THE PHYSICAL PROPERTIES OF CYTOPLASIM 73 



conditions. Studies of this kind have furnished considerable information 

 about some of its physical properties, 



1. Transparency.— Like the walls of most living cells protoplasm is usu- 

 ally transparent to the wave lengths of the visible spectrum. Photochemical 

 reactions can therefore occur in cells located well below the surfaces of many 

 plant organs such as leaves. 



2. Elasiicity.—Some authorities consider cytoplasm to be highly elastic. 

 Seifriz (1936) has shown that it is possible to draw out the cytoplasm of 

 living plant cells into long threads which behave as elastic bands and snap 

 back into the cytoplasmic mass when released (Fig. 16). It is difficult to 

 determine, however, whether the elastic properties of such cytoplasmic threads 

 are characteristic of the cytoplasm as a whole or only of the membranes which 

 enclose it. The phenomenon may be due to surface tension forces rather 

 than to a true elasticity of the cytoplasm proper. Experiments designed to 



Fig. 16. Demonstration of the "elastic" property of cytoplasm. 



Seifriz (1936). 



Redrawn from 



measure the elasticity of the cytoplasm within its limiting membranes indicate 

 very low values. Probably active cytoplasm as a whole possesses elastic prop- 

 erties because of the presence of elastic surface membranes but the bulk of the 

 cytoplasm which lies within these membranes behaves as a liquid and is essen- 

 tially non-elastic. The entire protoplasm of a cell may become elastic, how- 

 ever, when the viscosity is greatly increased, particularly when it is in the 

 gel state. 



3. Viscosity. — Numerous ingenious methods of measuring the viscosity of 

 cytoplasm have been devised but the results obtained are not in agreement. 

 The one conclusion which emerges most clearly from all of these studies is 

 that the viscosity of the cytoplasm of living cells may vary greatly. It has 

 also been found that not all of the cytoplasm in one given cell has the same 

 viscosity at the same time. The surface layers of the cytoplasm are more 

 viscous than the interior cytoplasm. In general cells that are physiologically 

 active have cytoplasm of low viscosit>^ while in dormant cells the cytoplasm 



