other environmental 
factors, notably precipitation, since this is the chief determinant of the moisture 
supply in this region (75, 76). 
Since winter sap densities are materially altered by transformations in the 
food reserves, any determinations which are made during the winter must be 
interpreted accordingly. For example, higher winter sap concentrations should 
not be interpreted as necessarily indicating greater drought resistance in one 
species than in another. Because of the material changes in the food reserves 
following the advent of cold weather it would appear inadvisable to use winter 
determinations alone in a correlation of sap density with environmental conditions. 
In the Intermountain Region, at least, the dry part of the growing season is 
the most critical from the standpoint of plant distribution and succession. Sap 
densities common to this period are therefore of greater significance than those 
of winter for correlation with the phenomena, which are largely dependent upon 
environmental conditions. 
SAP DENSITY IN RELATION TO FOREST DISTRIBUTION AND 
PLANT SUCCESSION 
The sap density of any single species shows variations corresponding to the 
degree of xerophytism of the forest types or associations in which that species 
is growing, especially when the determinations are made at or near the critically 
dry part of the growing season. Although a plant with low sap concentration 
can procure the necessary water from soil having a high moisture content, 
the total amount of available soil moisture is not always in itself the limiting 
factor in the struggle of a seedling to become established. Its success often 
depends principally on its ability to exert, through the concentration of the cell 
sap, an osmotic pressure sufficient to absorb water from the soil and at the same 
24 These observations have been confirmed by Mer (92 ). 
25 Pfeifer (102, p. 134-147) has pointed out that reserve food materials may be stored up in an insoluble 
form as starch, oil or proteid without causing any increase in the osmotic concentration of the cell sap, but 
that even the concentration of two molecules of a monosaccharide to one of a disaccharide may result in 
reducing the osmotic concentration by one-half. s 
