228 CHEMISTRY OF PLANT LIFE 



or the so-called " ionization theory," which has done so much to 

 clear up otherwise unexplainable properties of solutions, would be 

 out of place here. But it may be noted that the ionized condition 

 of salts in solution accounts for the avidity, or " strength," of 

 acids and bases; for the increased osmotic pressure of such solu- 

 tions; for the conduction of the electric current through solutions; 

 and for the effects of these dissolved electrolytes upon the col- 

 loidal condition of many substances, since this is due to the elec- 

 tric charge on the dispersed particles. 



Hence, the presence of salts in solution in the water of the 

 protoplasm has a tremendous influence upon the osmotic pressure 

 (which governs the movement of dissolved materials into and out 

 of the cell protoplasm); upon the colloidal condition of the cell 

 contents (which controls all the effects due to the surface boundary 

 phenomena which are discussed below and which are responsible 

 for a large part of the remarkable chemical activity of the proto- 

 plasm) ; upon the electrical phenomena (which constitute many of 

 the stimulations which the protoplasm receives); and upon the 

 acidity or alkalinity of the cell contents (which determine the 

 nature of the respiratory, or oxidation, reactions of the protoplasm 

 and, indirectly, its life or death). 



The general nature of these physical-chemical properties of 

 the protoplasm and of the relation of electrolytes in solution to 

 them may now be considered in some detail. 



OSMOTIC PRESSURE 



Osmotic pressure is one of the chief factors in controlling the 

 amount of water in the protoplasm. As in well known, the 

 phenomenon known as " osmosis " is the passage of solvents, or of 

 dissolved substances, into or out of any tissue, or substance, 

 through the membrane which surrounds it. In the case of a cell, 

 the membrane in question may be either the cell-wall or the internal 

 colloidal films which are distributed throughout the entire mass of 

 the cell contents. 



From the standpoint of their relation to osmosis, membranes 

 may be either impermeable, in which case neither solvent nor dis- 

 solved materials can pass through them; semi-permeable, which 

 permit the passage of the solvent, but not that of dissolved crys- 

 talloidal substances; or permeable, which permit the free passage 



