CHAP, i Processes of Absorption of IVater 253 



of protein substances, thrown out of solution by contact 

 with the water, and so precipitated on the surfaces of the 

 protoplasm. This view enabled him to suggest a hypothesis 

 to explain the varying powers of entry of different substances, 

 and he showed that the membranes are semi-permeable to 

 some substances, and permeable in various degrees to others. 

 Pfeffer suggested that a functional division of labour may 

 exist in the protoplasm with regard to the processes involved 

 in osmosis, but denied that such division of labour disturbs 

 the general principles of the regulation of osmosis and 

 absorption. According to his views the passage of salts 

 in solution through the membrane is mechanical, depending 

 on the relative sizes of the molecules of the salts and the 

 molecular interstices of the membrane, a view held as we 

 have seen by Traube. The action of the protoplasm, so 

 far as vital action is concerned at all, seems, therefore, to 

 be the regulation of the composition of the plasma mem- 

 branes. Pfeffer did not suggest the more modern view, 

 that the living substance exercises any process of active 

 imbibition or vital filtration of the salts, a view traceable 

 to the researches of Waymouth Reid in 1890. 



Pfeffer's work included the investigation of the osmotic 

 pressures exercised by different salts and the consequent 

 tensions existing in the cells. He discussed also the rela- 

 tion between the internal osmotic pressure and the force 

 of imbibition exerted by the cell wall, showing how the 

 two co-operate in setting up and maintaining turgor in the 

 cells, and discussing the conditions of the forces in their 

 interior and the physical principles involved. 



In a later paper, published in 1886, Pfeffer showed that 

 certain aniline dyes, when in very dilute solution, possess 

 the power of penetrating through protoplasm without killing 

 it, a discovery which has been of great importance in the 

 conduction of many subsequent researches. 



The second work alluded to as marking the year 1877 



