CYTOPLASM 



171 



hypothesis of a true liquid; they point rather to some fibrous sub- 

 microscopic structural element. 



The inner elasticity can be demonstrated by suspending iron filings 

 in the cytoplasm and moving them by means of a magnetic field. As 

 soon as the field is switched off, the particles swing back elastically to 



Fig. 107. a) Plasmic strands of epidermic cells from the bulb of Allium, plasmolyzed by 

 CaClj, (according to Kuster, 193 5^) ; h) plasmic strand oiSpirogyra, extruded in plasmoptysis 



(from Frey-Wyssling, 1940^). 



their original positions (compare Heilbronn, 1922). This method 

 has been further developed by Crick and Hughes (1950) to measure 

 the internal elasticity of cytoplasm quantitatively. They find the 

 modulus of rigidity of chick fibroblasts in tissue culture to be of the 

 magnitude of 100 dynes/cm^. At the same time they give evidence of 

 the thixotropic behaviour of the cytoplasm which can change its state 

 reversibly from solid to fluid when stirred. In this respect it gives 

 similar results to elastic gels of sodium oleate or bentonite. 



The reversible gel-sol transition is one of the most important proper- 

 ties of cytoplasm, as it is the basic phenomenon in protoplasmic flow 



