110 SOME RECENT RESEARCHES IN PLANT PHYSIOLOGY 



which plasmolyzes to the same* extent in the same tune 

 and at the same temperature, a measure of the relative 

 rates of penetration is furnished. The results of these two 

 and of the electrical method agree well. 



CRITERIA OF PENETRATION OF SALTS AND IONS. 



Overton considered inorganic salts as incapable of pene- 

 trating living cells. They are, it must be remembered, 

 almost entirely insoluble in lipoids. Osterhout (1913, 1), 

 repeating such experiments on Spirogyra, the material 

 used by Overton, found that very many salts enter cells 

 readily. That they should do so is quite easily under- 

 stood if Czapek's view of the nature of the protoplasmic 

 surface is adopted. The recovery from plasmolysis of 

 cells left in solutions of salts of ammonium, rubidium, 

 sodium, caesium, potassium, lithium, magnesium, calcium, 

 strontium, and aluminium, shows that all these salts can 

 enter. 



The most striking proof of the penetration of the salt is 

 given by the following experiment devised by Osterhout: 

 A filament of Spirogyra, divided into several portions, was 

 found to be plasmolyzed by 0-2 N calcium chloride and by 

 0-38 N sodium chloride, but not in solutions of these salts 

 of normalities 0-195 and 0-375 respectively. On mixing 

 100 c.c. of the sodium salt with 10 c.c. of the corresponding 

 calcium solution, other portions of the same filament 

 underwent rapid plasmolysis on immersion. Thus by 

 mixing two solutions, neither of which is able to plas- 

 molyze, there results a solution which plasmolyzes strongly. 

 The calcium chloride solution has, be it noted, a much 

 lower osmotic pressure than the sodium chloride solution, 

 but owing to its slower rate of penetration a smaller con- 

 centration is as efficient a plasmolyzing agent as a solution 

 of sodium chloride having a higher concentration. 



