12 MISC. PUBLICATION 257. U. S. DEPT. OF AGRICULTURE 



in osmotic pressure with elevation above the ground has never been 

 seriously questioned. 



Later workers in this field have done little but accumulate supporting 

 evidence from other trees and types of plants. Thus Regli (174) 

 reports that in Fraxinus excelsior L. and Pyrus communis L. the higher 

 above the ground the leaves are attached the higher is the osmotic 

 pressure. This is also true for vines like Aristolochia and Hedera; and 

 in compound leaves {Fraxinus and Robinia) it was found that there is 

 even a progressive increase in the osmotic pressure of the leaflets from 

 the base to the tip of the leaf. Huber (107) goes so far as to divide 

 the tree crown into zones of iso-osmotic pressure and on this basis 

 explains the dying back of the tips of branches in dry weather; since 

 the tips are farthest from the supply, they are the first to feel the 

 effects of drought. 



SEASONAL VARIATIONS AND RELATION TO HARDINESS 



Seasonal variations in the osmotic pressure of the cell sap of trees 

 have been the object of considerable study. Dixon and Atkins (50) 

 were among the first workers in this field also. They found higher 

 osmotic pressure in lilac and holly roots and leaves in the winter; in 

 the case of leaves this was attributed to an increase in electrolytes, 

 but in the case of roots to an increase in carbohydrates. 



Lewis and Tuttle (185) found a maximum osmotic pressure in 

 leaves of evergreens in March; this was followed by a sudden decrease, 

 which they considered was associated with a decrease in sugar content 

 brought about by the increased temperatures of spring. These 

 workers, as well as others, associated the increased osmotic pressure 

 of plant cells in winter with their increased hardiness, and showed 

 (136) that the osmotic pressure of the sap of Picea glauca needles 

 near Edmonton, Alberta, dropped from 20 atmospheres in late April 

 or May to 16 or 17 atmospheres as the leaves passed into the summer 

 condition. These changes seemed to be quite independent of altitude 

 and were found in other trees; such as Picea engelmanni (Parry) 

 Engelm., Pinus albicaidis Engelm., P. contorta, Dougl. (syn. P. Murray- 

 ana Balf.), etc., which grow at various altitudes. 



Goldsmith and Smith (72) showed that the water content (based 

 on fresh weight) of the leaves of Picea engelmanni at four typical 

 stations throughout the altitudinal range of this tree (6,500-11,500 

 feet) in the Pike's Peak section, was relatively high (60-80 percent) 

 in late spring, falling rapidly to a summer level and then slowly 

 diminishing in winter to 50-56 percent. The osmotic pressure 

 changed in a reverse fashion, being lower in summer and at a maximum 

 in April or May. 



Gail (63) found somewhat similar results in various conifers in the 

 northwestern United States. In Pinus ponderosa Dougl. the osmotic 

 pressure was highest in midwinter and least at the end of June and 

 early July. Gail found also that the osmotic pressure increased with 

 the altitude. TVhen sudden cold spells occurred, the plants did not 

 have a chance to increase the osmotic pressure by changing materials 

 that are not osmotically active, such as starches, into osmotically 

 active substances, such as sugars, and under these circumstances the 

 plants were sometimes killed. That the high osmotic pressures are 

 due to soluble carbohydrates is indicated by the fact that when the 

 food-manufacturing tissues — the leaves — stopped functioning or 



