568 RADIATION BIOLOGY 



this can give rise to rapid changes in the permeabiUty which cannot be 

 registered with methods with a slow action. 



The method used by Blackman and Paine (1918) to determine the 

 exit of electrolytes from the cells by measuring the electrical conductivity 

 of the surrounding fluid therefore presents considerable advantages over 

 slower methods. However, it permits a determination only of the dif- 

 ference between the intake of electrolytes by the cells and their output. 

 If both processes are eciual, no results are obtained with the method. 

 Blackman and Paine used the method to study the photonastic reactions 

 of the pulvini of mimosa, for pulvini immersed in water cause a 

 change in its electrical conductivity. 



It was shown that contraction of the pulvinus was associated with an 

 exit of electrolytes from it and that the permeability of the cells was 

 markedly increased by exposure to light, the effect decreasing rapidly 

 with time. A sudden change from light to darkness also increased the 

 permeability. It was therefore not the light in itself, but the changes in 

 light, which induced the changes in permeability. 



The same method was used by Dillewijn (1927). He placed severed 

 hypocotyls of Helianthus in distilled water and measured the electrical 

 conductivity of the water. The experiments were started 8-10 hr after 

 severing of the hypocotyls in order to avoid the interferences accompany- 

 ing the intervention. In objects that had stood in the dark and had 

 then been illuminated with 200 m-c X 400 sec, the permeability showed 

 a decrease during the first 20 min, with a rapid increase during the subse- 

 quent 7 min. This in turn was followed by a decrease and some vari- 

 ations, which gradually ceased. These changes in permeability are 

 reminiscent of the changes in viscosity caused by illumination of objects 

 that have earlier stood in the dark. It would scarcely have been possible 

 to demonstrate their occurrence with the use of slowly working methods 

 such as that of plasmolysis or an analysis of the cell sap. 



Lepeschkin (1948) placed leaflets of Sambuciis nigra and Parthenocissus 

 quinquefoUa on the surface of dilute solutions of CaCU (0.0006-0.0009 M) 

 or of a mixture of tap water and water distilled in quartz, with the upper 

 side (without stomata) downward. He then determined the electrical 

 resistance of these solutions and its variations. The exit of salts from 

 the leaves was greater by day than by night and was accelerated not 

 only by direct but also by dispersed sunlight. 



Summary of the Experiments. The investigations described in the 

 foregoing amount to about 40 and were made with at least 7 different 

 methods, the number of test objects being approximately 25. In general, 

 they showed that light increases the passage of substances through the 

 protoplasm and membranes, i.e., that it increases the permeability of the 

 cells. In only a few cases was it impossible to show any such effect. 

 The agreement between the results must be denoted as good, if the con- 



