OSMOTIC PROPERTIES OF THE CELL. 43 



tannic acid, a membrane of precipitation is formed which is 

 infiltrated with barium sulphate. This membrane, unlike the 

 former, is impermeable to ammonium sulphate, and will only 

 allow the smaller molecules of such substances as ammonium 

 chloride and water to pass through it. This consideration 

 affords an explanation of the fact that colloidal substances 

 cannot diffuse through membranes. According to Stras- 

 burger's hypothesis, these substances consist of groups of 

 molecules linked together by multivalent atoms; when such 

 a group is larger than the intermolecular interstices of a 

 membrane, it cannot diffuse through it. 



In discussing the absorption of substances in solution by 

 cells, we must bear in mind that they have to pass through, 

 firstly, the cell-wall, and secondly, the primordial utricle. If a 

 substance cannot traverse the cell-wall it is impossible for it 

 to be absorbed by the cell, but this is a matter of only second- 

 ary importance. The matter of primary importance is the 

 passage of substances through the primordial utricle. We 

 shall see that of the many substances which readily pass 

 through the cell-wall, some cannot traverse the primordial 

 utricle at all, and others only in small quantity, and the con- 

 clusion that we shall arrive at will be that it is the primordial 

 utricle which determines what substances and what quantity 

 of them shall enter the cell. 



If a section of parenchymatous tissue be treated with a 

 6 per cent, nitre solution, it will be seen that the cells undergo 

 a diminution in size, and that the primordial utricle becomes 

 detached from the cell-wall at the angles of each cell (Fig. 

 9, 3); if a 10 per cent, solution be used, the primordial utricle 

 will assume the form of a spheroidal vesicle almost entirely 

 free from the cell-wall, the space between the primordial 

 utricle and the cell-wall being filled with the nitre solution 

 (Fig. 9, 4). A cell in this condition is said to be plasmolytic 

 (de Vries). This behaviour of the primordial utricle is the 

 expression of the fact that water has been withdrawn from the 

 cell-sap, and it appears that none, or at most a, very small 

 quantity of the nitre solution has penetrated through the 

 primordial utricle into the vacuole, although it readily passes 



