156 
Journal of Agricultural Research 
Vol. XXIV, Na a 
that of the Rocky Mountain species one is distinctly more drought-re¬ 
sistant than the others. The mositure relations apparently vary much 
with the other environmental conditions, and it is perhaps the most 
important feature of this paper that a somewhat logical relationship has 
been shown to exist between moisture requirements and other require¬ 
ments of each species. 
We may, then, bnefly outline the theory and at the same time observe 
how closely it applies to the behavior of each species under each of the 
situations that has been presented. We may take as our starting point 
the relative “tolerance to shade” of the several species, because this is a 
diaracter which always has been quite closely observed by foresters and 
in which, empirically, rather definite lines have been drawn. 
Briefly the physiological requirements appear to be related on this 
basis: 
1. The species of greatest shade tolerance or greatest ability to make 
effective use of sunlight in photosynthesis will possess, other conditions 
remaining equal, after a period permitting accumulation the greatest 
amount of soluble carbohydrates in the leaves. In this fundamental 
respect we shall adhere, at least tentatively, to the classification in¬ 
dicated by the December, 1917, sap densities, as shown in Table XIII, 
placing spruce at the head of the list of our indigenous species, followed 
by Douglas fir, lodgepole, bristlecone, yellow, and limber pines. 
2. The presence of considerable quantities of carbohydrates augment¬ 
ing other solutes creates a dense sap, or solution, which does not evaporate 
so readily as a dilute solution. Because of the osmotic pressure exerted 
by a dense solution, there should at the same time be greater ability to 
extract water from the soil, though there is no evidence that at the end 
of the struggle o;ne species tolerates appreciably greater drought than 
lie others. 
^ 3. The presence of these solutes in large quantities is also, naturally, 
conducive to a high growth rate. 
4. By restricting evaporation, the soluble carbohydrates may increase 
the net amount of the light energy available for photosynthesis, so that, 
whatever the original quality which made the plant effective, this quality 
is augmented by its own results. 
. 5. By restricting the use of heat in evaporation, however, the dense 
cell sap may not only reduce the relative heat or light requirement of the 
species but may subject it to the danger of superheating. Of all the 
possible influences of the specific differences which give rise to the cell- 
sap differences, it is believed this is the most important ecologically and 
thfi most potent in its effect on the distribution of the species. If we 
assume distribution to be controlled primarily by this physiological 
factor, it becomes fairly simple to see how adjustments have been made 
to meet other conditions of the environment, principally in the form of 
structural adaptations, which differentiate the species beyond that 
difference which may arise from photosynthetic efficiency, and which 
may to a certain extent compensate for the physiological deficiencies. 
If we accept the heat hypothesis as fundamental, we mean that each 
species ydll be limited in its distribution rather sharply by the maximum 
tOTperatfUres which it can tolerate (probably in the early seedling stage) 
mid also limited in its growth by its minimum requirements, so that at a 
certain low temperature it is unable to compete with more highly devel¬ 
oped species apd hence loses its dominance in the forest. In the moun¬ 
tain forests, therefore, we should expect to find the six species zonated 
according to temperatures, in the order named just above. 
