OSMOTIC PRESSURE. 5 



if we find that A for a gram molecular solution of NaCl is 3.40 then we have 

 according to the above 1 : (l+[n l]a) =1.86 : 3.40. In the dissociation of 

 NaCl two ions are formed, therefore n =2, and from the above equation the degree 

 of dissociation can be calculated, a =0.83. The degree of dissociation can also 

 be calculated from the electrical conductivity. Only the ions take part in the con- 



/,...., , ,. -, / conductivity \ 



duction of electricity, and the molecular conductivity I = , -, 



J \ molecular concentration/ 



is proportional to the degree of dissociation. The dissociation increases with the 

 dilution and at infinite dilution all molecules are dissociated (a = l). If we desig- 

 nate with )uoo the limit value which the molecular conductivity approaches in 

 infinite dilution and with ^ v the molecular conductivity at some definite dilution 



p 



v, then the degree of dissociation at this dilution is a = : . 



The positively charged ions are called cations, and the negatively 

 charged ones anions. Common for all acids are the positively charged 

 H-ions while the negatively charged OH-ions are common for all bases. 



Osmotic Experiments with Plant Cells. We often meet the 

 word osmosis in literature without understanding exactly what is meant 

 thereby. As a rule diffusion streams are meant, which are modified 

 by means of the permeability conditions of an inclosing membrane. 

 We now know that the driving force, namely, the streaming, is brought 

 about by the differences in concentration, i.e., by difference in the osmotic 

 pressure on the two sides of the membrane. 



After NAGELI found that certain plant cells, when they were treated 

 with a sufficiently concentrated solution of certain substances, changed 

 their appearance so that the protoplasm retracted, 1 DE VRIES studied 

 this phenomenon further. 2 He called it plasmolysis. The most important 

 substances for bringing about plasmolysis are the salts of the alkalies and 

 alkaline earths, varieties of sugars, polyatomic alcohols, and neutral amino- 

 acids. An indispensable condition for bringing about plasmolysis is that 

 the solution must not have any destructive action upon the cells. NAGELI 

 gave the correct interpretation of plasmolysis, which is that those bodies 

 which plasmolyze plant cells pass through the cell membrane of the cell, but 

 not through the protoplasmic layer which follows. Instead of this the sub- 

 stance attracts water from the inner parts of the cell. The cell contents 

 surrounded by protoplasm therefore diminish in volume and the protoplasm 

 recedes more or less from the cell membrane. From this it follows that 

 only those solutions whose power of attracting water is greater than that 

 of the cell contents can bring about plasmolysis. As the ability to attract 

 water (or the osmotic pressure) increases with concentration, there must 

 be a limit solution for every substance above which all higher concentra- 

 tions plasmolyze. The limit solution is called isotonic with the cells; 



1 Pflanzenphysiol. Untersuch., 1855. 



2 Sine Analyse der Turgorkraft, Jahresber. f. Wissensch. Botanik, 14, 427 (1884). 



