152 
Journal of Agricultural Research 
VtA. XXIV, Na a 
With this confirmatory evidence, no hesitancy need be felt in placing 
these four species in the following approximate relationships: 
Approxi- 
Approxi¬ 
mate mean 
mate mean 
ratio of 
ratio of 
wilting co¬ 
wilting co¬ 
efficients to 
efficients to 
Species. 
moisture 
Species. 
moisture 
equiva¬ 
equiva¬ 
lents on 
lents on 
common 
common 
basis of soil 
basis of soil 
qualities. 
qualities. 
Lodgepole pine.. 
0*35 
Engelmann spruce... .. 
0.32 
Yellow pine. 
•33 
Douglas fir. 
•31 
Certainly, from all the evidence available, the differences between the 
species are not any greater than here indicated. From all that we have 
found, it would probably be fair to say that in actual ability to stand 
drought, at least for the conditions existing in these pan tests, there is no 
essential difference between yellow pine, Engelmann spruce, and Douglas 
fir, the greater frailty and slow rooting of the spruce as compared with 
fir or pine being balanced by an actually stronger affinity of the spruce for 
any water within reach of its roots. On the other hand, lodgepole seems 
to stand out both as frail and slow-rooting, and with no compensating 
development of high sap density or osmotic pressure, so that it does 
succumb to drought much sooner than the others. In one test only, 
limber pine and bristlecone pine have shown themselves in practically 
the same class as yellow pine. 
RESISTANCE TO EXCESSIVE HEAT 
The sap densities observed in seedlings, and the relative rates of tran¬ 
spiration as apparently affected thereby, gave rise to the suggestion that 
there might be a specific difference in heat requirements based on this 
same set of internal conditions. While, on the one hand, the freely 
transpiring species of low sap density would seem to require a warm 
environment to counteract the cooling effect of this transpiration, on the 
other hand, the species of high sap density, which also seem to function 
more fully than others without full direct sunlight, appear to be always 
in danger of becoming overheated because, for some physical reason not 
fully explained, the heat absorbed is not so fully utilized in evaporation. 
One thing which the close observation of seedlings in the wilting tests 
has made very plain is that at an early age all seedlings are very sus¬ 
ceptible to injury just where the stems are in contact with the surface 
soil. At times it has seemed as though moisture absorbed by the roots 
might be extracted from the stems at this point, so blanched and shrunken 
do they become as soon as the surface of the soil becomes dry. On 
the other hand, it is perfectly evident that, as soon as the surface soil 
ceases to possess moisture to keep its temperature down, it may in sun¬ 
light easily become by far the warmest part of the environment. The 
measurement of the temperature at the warmest point is exceedingly 
difficult, but the showing of thermometers more or less submerged 
indicates that the soil surface not infrequently attains a temperature 
