26 BIOLOGY AND TECHNIQUE 



equilibrium may eventually be restored and the normal morphology 

 of the cell resumed. In all cases the speed and completeness of the 

 return to normal depends upon the permeability of the cell mem- 

 brane 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, 39 flagella are much less sus- 

 ceptible 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. " Plas- 

 moptysis leads to cell death, and is probably the cause of the death 

 of microorganisms 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 refract- 

 ive 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 Bolt-on, 40 Rubner, 41 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 



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



40 Bolton, Zeit. f . Hyg., i, 1886. 



41 Rubner, Arch. f. Hyg., xi, 1890. 



