May3i, 1924 Cell Sap Density and Environmental Conditions 
889 
solution. 20 Species unable to thus adapt themselves will disappear from the 
type. As autumn approaches with its lower temperatures and reduced trans¬ 
piration the fall rains augment the moisture supply. With the return of these 
favorable conditions the density of the cell sap is again materially lowered. 
The September determinations were made after the occurrence of several fall 
rains, of which the results are apparent in the prevailingly low densities com¬ 
pared with those of July. 
While winter is the dormant season for growth and, in the main, for photo¬ 
synthesis also, transpiration continues except during such times as the leaves 
are actually frozen. In winter the water losses of coniferous evergreen trees 
with their leaves intact have been shown by Weaver and Mogensen {120) to be 
relatively no greater than the losses from deciduous trees after leaf-fail. This 
indicates great water-retaining capacity in the leaves of conifers, many of which 
are verj T hardy. With some species, at least, a small amount of photosynthesis 
doubtless occurs on some of the warmest days, but it is necessarily of short dura¬ 
tion. 
Table XV .—Sap densities in first and second year leaves of conifers 
Species 
Depression of freezing 
point in degrees C. 
Osmotic pressure in at¬ 
mospheres 
First-year 
leaves 
Second-year 
leaves 
First-year 
leaves 
Second-year 
leaves 
Pinus ponderosa __ __ 
1.03 
1.11 
12.4 
13.4 
Pin ns ponderosa scop u lor tun . _ . _... 
1. 35 
1.38 
16.2 
16. 6 
Pinus flexilis __ . _ ___._ 
0. 91 
1.29 
11.0 
15.5 
Do_ _ 
0.62 
0.79 
7.5 
9.5 
Do.. ._____ 
0.51 
0. 54 
6. 1 
6.5 
Do.._ _ ___ 
0. 64 
0. 67 
7.7 
8. 1 
Pinus sylvestris . ._ _ ..___ 
1.17 
1.25 
14. 1 
15.0 
Do.. ___ 
0.66 
0. 75 
8.0 
9.0 
Pinus austriaca.. _____ 
0.81 
1. 19 
9.8 
14.3 
Pseudotsuga taxifolia __ .. 
0. 64 
0. 66 
7. 7 
8.0 
Do...._1... 
0.76 
0. 92 
9. 2 
11. 1 
Abies coneolor ___ . . _ 
0.70 
0.83 
8.4 
10.0 
Do____ 
1.02 
1.35 
12.3 
16.2 
Do___ 
1.18 
1. 28 
14.2 
15.4 
Do_____ 
1. 39 
1.47 
16.7 
17.7 
Do___ 
0. 96 
1.04 
11.6 
12.5 
Do_____ 
0.57 
1.06 
6.9 
12.8 
Do_ _______ 
0.54 
0. 64 
6.5 
7.7 
Abies lasiocarpa . ... _ _ 
0. 75 
0.86 
9.0 
10.4 
Do..__ __.__ 
1.20 
1.35 
14.4 
16.2 
Do... . ________ 
1,21 
1.30 
14.6 
15.6 
Do . . . ___ 
0.8 
0.85 
9.8 
10.2 
Do.. . ____ 
0.85 
1.12 
10.2 
13.5 
Do . . .. 
0.60 
0.74 
7.2 
8.9 
Pi cea parry ana .. 
0.80 
1.02 
9.6 
12.3 
Do ' . _ _ 
1.21 
1. 34 
14.6 
16.1 
Do_ ____ 
1.09 
1.40 
13.1 
16.8 
Do 
1.05 
1.16 
12.6 
14.0 
Picea engelmanni ___ . .. .. 
1.17 
1.29 
14.1 
15.5 
Do "___ 
1.19 
1.40 
14.3 
16.8 
Do..... 
0.86 
0.95 
10.4 
11.4 
Picea excelsa . ...... 
0.86 
1.00 
10.4 
12.0 
The December and January collections were made while the leaves of the 
conifers and evergreen shrubs were frozen solid; the temperatures varying from 
5° to 15° C. below the freezing point. These winter determinations were found 
to be at variance with the summer and autumn series in that the conifers showed 
low concentration, while the closely associated evergreen shrubs exhibited high 
concentration. Furthermore, the sap densities of the shrubs increased with 
increase in elevation, a complete reversal of the results secured in the series 
20 The increasing physical grip of the soil particles on the soil solution may also be important. 
