SELECTIVE PERMEABILITY 335 



solution, it is observed that the dye begins to penetrate the 

 . cell from certain points on its surface and that it is then dis- 

 tributed gi'adually all over the protoplasm. The explanation 

 is that the protoplasm which is permeable to the dye can not be 

 stained till the plasma-membrane is ruptured by the influx of 

 water. 



The question is, asks Hober, why a healthy cell does not behave 

 in the same way when introduced in a colored hypotonic solution?^ 



(2) When a thread of Spirogyra nitida is placed in a 10% 

 solution of potassium nitrate colored with eosin, the vacuoles 

 resist for a time the entrance of the dye after the entire plasmo- 

 lysed contents of the cells are killed and stained. This would 

 tend to indicate the existence of a vacuolar plasma-membrane 

 as the protoplasm in immediate contact with the vacuole seems 

 to behave differently from the entire cell content.^ 



(3) Lepeshkin^ has observed that when a thread of Spirogyra 

 is pressed under the microscope by the cover glass, the external 

 layer of protoplasm seems to die, while the remaining mass of 

 protoplasm falls apart into balls some of which contain con- 

 siderable quantities of cell sap. The explanation is that the 

 entire mass of protoplasm was held together by the plasma- 

 membrane and fell apart when it was killed. 



The evidence, as it is seen, is too indirect and inconclusive. 

 The facts reported could be explained in a variety of ways be- 

 sides the existence of special plasma-membranes. Thus, the 

 behavior of the dead cells of the root hair or Hydrocharis when 

 transferred from a hypertonic to a hypotonic solution can be 

 explained by the rupture of the entire cytoplasm, as well as by 

 the rupture of the plasma-membrane. The evidence in favor 

 of the existence of vacuolar membranes can be explained by the 

 fact, as it was suggested by McClendon,^ that the granular 

 protoplasm absorbs the dye and thus prevents it for some time 



8 Hober, 1. c, p. 54. 



^ DeVries, Pringsheims Jahrb. 16: 465. 1885. Hober, 1. c. p. 56. 



* Lepeshkin, W. W., Zur I^^nntnis der Plasma-membrane. 1 Ber. d. Deut. 

 Bot. Ges. 28: 91-105. 1910. 



' McClendon, J. E., The osmotic and surface tension phenomena, etc., Biol. 

 Bui. 22: 113-162. 1912. 



