74 PLANT CELLS 



may be as stiff as a rigid gel. According to Seifriz (1936) fluid cytoplasm 

 usually has a viscosity ranging from 800 (the consistency of glycerine) to 

 8000 (the viscosity of a thick sugar syrup) times that of pure vi^ater. On 

 the other hand Heilbrunn (1928) considers active cytoplasm to have a vis- 

 cosity only four or five times that of water. Probably most workers would 

 consider the latter estimate a closer approximation to the facts than the 

 former. The viscosity of the cytoplasm in active cells may change rapidly 

 in response to mechanical injury or electric shock, to changes in temperature, 

 differences in acidity, and exposure to various chemical compounds. Dehy- 

 dration increases the viscosity of the cytoplasm and death of the cell results 

 in a marked increase in viscosity. 



4. I nunis ability with Water. — The cytoplasm of cells that are physiologi- 

 cally active is invariably composed predominantly of water, yet when the 

 protoplasts are extruded from such cells into an aqueous medium they do not 

 ordinarily mix with the water. The failure of cytoplasm to become dispersed 

 through the water is largely, if not entirely, due to the presence of a surface 

 membrane containing fat-like constituents which are insoluble in water. 

 When this surface film is punctured a new membrane quickly forms across 

 the broken surface. If substances are present that prevent the development 

 of this surface film the cytoplasm of active cells readily disperses in the water. 



5. Gelation. — Most of the cytoplasm of metabolically active cells exhibits 

 the properties of a hydrophilic sol. In dormant or inactive cells, however, the 

 cytoplasm apparently is often in a gel condition. No sharp line of division 

 can be drawn between hydrophilic sols and gels and every gradation may exist 

 in a given system between a well marked sol state and a well marked gel 

 state (Chap. V). Every such gradation in physical condition may also exist 

 in cytoplasm. It may range in physical state from a highly fluid sol through 

 a viscous sol condition to a stiff elastic gel. 



6. Coagulability. — The cytoplasmic system of most physiologically active 

 cells is destroyed by temperatures of 60° C. or above. Death of plant cells 

 by such temperatures is generally considered to result from a coagulation of 

 some of the protcinaceous constituents of the protoplasm (Chap. XXXIII). 



A number of other factors may bring about coagulation of the protoplasm, 

 at least in the cells of certain species. Among these are certain electrolytes, 

 electric currents, freezing, mechanical pressure, and certain wave lengths of 

 radiant energy (especially ultraviolet radiations, X-rays, and radium radia- 

 tions). 



7. Electrical Properties. — Numerous attempts have been made to deter- 

 mine the isoelectric point of cytoplasm. It was soon discovered that cyto- 

 plasm does not have a definite isoelectric point but rather an isoelectric range. 



