4:6 Introduction. 



Using a 3.3 per cent, solution of potassium nitrate in bis 

 apparatus Pfeffer secured a pressure of nearly 85 pounds 

 per square inch or enough to sustain a column of water 

 195 feet high. This force has been looked upon as the 

 cause of the movement of sap in plants and it was a search 

 for a cause for this movement which led Pfeffer to make 

 the observations here referred to. 



62. Influence of Temperature on Osmotic Pressure 



PfefFer extended his observations so as to measure the 

 influence of different temperatures on the osmotic pressure 

 of the same solution in the same piece of apparatus and 

 some of the results he obtained are given in the next 

 table. 



Table showing the influence of temperature on the in- 

 tensity of osmotic pressure. 



1 With temp. 14.2C. pressure=51 c. m. but with temp. 32C.pressure= 54.4 c. m. 



2 ' " 6.8 " =50.5 " " 13.7 " = 52.5 ' 



3 " " 15.5 =52. " 36.0 " =56.7 



In order 4 to understand the relation of osmotic pressure 

 to temperature it is necessary to state them in terms of 

 degrees above absolute zero (32) rather than above the 

 temperature at which water freezes. When the results are 

 stated with reference to the absolute zero of temperature 

 they stand as below: 



1 With temp. 287.92C pressure=51 c. m. but with temp. 305.72C pressure-54.4 c.m. 



2 " 280.52 =50.5 ' " 287.42 =52.5 " 



3 289.22 r=52. " " 809.92 " =56.7 



These observations are in harmony with others regard- 

 ing plant growth which show that a low soil temperature 

 may cause plants to wilt even in the night when evapora- 

 tion from the leaf surface is small, while a high soil tem- 

 perature may increase the root pressure to such an extent 

 as to cause drops of water to form at the tips of leaves in 

 a bright day. 



62a. Osmosis and Diffusion in Plant Feeding. If in a 

 plant cell water is being used in the production of some 

 substance such as starch, sugar or cellulose, the water 



