CHAPTER X 



OSMOTIC PRESSURES IN PLANTS 



Having determined the freezing-point of the unaltered 

 sap, it remains to calculate from it the osmotic pressure 

 developed in the cells. The relation connecting these 

 two quantities has been determined experimentally and 

 also deduced theoretically. Nernst gives the equation : 



A x 12 03 = osmotic pressure in atmospheres at 0. 



In Table 30 (p. 192), the depression of freezing-point 

 and the calculated osmotic pressures are given under 

 A and P respectively. 



Factors tending to lower results. It should 

 here be mentioned that the osmotic pressure recorded 

 is probably in no case so high as that obtaining in 

 the cells of the leaves at the moment when they were 

 collected ; for when the leaves are plucked the tension 

 in the water columns is destroyed and the cells of the 

 leaves are free to gorge themselves with water from the 

 trachea?. The dilution may be considerable if the cells 

 were not fully distended when the leaves were gathered, 

 and if they were severed from the branch with a consider- 

 able mass of the conducting tracts. 



Even when this dilution may be neglected the osmotic 

 pressure of the sap of the fresh leaf does not represent the 

 maximum tension the osmotic substances in the cells can 



190 



