18 SOME RECENT RESEARCHES IN PLANT PHYSIOLOGY 



root, sets a limit to the osmotic pressures attainable by the 

 cells of the leaf parenchyma. 



Maquenne (1895) investigated the causes of the passage 

 of sucrose from leaf to stem in the beet, and found that, by 

 pressing sap from each organ and determining its freezing- 

 point, values were obtained for the osmotic pressures in 

 which those of the leaf were usually a little lower than those 

 of the root. His table of results shows, moreover, how 

 closely osmotic equilibrium is maintained in this plant. 

 It must be noted, however, that his figures are only 

 relative, not absolute, for it has been pointed out by 

 Dixon and Atkins (1913, 1) that, unless the protoplasmic 

 membranes of cells are rendered permeable by some 

 method, treatment with liquid air being the most reliable, 

 the concentration of the expressed sap varies with the 

 degree and duration of pressure. In contradistinction to 

 Maquenne these authors found that as a rule the osmotic 

 pressure obtaining in the cells of leaves is greater than that 

 in the roots. Maquenne gives it as a general rule that 

 any substance formed in a cell may accumulate when its 

 formation occasions a fall in osmotic pressure. 



TABLE VIII. 



Maquenne's determinations were made, presumably, in 

 the height of summer. The above measurements by the 

 author (1910) agree closely with them. The point of 

 special interest is the low value for the mean molecular 



