IMPORTANCE OF WATER 107 



high. Starch, when heated, becomes pasty, turning into a col- 

 loid of unlimited swelling capacity. 



The phenomenon of swelling and the reciprocal phenomenon 

 of desiccation play an important role in the life of the cell. The 

 majority of vital processes can proceed only in a protoplasm suf- 

 ficiently saturated with water. 



35. The Cell as a Colloidal System. Osmotic Properties of 

 Cell Membranes. General Laws of Osmosis. — The plant cell 

 consists of two principal parts, the cell wall and the protoplasm, 

 each of which may be regarded as a colloidal system. The cell 

 wall, is made up largely of cellulose — a gel of limited swelling 

 capacity and of a highly porous structure. The protoplasm within 

 the wall is a sol, though very dense, still exhibiting the character- 

 istics of a liquid body. This is shown by a number of properties. 

 Protoplasm possesses a marked surface tension. When escaping 

 from the wall, for instance, when zoospores are formed by algae, 

 it assumes a spherical shape, typical of drops of liquid. The fre- 

 quently observed movement of protoplasm within the cell also 

 points to its liquid nature. As protoplasm does not mix with 

 water, it belongs to colloids of limited swelling. 



Living protoplasm is always saturated with water. Its lim- 

 ited swelling capacity, as well as the characteristic permeability 

 for substances dissolved in water, make us suppose, however, that 

 the water in it is not a continuous but a dispersed phase. The 

 dispersion medium is probably a mixture of protein and fatlike 

 substances. This question, however, has not yet been solved 

 completely. 



In very young cells, the whole cavity is filled with a solid mass 

 of protoplasm. In time, vacuoles containing a watery sap arise 

 in this mass. These vacuoles finally merge, forming one central 

 vacuole. The typical vegetative cell, therefore, may be con- 

 sidered as a double-walled sac filled with cell sap. The cellulose 

 membrane of the cell is the outer wall of this sac, the layer of semi- 

 liquid protoplasm, forms a lining within it (Fig. 42). 



These two membranes, separating the cell contents from the 

 surrounding medium, are endowed with very different properties. 

 The wall is a stiff porous gel. Owing to its solidity, it easily pre-, 

 serves its shape and shows but a limited capacity of distention. 

 Being a liquid, though very dense, the protoplasmic sac shows an 

 almost unlimited distention capacity. Not being able to maintain 



