FACTORS DETERMINING OSMOTIC PRESSURES 139 



tic pressure of cells in their normally distended condition. It is generally 

 considered that the results of determinations by the cryoscopic method repre- 

 sent approximately the osmotic pressure of the cells in this condition. 



Different organs or tissues of the same plant may exhibit a wide range of 

 osmotic pressures. Even similar organs on the same plant — leaves for example 

 ■ — may vary considerably among themselves in the average osmotic pressure 

 of their cells. Furthermore the tissues w^ithin the same organ usually show 

 a considerable variation in osmotic pressures. The mesophyll cells of most 

 leaves, for example, show higher values than the epidermal cells. 



In general the osmotic pressures of the plant cells and tissues ^ of the 

 mesic species of North America vary in their extreme range from a fraction 

 of an atmosphere to about 50 atmos. Most of the values lie, of course, within 

 a narrower part of this range, probably within the limits of a 4 to 30 atmos. 

 span of values. In general the cells of ligneous species of plants as a group 

 appear to have somewhat higher osmotic pressures than herbaceous plants as 

 a group. This generalization cannot of course be extended to a comparison 

 of any individual ligneous species with an individual herbaceous species. The 

 range of osmotic pressures in the leaf tissues of plants of the eastern United 

 States is illustrated by the data in Table 18, which is self-explanatory. It 

 should be clearly understood that values such as those given in this table by 

 no means represent constants. Considerable variation may be found in any 

 one species depending upon environmental conditions, position of leaf on the 

 plant, etc. 



Factors Determining the Osmotic Pressures of Plant Cells. — Any 

 factor which influences either the water content of a plant cell or the solute 

 content of its cell sap will have an effect on the magnitude of the osmotic 

 pressure of that plant cell. The water content of the plant as a whole, and 

 hence of its constituent cells, is controlled principally by the relative rates 

 of transpiration and the absorption of water (Chap. XVIII). The latter 

 process is influenced very markedly by the water content and other conditions 

 prevailing in the soil. Individuals of the same species invariably have a higher 

 osmotic pressure when growing under drought conditions than when provided 

 with a favorable water supply. This is at least partially due to the low water 

 content of the leaves which results when the available soil water supply 

 becomes low. Other factors which are also probably involved are a decrease 

 in the growth rate of the plant which often permits an accumulation of min- 

 eral salts and soluble foods, and a shift of the starch ^ soluble carbohydrates 



^ It should perhaps be emphasized that the term "osmotic pressure of a tissue" 

 can possess meaning only in the sense of the average osmotic pressure of the cells 

 composing that tissue. 



