Apr. Z4« 1933 
Physiological Requirements of Rocky Mountain Trees 141 
atures and little light and the movement of water stopped by freezing 
of the soil, such a condition does not describe the winters of the region 
in which we are particularly interested in this study. Here the winter 
days are often warm and bright enough to permit possibly some photo- 
S3mthesis in the evergreens and certainly very considerable transpiration. 
Such days may be followed by severe cold of sufficient duration to freeze 
the tree and the soil to a depth of i or 2 feet. These cold waves are not 
uncommonly followed by warm winds which immediately thaw the foli¬ 
age and may make great demands on its moisture before the tree stems 
and the soil thaw out enough to provide a new supply. Thus, in extreme 
cases great injury may be done, and in the usual weather cycles the tree 
is subjected to very marked changes in water supply and in the sap 
density of its foliage. At this season the sap density, it will be readily 
seen, may depend very greatly on the exposure of the tree, since the 
primary factor in (hying is wind. A tree growing in a dense stand with 
a northern exposure passes through much more moderate changes than 
one in an open stand where both sunshine and wind may have full effect. 
Sap Density in Period of Vernation 
We have already pointed out, in connection with Table XIV, the pos¬ 
sible bearing of different responses to temperature on the comp^ative 
sap densities in the spring of 1920. In Table XV are given a(iditional 
data which, with the explanatory notes, make a very clear case. It is 
not surprising to find that a control test made with nursery material 
collected June i, 1918, when growth of most species had begun, shows 
Complete disarrangement of the December, 1917, values. 
The material was collected as before, complete aerial portions being 
taken. These were placed immediately in stoppered bottles and were 
exposed each to the others* vapor until June 18, in order that from the 
losses or absorptions some idea as to relative osmotic pressures might 
be gained. The exposure was not long enough to ^ve more than an 
indication. After this period the material was dried in the bottles, 
then leached and redried. Spruce specimens were inadvertently omitted 
from this collection. 
TablB XVI .—Sap densities of nursery specimens at beginning of growing season, jQiS 
{tops only) 
Species. 
Age. 
Num¬ 
ber of 
tests. 
Average sap 
density. 
Relative 
osmotic 
pressure in¬ 
dicated by 
vapor 
tran^ca*s. 
I/ixnber pine .. 
Years. 
7 
2—6 
2 
Per cent, 
iS-S 4 ±i -37 
15. io±o. 5S 
20-37 
12. 22 ±0. 53 
16. 9i±o. 41 
10. 61 ±0. 98 
20.33 
13.48 
14.18 
Per cent. 
0.978 
.998 
. 980 
• 979 
.969 
.972 
1.014 
.984 
r. 012 
Yellow pine...... 
2 
Bristlecone piiie ®. 
7 
1 
Lodzepole pine... . . . 
4 
2 
Douglcw fir . 
6 
2 
Siberian l^ch &. ... 
4 
2 
Western white pine ® . 
I 
Scotch pine . . . .. 
0 ■ 
1 
i 
Aspen (newleaves) . . .... 
0 
0 
I 
® Buds not yet opened. 
^ One specimen with and the other without roots. The former showed the lower density. 
