Apt. X4,1933 
Physiological Requirements of Rocky Mountain Trees 145 
of protection. The high density of the last spruce in the test, on May 18, 
is difficult to explain, in view of the exposure on a southwesterly bank 
and the apparent abundance of moisture. This may be due, however, 
both to the full exposure of the tree to light and to a possible high degree 
of nonavailability of the moisture as a result of acidity and lack of 
aeration. 
^ Suminarizing, it is evident that the sap density of any species or indi¬ 
vidual is not a stable quality but varies according to the amount of 
photosynthesis which is permitted and according to current conditions 
affecting water supply and transpiration. Nevertheless, there is found 
in these data no reason for changing the original conclusion that, given 
equal opportunities and exposures, the sap density of spruce will be 
higher than that of any of the other species; that spruce will tolerate a 
very great loss of moisture, and a resultant high sap density in the 
needles, without injury; and that it can, therefore, be said that spruce 
is not only better equipped to resist transpiration, other things being 
equal, but that the ability to resist transpiration and its possibly injurious 
effects makes spruce in reality the least moisture-demanding of all of 
the Central Rocky Mountain trees. 
These conclusions, however tentative, must at least develop a wariness 
to accept average or temporary moisture conditions of the site as prima 
facie evidence of the relative moisture requirements of the species occu¬ 
pying it. 
On the other hand, we have as yet no evidence that spruce is more 
drought-resistant than other species or that the sap density and the speci¬ 
fic qualities that affect it react upon distribution through the water re¬ 
quirements. All that we have so far been able to show is that low sap 
density permits a species to occupy warm sites where the exposure is 
very great most of the time, while high sap density appears to hold the 
species to cool sites, where the winter drought may be severe, especially 
at high altitudes. 
WII.TING COEl^FlcmNTS FOR DIFFKRISNT SOII^ AND SPl^CmS 
If it could be shown that one species is capable of extracting the mois¬ 
ture of the soil to a lower point than other species before wilting or other 
injury to the plant was apparent, this would constitute direct evidence 
that the first species not only was less likely to experience fatal drought 
conditions but also was capable of sustaining higher internal osmotic 
pressures without injury to the protoplasm. When wilting occurs, if 
the condition has been approached gradually, it may be assumed that the 
osmotic pressure in the plant is essentially the same as in the soil, and the 
latter, of course, increases as the moisture content decreases and the con¬ 
centration of the soil solution increases. 
It has already been indicated in connection with transpiration in 1920 
that spruce appeared to be able, under all conditions, to obtain the 
water required for free transpiration more nearly than any of the pines 
or Douglas fiir. A similar test with the moisture gradually reduced to 
complete nonavailability would, perhaps, be preferable to wilting tests, 
which must be conducted with seedlings in order that the end-point 
may be observed ocularly. It is fairly evident that the seedlings may not 
show development of the internal characters which are important in this 
connection to the same extent as older trees. However, it can not be 
gaipsaid that it is the seedlings which are subjected to the greatest dan- 
■ 80616 — 23—4 
