254 The Physiology of Plants BOOK in 



was De Vries's paper entitled Untersuchungen tiber die 

 mechanischen Ursachen der Zellstreckung. Though of less 

 importance than Pfeffer's, it was a very valuable contribu- 

 tion to our knowledge of the subject. It contained a 

 number of determinations of the relative osmotic values 

 of the different substances which are constitutents of the 

 cell sap, in the course of which De Vries established a general 

 relationship between molecular weight and osmotic pressure. 

 He also investigated the phenomenon to which he gave 

 the name of plasmolysis, and showed how it can be applied 

 to ascertain the osmotic pressure of various substances 

 and solutions. Of this work Sachs says, ' the relations 

 between turgescence, the protoplasm, and the cell wall, 

 established by Naegeli, Pfeffer, and myself, were first clearly 

 explained by De Vries in 1877.' 



Subsequent contributions to the subject which appeared 

 at intervals dealt especially with the osmotic pressures 

 in the cells of various plants, both terrestrial and 

 aquatic. The chief were those of Westermaier (1883), 

 De Vries (1884), Krabbe (1884), Janse (1888), Stange (1892), 

 Pfeffer (1893), Copeland (1896), and Dixon (1896). De 

 Vries determined the osmotic pressure in the cells of the 

 beetroot to range between 15 and 21 atmospheres. Wieler 

 found almost the same amount in the cells of developing 

 medullary rays of Pinus sylvestris. Dixon determined that 

 of the cells of leaves of Acer to exceed eight atmospheres. 



Pfeffer made a further important contribution to the 

 subject in 1890, when he published his memoir Plasmahaut 

 u. Vacuolen, which was an extension of the former paper 

 and dealt especially with the plasma membranes and their 

 relations to the vacuoles. 



The varying permeability of protoplasm by different 

 substances, and its conditions, were investigated very fully 

 by Overton at the end of the century. Without actually 

 endorsing Pfeffer's views, he supported the idea of there 



