May 31, 1924 
Cell Sap Density and Environmental Conditions 
899 
The young succulent shoots and leaves are much more susceptible to early 
fall frosts which occur before the process of lignification has been completed. 
Late spring frosts after the buds have begun to swell are also very injurious and 
are increasingly harmful as the new leaves and shoots develop. The greater 
susceptibility of Douglas fir to frost injury than western yellow pine is probably 
ascribable to the lower temperature at which Douglas fir, as compared with the 
pine, reconverts the stored sugars into starch in the spring. As pointed out 
above, this conversion results in lower sap concentrations, which also offers a 
possible explanation of the relatively greater susceptibility to frost of the true 
firs than of Engelmann spruce (7^). 
On the whole, our native trees and shrubs are so intimately adjusted to the 
environmental conditions to which they have been long subjected that they are 
well protected from injury by ordinary freezing. On the other hand, many of 
the species introduced from regions with different environmental conditions are 
only imperfectly adapted to withstand the local conditions. They frequently 
grow at times when the native species, through adjustment and adaptation, 
have become dormant. As a result they suffer severe injury. For example, 
hardwood species such as green ash, box elder, black locust, and Russian olive 
have been completely killed by frost at times when the native conifers remained 
uninjured. 
It is probable, therefore, that the amount of sugars present in the leaf, and 
consequently the density of the cell sap, has a bearing on frost hardiness. Fur¬ 
thermore, it is likely that the different species convert starch to sugars at different 
temperatures. By experimentally determining the relative temperatures at 
which this conversion occurs in the different species, a further guide should 
become available for determining their relative frost resistance and their suita¬ 
bility for planting on sites subject to early fall and late spring frosts. 
SUMMARY 
The investigation of the density of the cell sap of a large number of plants 
common to the important forest types of the Wasatch Mountains has yielded 
results of direct application in forest research. It shows that sap density may 
be used as an index of site in correlating the great complex of environmental 
factors with the physiological responses of the plant. 
The concentration of the sap of a species is not constant. It may be influenced 
by any of the environmental conditions affecting transpiration, the products 
of photosynthesis, or the supply of available soil moisture. Osmotic pressure 
in plants is more rapidly changed by fluctuations in the moisture conditions of 
the site than by temperature or light. 
Because of the wide range of sites covered in this investigation and the general 
agreement with the results of studies by other investigators, the following bio¬ 
logical principles have been confirmed: 
Annual herbaceous plants which complete their life cycle before the criti¬ 
cally dry part of the growing season, and are therefore not subject to drought 
conditions, have low sap densities. 
2. The concentration of the sap of woody species is much higher than that of 
herbaceous species. 
3. During the growing season the lowest sap densities occur in those forest 
types which are well supplied with available moisture, whose plants are best 
adapted to secure it, and in which the complex of conditions is most favorable 
to plant growth. On the other hand, the highest densities occur on the most 
adverse (dry or saline) sites. 
