WATER UTILIZATION BY TREKS 15 



view of moisture, the lower, by and large, was the sap concentration. 

 Korstian also found that cell sap of trees had a higher osmotic pres- 

 sure than that of herbs and that the osmotic pressure was higher in 

 the sap of leaves than in that of the roots. In Pinus ponderosa, for 

 example, the osmotic pressure of the leaves was 16.2 atmospheres in 

 July and that of the roots 12.4 atmospheres. Similarly, 2-year-old 

 leaves (average of 10 trees) showed an osmotic pressure at the base 

 of the crown of 17.3 atmospheres, while at the top of the crown, 40 

 feet above the ground, the osmotic pressure was 18.5 atmospheres. 

 The second-year leaves had a consistently higher osmotic pressure 

 than the first-year leaves, but these latter also show distinct differences 

 with age as shown in table 4. 



Table 4. — Variation in osmotic pressure with age of needles in four species of pine 





Species 



Osmotic pressure in— 





1-month-old 

 needles 



1-year-old 

 needless 



Pinus ponderosa Dougl . 



Atmospheres 

 16.7 

 17.6 

 15.5 

 12.9- 



Atmospheres 

 18.6 



P. ponderosa scopulorum Engelm 



18.9 



P. contorta Dougl. . . . 



17.1 



P. monticoln DougL _ . 



17.0 







Korstian also found that the osmotic pressure of the sap of colored 

 leaves at time of leaf fall was always less than that of green leaves, 

 which agrees with the results of Gail (63) mentioned above; he gives 

 the following figures: 



Osmotic pressure 

 Populus tremuloides Michx.: (»» atmospheres) 



Yellow leaves 16. 7 



Green leaves 22. 



Acer grandidentatum Nutt.: 



Red leaves 17. 3 



Green leaves 18. 3 



Prunus melanocarpa Shafer: 



Red and yellow leaves 15. 4 



Green leaves 21. 



Similar correlations between osmotic pressure and site factors are 

 reported by Regli (174) and also by Walter (224) in his excellent 

 monograph on water relations ("Hydratur") of plants with special 

 reference to osmotic pressure and drought resistance. 



In more recent work with Covillea tridentata Vail, Mallery (146) 

 has attempted to correlate the osmotic value (A) of expressed sap 

 from leaves and twigs of this xerophytic shrub with various environ- 

 mental factors. It was found that the osmotic value of the expressed 

 sap varied with the rainfall and soil moisture more than with soil 

 type or salt content of soil. Osmotic values were always highest at 

 the end of the dry season and lowest at the end of the rainy season 

 and thus might serve as a moderately good index to the available 

 water supply. 



SUMMARY AND SUGGESTIONS FOR RESEARCH 



While the chief function of the stems and leaves is transportation 

 of water and manufacture of food, respectively, they also (especially 

 the stems) store varying amounts of water. This is reflected in the 



