OSMOTIC PRESSURE: SUCTION FORCE 53 



steppe had a pressure of about 2 atmospheres more than 

 that of an individual of the same species growing in the 

 meadow. 



Investigations in the coastal deserts of Jamaica by Harris 

 and Lawrence (19 17), using a freezing-point depression or 

 cryoscopic method of determining the osmotic pressure of 

 the leaves, gave values typically over 20 and usually between 

 30 and 50 atmospheres for plants with leathery leaves. 

 Cactuses and other succulents had low osmotic pressures, 

 less than 10 atmospheres. 



The actual suction force has been measured by Ursprung 

 and Blum (1921a). The measurement is made by deter- 

 mining the concentration of sugar solution at which no 

 change occurs in the volume of the cell, measured in the 

 first place under liquid paraffin ; in other words, the 

 concentration at which water passes neither into nor out of 

 the cell gives a measure of the suction force of the cell sap. 

 The suction force is not a constant for a given plant ; it 

 rapidly grows less as the cell is supplied with water, and it 

 thus depends largely on the forces withholding water from 

 the plant ; indeed, as we shall see, it has been used to 

 measure these. Table XI gives values obtained for the 

 epiderm of roots of Phaseolus vulgaris and Vicia Faba, with 

 the corresponding osmotic pressure. 



TABLE XI 

 Suction Forces of Root Cells 



Such measurements, then, show us that the root hair 

 exercises a suction force on the soil water which is closely 

 accommodated to the water supply available at the moment. 



