Chapter III — 13 — Physical Properties 



The centrifuge has also been used on living cells. This brings 

 about a displacement of heavy inclusions, such as starch grains, 

 or of light inclusions, such as lipide granules, with a speed 

 and facility varying with the consistency of the cytoplasm. This 

 method is much less precise since the exact densities of the gran- 

 ules and cytoplasm are not exactly known. 



These various methods have shown that the most fluid cyto- 

 plasm has a viscosity which is only 3-5 times that of water and 

 that the most dense cytoplasm (that of animal cells) reaches nearly 

 10,000 times the viscosity of water. Thus from the results obtained 

 by means of centrifuging, microdissection and other techniques, an 

 essential and very general conclusion may be drawn : the cytoplasm 

 of plants does not present at all times the same viscosity, for this 

 varies essentially with the physiological state of the organ under 

 consideration. 



Rigidity:- The cytoplasm possesses, at the same time, a property 

 which, as will be seen later, is tied up with its physical state and 

 which is characteristic of cells, namely, a certain rigidity which 

 gives to it an elasticity of torsion. Rigidity expresses the physical 

 ties between the particles of the system in question, ties which are 

 lacking in true liquids. This rigidity can be brought out by micro- 

 manipulation. Thus, in displacing cellular inclusions within the 

 cell, it was observed that sometimes these return to their places 

 when pressure is released, the rigid surroundings acting as a spring, 

 and sometimes the inclusions are displaced as from a liquid without 

 rigidity. The stability or instability of form of these inclusions 

 after being deformed, for instance being drawn out between two 

 needles, also teaches us something of their rigidity and their varia- 

 tions. In this way, Scarth, for example, demonstrated the elastic- 

 ity of the cytoplasm of Spirogyra. The nucleus of one cell of this 

 alga, when pushed by a microneedle from one side of the cell to the 

 other and then left alone, was observed to return of itself to its 

 original position. Like viscosity, rigidity seems to be variable in 

 the cell. Microneedles sometimes penetrate very easily into a fluid 

 cytoplasm without reaction, and sometimes with difficulty into a 

 thick gell. There is no method for measuring the rigidity of the 

 cytoplasm. 



Density:- By a micropycnometric method, Leontjew succeeded 

 in measuring the density of the Plasmodium of Fuligo septica and 

 has shown it to be, on the average, 1.040 for individuals collected in 

 dry weather while it does not surpass 1.016 for those collected in 

 wet weather, but eleven hours after sporulation it rises to 1.065. 



Ectoplasmic layer:- The cytoplasm is not miscible with its ex- 

 ternal surroundings but remains always very sharply separated 

 from them. In cells with no skeletal walls, the cytoplasm is sur- 

 rounded by an external zone presenting a consistency greater than 

 that of its central part. This zone is called ectoplasm, or ectopias- 



