May 3 i, 1924 Cell Sap Density and Environmental Conditions 
895 
Sap density also vitally affects the success of artificial reforestation in regions of 
deficient and poorly distributed rainfall. Only those species can succeed which 
can maintain a sap concentration high enough to provide an osmotic gradient 
sufficient to maintain the transpiration stream. In other words, the plant 
must be able to absorb moisture from the soil to replace the water lost through 
evaporation from the leaves. It would, therefore, appear unwise to attempt to 
grow on any site a species having a smaller range in sap concentration than that 
exhibited by native species normally present on the site; otherwise the introduced 
species will be unable to succeed in its new environment. 
GROWTH AND REPRODUCTION IN RELATION TO SAP DENSITY 
Osmotic pressure may exert a decided influence on the growth and reproduc¬ 
tion of plants. Livingston (83) has shown that growth is accelerated in weak 
soil solutions and is retarded in concentrated ones. Reproduction in some cases 
appears to be almost entirely dependent upon the osmotic pressure of the soil 
solution surrounding the roots. The density of the sap increases as the season 
advances due largely to the accumulation of solutes, although there is some 
evidence that diminished absorption of moisture from the soil is partially responsi¬ 
ble for the increase of sap density. Reed (106) has found that the growth and 
sap concentration in young trees vary in opposite directions; that is, low concen¬ 
trations of the sap in the shoot appeared to be associated with abundant water 
intake and rapid vegetative growth, while higher concentrations were associated 
with slow growth and bud formation. 
It was found in the present investigation that both age and the conditions of 
growth exercise a profound influence on the osmotic pressure of the cell sap. 
Newly developed leaves of the two varieties of western yellow pine, lodgepole 
pine, and western white pine showed lower concentrations in July than one-year- 
old leaves taken from the same plants, all of which were growing in the Cotton¬ 
wood Nursery under identical exposure. The results are given in Table XVIII. 
Table XVIII .—Effect of maturity on sap density in pine leaves 
Species, and age of leaves 
Depression 
of freezing 
point 
Osmotic 
pressure 
Pinits ponderosa: 
Month-old leaves __ ___ 
Degrees C 
1.39 
Atmospheres 
16. 7 
Vear-old leaves _ _; 
I. 00 
18. 6 
Pinus pondcrosa sc.opulorum: 
Month-old leaves - _ __ 
! 
1. 46 
Year-old leaves __ 
1.57 
18.9 
Pinus coniorta: 
Month-old leaves _____ 
1.29 
15.5 
Year-old leaves _ _ __ 
1.42 
17.1 
Pinus monticola: 
Month-old leaves. ...... 
1. 07 
12.9 
Year-old leaves______.___ 
1.41 
17.0 
The young succulent leaves, when subjected to the regular technique in ex¬ 
pressing the sap, yielded three to four times the quantity that was secured from 
the same amount of mature leaves under the same pressure. The sap of the 
young leaves was also thin and almost colorless, resembling a sugar solution and 
largely lacking the characteristic resinous odor of coniferous leaves, while the sap 
from the mature leaves was thicker, possessed the resinous odor, and was of a 
bright green color due to the presence of some of the chlorophyl matter which was 
expressed with the sap. 
It is evident not only that periods of retarded growth correspond to periods ol 
high sap concentration but also that in trees found typically in regions of little 
