THE PERMEABILITY OF PROTOPLASM 107 



protoplasm readily. Salts of barium and of caesium, and 

 iodides in general, only entered with difficulty. Certain 

 salts of aluminium, yttrium, and the majority of the salts 

 of the heavy metals, did not penetrate at all. 



He further ascertained that the permeability of proto- 

 plasm to solutions of low concentration is quite different 

 to that with respect to high concentrations. The latter 

 enter easily, without killing the protoplasm. Thus it 

 acquires certain conditions of dead protoplasm, and con- 

 sidered as a membrane it is a modifiable one. In this 

 connection Osterhout's researches quoted farther on are 

 of interest. 



Endler (1912) also has investigated the permeability 

 of protoplasm by following the influence exerted by 

 various salts upon the penetration of a dye, such as 

 neutral red. He showed that neutral salts, in dilute 

 solution, favoured the entry of the dye into living 

 cells of Ulva lactuca, Vaucheria sp., Nitophyllum punc- 

 tatum. Increase in concentration, however, resulted in a 

 maximum for dye absorption being reached, after which 

 the penetration of the colouring matter fell off and finally 

 ceased. The curves given by various salts resembled one 

 another in form, but exhibited qualitative differences. A 

 table showing the order in which the various aiiions came 

 with regard to their effect upon stopping the entry of the 

 dye was also drawn up by Engler. The behaviour of the 

 anions lent itself more readily to comparison inter se than 

 did that of the kations, for the differences between the 

 latter are not so sharply defined. Endler later on (1913) 

 tried to measure the iso-electric point of protoplasm, by a 

 method based upon the influence of hydrion and hydrox- 

 ylion upon the coloration of cells. 



The penetration of dyes, acids, alkalies, and other sub- 

 stances, has been studied by various physiologists, but 



