144 
Journal of Agricultural Research 
Vol. XXIV. No. s 
Tabls XVIII .—Sap densities in exposed situations, at high elevations, iqi8 
Species. 
Location. 
Sap densities. 
Jan. I. 
Feb. 6. 
May 7. 
May s8. 
Engelmann spruce.. 
BristleGone pme. 
limber pine. 
Station F-16. 
Station F-13. 
Per cent. 
“ 37 - 30 
30- 75 
31.00 
30. 56 
Per cent. 
31. 29 
28.68 
24. 00 
30*37 
23. 66 
21. 45 
Per cent. 
633. 60 
C19. 71 
Per cent. 
22. z6 
20. 84 
^[istlecone pine. 
Engelmann spruce.. 
Alpine lir. . . 
Cabin Creek. 
Gulch near F-13 . 
<*21. 70 
21. SO 
19. 02 
20. 62 
21.07 
24. 51 
Ehnber pine. ...... 
Cabin Creek . 
Engelmann spruce .. 
Cabin Creek, edge of 
water. 
® Similar specimen obtained Jan. 15, 1921, from a very exposed site showed 33.5 atmospheres of osmotic 
pre9sure by freezing-point method with 27.7 per cent sap density. 
^ Twigs from which needles were cUpped show«l at same time 19.95 Pcr cent. 
« Twigs from which needles were clipped showed at same time 15.84 per cent. ♦ 
d Twigs from which needles were clipped showed at same time 15.19 per cent. 
At Station F-i6 there is at all times a contrast between the spruce 
and bristlecone pine specimens, until May i8, when thawing had become 
general. This is probably not altogether a specific difference but is 
due in part to tte fact that the spruce was located in a hollow which 
collected snow and did not permit even temporary thawing of the soil 
until v^y late, while the bristlecone was on high ground only a few feet 
dis^nt, from which the snow is usually swept away, and which might, 
therefore, thaw in a brief period. 
It may be a very significant fact that although the soil temperatures 
at Station F-i6 on February 6 were 21.5° F. at i foot, and 23.6® at 4 feet 
(as against 27.0^ and 30.5® on January i), and although there had prob- 
^ly been no thawing whatever during January, the sap densities of both 
spruce and bristlecone pine decreased during the month. The evapora¬ 
tion rate from February i to 6 was only about one-fifth as great as from 
December 26 to January i, meaning, of course, much less current desic¬ 
cation in the later period. But it is also indicated that at some time 
between January i and February 6 the leaves must have obtained mois¬ 
ture from some source. This might have been by transfer from the 
stems, if thawing of aerial parts occurred; but there is no apparent reason 
why the leaves should not have taken up vapor from the atmosphere 
dtfring periods when the vapor of the atmosphere was practically sat¬ 
urated. The possibilities of such absorption, as a relief for winter 
drought conditions, are too important to be overlooked. 
The material collected May 7 showed in all cases much lower sap den¬ 
sity in the twigs than in the more exposed.half of the needles, the latter 
being selected because logically subject to the greatest drying. This 
indicates that small variations in the results may be expected from 
clipping slightly more or less than half of the needles. ^ 
rAll.s^cimens showed a decided drop in densities on May 7, by which 
time there was a great deal of surface thawing, except, as explained, 
around the roots of the spruce at Station F-16. This tree responded 
ilDL lie next period. 
The spruce and alpine fir obtained from a protected stand in a gulch 
near F-13 both on February 6 and May 18 show plainly the advantages 
