SEASONAL VARIATIONS IN OSMOTIC PRESSURES 143 



The rather consistent increase in the osmotic pressure of leaves which usually 

 occurs during the daylight hours is probably conditioned primarily by two 

 factors: the accumulation of soluble carbohydrates or other organic com- 

 pounds resulting directly or indirectly from photosynthesis, and the decrease in 

 the water content of the cells due to an excess rate of transpiration over the 

 rate of absorption of water resulting in a shrinkage in the volume of the cells. 

 The minimum leaf osmotic pressure, which is usually attained between mid- 

 night and dawn, probably corresponds to the period when the leaf cells are at 

 their maximum water content, and at their minimum organic solute content, 

 due to the continuance of translocation out of the leaves during the night. 

 Seasonal Variations in the Osmotic Pressures of Plant Tissues. — • 

 Pronounced seasonal changes also occur in the osmotic pressures of plant 



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a. 



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(jj 

 a. 



Q. 



o 



H 

 O 

 2 10 

 I/) 

 O 



ABIES GRANDIS 



PINUS PONDEROSA 



PSEUDOTSUGA TAXIFOLIA 



1 



J L 



FEB. 



MAR. APR. 



JUNE JULY 



AUG. SEPT. 



Fig. 29. 



Seasonal variations in the osmotic pressure of the leaves of evergreens. 

 Data of Gail (1926). 



tissues. In temperate regions the osmotic pressures of at least the exposed 

 organs of plants are usually higher in the winter than in the summer. Sea- 

 sonal variations in the osmotic pressures of temperate zone plants have been 

 studied principally in the leaves of evergreen species (Fig. 29). Such varia- 

 tions are also known to occur in the exposed woody stems of plants, although 

 information about them is much less complete. The increased winter osmotic 

 pressures are undoubtedly correlated principally with an increase in the solu- 

 ble carbohydrates in the cell sap due to a shift in the starch ^ soluble carbo- 

 hydrate equilibrium caused by low temperatures (Chap. XXII) and to a lesser 

 extent to the reduced water content of the leaf tissues which is usually char- 

 acteristic of the winter condition. In some species the first of these factors is 



