THE STRUCTURE OF PROTOPLASM 265 



fluidity and assumed water miscibility of protoplasm are usually 

 cited in support of the idea that protoplasm is a pure solution. 

 Liquids flow freely and round up readily into droplets. As 

 protoplasm does the same, it is assumed that it must be a pure 

 liquid or a true solution throughout. But we have seen how 

 fluidity is also characteristic of gel-forming systems which show 

 elastic qualities even when quite thin. 



Belief in the water miscibility of protoplasm dates back many 

 years, though its discoverer denied that such a property char- 

 acterized it. Dujardin described protoplasm as "immiscible 

 in water." The opposing point of view rests upon a miscon- 

 ception. Those who believe that proiioplasm mixes freely in 

 water maintain that it is ordinarily kept from doing so by the 

 presence of a membrane — an outer oily layer. Protoplasm, 

 when cut or torn, immediately forms a new surface over the 

 wounded part, if it is to remain alive. That a protective mem- 

 brane was there and is again formed over the wound is certainly 

 true, but torn protoplasmic surfaces may remain ragged and 

 exposed for some time. There may then be no indication of 

 miscibility at the unprotected surface until a complete break- 

 down occurs. At death, the protoplasm quickly diffuses into 

 the water or coagulates. 



The colloid chemist readily grasps the idea of a jelly liquid 

 enough to flow yet possessing gel (solid) qualities. He can also 

 visualize a system like gelatin which takes up water with avidity 

 and yet gives no indication of miscibility with it (when cold). 

 Water enters by imbibition in gelatin and in protoplasm. Mis- 

 cibility is prevented by structural continuity which holds the 

 gel together though permitting swelling. Loose contact between 

 the structural units allows for readjustment, but the bonds are 

 too strong to sever completely, unless, as in the case of gelatin, 

 freed by heat. 



The osmotic properties of protoplasm appear to be other 

 evidence in support of the idea that protoplasm is a true solution 

 comparable to salt in water. Heretofore, it has been customary 

 to refer to the taking up of water by protoplasm as imbibition 

 rather than osmosis. In plants, osmosis is the predominating 

 force involved in water intake because of the large cell vacuole. 

 In animal cells, the situation was thought to be the reverse, 

 imbibition predominating. But the researches of Lucke indicate 



