Apr, 14,1923 Physiological Requirements of Rocky Mountain Trees 143 
minations have also shown that with all of our conifers a sap density of 
I per cent is approximately equivalent to i atmosphere of osmotic pres¬ 
sure, this relation holding at least up to 20 per cent. Such tests have not, 
as yet, been sufficient to bring out any consistent differences in the saps 
of different species. 
Winter Sap Densities in Natural Habitats 
Attention may now be turned to determinations of the sap density of 
trees growing in their natural habitats, as made at the end of December, 
1917. The foliage specimens were collected on the afternoons of 
December 30 and 31, both days being warm and the soil not yet frozen 
in any instance at a depth of a foot. The preceding week had been warm 
and dry, with a high evaporation rate for that season. Hence it may be 
expected that the results will show the influences of different exposures. 
All material was from limbs at a height of about 4K above the 
ground. 
In, this, as in all the following cases where only foliage is sampled, the 
outer half or two-thirds of the needles was clipped off with shears in 
sections about one-half inch long. This material was leached before 
dr3dng Was otherwise treated as the ground pulps had been. 
When these results are compared with those obtained from nursery 
stock on December 3 (fig. 3), it is seen that a very great but regular 
difference in the value exists. The average sap density of limber pine 
has increased 8 per cent, of yellow pine 5 per cent, of lodgepole 4 per 
cent; that of Douglas fir has decreased i per cent and that of spruce 6 
per cent. These changes form almost a straight line when plotted with 
the original sap densities as abscissae. 
This shows that sap densities in a given species are subject to great 
v'ariations, but it does not mean that they have no significance. The 
differences between these field specimens and the nursery trees growing 
under uniform conditions reflect the fact that the pines had latdy been 
subjected to the strongest drying influences, while the fir in part, and 
spruce wholly, had recently been protected from any severe drying; 
Also, owing to the protection afforded the latter species at all times, they 
had probably never had the benefit of full light and, therefore, may not 
have accumulated as large a supply of carbohydrates as the pines growing 
in the open. 
To assume from this that spruce or fir is not subjected in the field to 
dr3dng stresses equal to those experienced by the pines, or that the 
former would not tolerate great stresses as well as the latter, is altogether 
erroneous. These tests were made before the soil was frozen and before 
the winter exposure had had opportunity to bring about any degree of 
equilibrium between different sites. It is greatly to be regretted that 
this series of specimens could not have been duplicated late in the winter. 
Winter Sap Densities Near Timber Line 
On the other hand, specimens collected at high elevations, on January 
I, 1918, only a day or two later, tell a very different story, for here the 
soil was already deeply frozen, and the exposure to evaporation had be^ 
very severe for the preceding six days. 
These results and others which show changes with season, weather, 
and soil conditions are given in Table XVIII. 
