30 PHYSICAL CHEMISTRY IN BIOLOGY. 



/ conductivity \ 



duction of electricity, and the molecular conductivity ( = mo i ecularconcentration ; 



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

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

 nate with /*> the limit value which the molecular conductivity approaches in 

 infinite dilution and with pv the molecular conductivity at some definite dilution 



U-V 



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



The positively charged ions are called cations, and the negatively 

 charged 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 modi- 

 fied 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 bodies, changed their 

 appearance so that the protoplasm retracted, 1 DE VRIES studied 

 this phenomenon further. 2 This phenomenon is called plasmolysis 

 by DE VRIES. The most important substances for bringing about 

 plasmolysis are the salts of the alkalies and alkaline earths, varieties of 

 sugars, monatomic 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 

 substance attracts water from the inner parts of the cell. The cell con- 

 tents 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 con- 

 centration, there must be a limit solution for every substance above 

 which all higher concentrations plasmolyze. The limit solution is called 

 isotonic with the cells; weaker solutions are called hypotonic, and stronger 

 hypertonic. DE VRIES, with the aid of equal cells (cells of the epidermis 

 of the lower side of the leaf of the Tradescantia discolor) has, for various 

 substances, determined the concentration of this limit solution. It 



1 Pflanzenphysiol. Untersuch., 1855. 



2 Eine Analyse der Turgorkraft, Jahresber. f. Wissensch. Botanik, 14, 427, 1884. 



