24 BIOLOGY AND TECHNIQUE 



the speed and completeness of the return to normal depends upon the 

 permeability of the cell membrane for the dissolved substances. There 

 is no evidence to support the view that the internal pressure of a cell 

 may be in any way increased by an inherent power of the protoplasm 

 independently of the laws of diffusion. As a general rule, old cultures 

 are more susceptible to plasmolysis than are young and vigorous strains. 

 Spores and, according to A. Fischer, 1 flagella are much less susceptible 

 to osmotic changes than are the vegetative bodies. 



When, on the other hand, bacteria are suddenly removed from a 

 medium possessing a high osmotic pressure to one comparatively low, 

 say, from a concentrated salt solution to distilled water, a bursting of the 

 cell may occur, a process spoken of as "plasmoptysis." Plasmoptysis 

 leads to cell death, and is probably the cause of the death of micro^ 

 organisms so often observed in distilled-water emulsions of bacteria. 



Other Physical Properties of Bacteria. The refractive index of the 

 vegetative bacterial body is low, in contrast to the highly refractive 

 character of the spores and flagella. According to Fischer, the ectoplasm 

 or cell membrane shows a higher index than does the endoplasm. 



The specific gravity of various microorganisms has been investi- 

 gated by Bolton, 2 Rubner, 3 and others. Some of Rubner's results are 

 the following: 



Gelatin fluidifiers . . . Sp.gr. 1 .0651 



Gas formers " " 1 .0465 



Cultures from potato " " 1 .038 



M. prodigiosus " " 1 . 054 



1 A. Fischer, quoted from Gottschlich in Fliigge, " Mikroorganismen," I, p. 91. 



2 Bolton, Zeit. f. Hyg., i, 1886. a Rubner, Arch. f. Hyg., xi, 1890. 



