WATER UTILIZATION BY TREES \\ 



by the action of the living cells in the root tissues. The chief contri- 

 bution of these workers was in showing that the tensile strength of 

 the water column was sufficient to permit the columns of water to be 

 lifted the distance required in the tallest trees. While the energy for 

 this process comes ultimately from the sun, other forces are thus seen 

 to come into action, including those of surface tension, osmotic pres- 

 sure, root pressure, etc. It is consequently natural to expect to find 

 an osmotic gradient in plants as one goes upward from the soil. Since 

 the active force required to extract water from a living cell (e. g., by 

 evaporation) and the force required to add water to that cell from 

 tissues below depends largely upon the osmotic forces active in the cell, 

 such an osmotic gradient from roots to crown may be anticipated. 



Ewart (58) was one of the first to show that the osmotic pressure of 

 leaves increased with their elevation on the tree, but at this early 

 period he thought that the sources of error in the methods then avail- 

 able were too great to permit any conclusions, since he got equally 

 great differences between leaves at the same level or with leaves of 

 different ages. It has later been found, however, that while the 

 difference in position is not the only factor bringing about differences 

 in osmotic pressure, there is in general a difference in the osmotic 

 pressure of leaves at different levels on the stem, other factors being 

 equal. This was shown by Dixon and Atkins (48) who obtained 

 differences in osmotic pressure in Magnolia, Fraxinus, and Ulmus 

 leaves from various levels in the tree; but they also got very varied 

 results and came to no general conclusions. Part of their difficulty lay 

 in the fact that the tissues were not given the preliminary freezing 

 treatment which they later found to be necessary. 



Harris, Gortner, and Lawrence (85), in their fairly complete series 

 of studies on this subject, showed very clearly that there was a distinct 

 difference in the osmotic concentration of leaf sap at different eleva- 

 tions on the tree. They measured 26 trees of 12 different species and 

 found that the osmotic pressure without exception increased from 

 lower to higher levels. Results from Quercus prinus L. are typical: 



. Osmotic pressure 



Height (feet): (in atmospheres) 



47 20.23 



36 20. 08 



30 19. 72 



19 19. 57 



These differences in osmotic pressure are considered as due chiefly to 

 carbohydrates, since the concentration of electrolytes commonly 

 decreased upward. In a later paper (86) these authors found that sap 

 of trees and shrubs consistently shows a higher osmotic pressure than 

 that of herbs in the same environment. The sap of 19 species of trees 

 from Long Island had an average A (depression of the freezing point) 

 of 1.29° C. ; that of 36 shrubs from the same section had an average 

 A of 1.18°; and that of 162 herbs from the same locality had a A of 

 0.85°. Similar work carried on in the deserts of Arizona and in the 

 rain forests of Jamaica showed similar differences between trees, 

 shrubs, and herbs, the osmotic pressure in these different localities 

 being 26.47 to 38.63 percent more in the cell sap of the trees and 

 shrubs than in that of the herbs. Here again is noted an increase in 

 osmotic pressure in trees with distance above the ground. Since the 

 appearance of these papers of Harris and his associates this increase 



