144 THE OSMOTIC QUANTITIES OF PLANT CELLS 



predominant, in other species the second (Steiner, 1933). Similar seasonal 

 variations in osmotic pressure occur in the leaves of evergreen herbaceous 

 species, as for example winter wheat. 



In many woody species of semi-desert regions of the southwestern United 

 States and adjacent Mexico the osmotic pressure of the leaves increases mark- 

 edly during the hot, dry season (May- July) but shows a rapid decrease with 



AUGUST 



Fig. 30. Seasonal variation in the osmotic pressure of the leaves of the creosote 

 bush {Larrea tridentata) under desert conditions. Height of black columns indicates the 

 rainfall in inches. Data of Mallery (1935). 



the advent of summer rains (Fig. 30). The increasing osmotic pressure of 

 the leaves of such species during the dry months is closely correlated with the 

 increasing unavailability of soil water. 



The Osmotic Quantities of Plant Cells. — Let us suppose three similar 

 cells, each in a state of incipient plasmolysis, and each with a cell sap osmotic 

 pressure of 10 atmos., to be immersed, the first in pure water, the second in 



ABC 



Fig. 31. Similar cells immersed in solutions of different osmotic pressures. 



a solution with an osmotic pressure of 6 atmos. and the third in a solution 

 with an osmotic pressure of 10 atmos. (Fig. 31). The volume of the liquid 

 in which each cell is immersed is supposed to be very large in proportion to 

 the volume of the cell. 



In order to simplify the discussion certain assumptions will be made re- 

 garding these cells and, unless stated to the contrary, regarding all other cells 



