226 GENERAL PHYSIOLOGY 



cannot diffuse into the cell. Now it is known that the molecules 

 of such soluble substances as salts, sugar, etc., attract water, 

 every molecule taking to itself a number of molecules of 

 water. The molecules of the former are said to act " osmotically." 

 As Van t'Hoff has recently shown by his important researches, 

 the osmotic pressure is proportional to the number of molecules 

 dissolved in the unit of volume. If, therefore, there are stored 

 within the cell-sap strongly osmotic substances, and outside the 

 cell in the water substances that are less osmotic, and if the wall of 

 the primordial utricle is impermeable to these dissolved substances, 

 an equalisation by diffusion cannot take place ; but, since the 

 primordial utricle allows pure water to pass through it unhindered, 

 water must be drawn by the osmotic substances of the sap into 

 the interior and held there permanently. The result of this 

 process is that the pressure in the primordial utricle becomes 

 constantly greater the more osmotic substances are dissolved in 

 the sap, i.e.. the more the concentration of the sap increases. The 

 primordial utricle of the cell, therefore, must be extended from 

 within outward ; and this tension, stretching the elastic cellulose 

 wall, is the turgor of the cell. It is evident that the turgor 

 will become greater, that the cell must be put more upon the 

 stretch, the more osmotic substances accumulate in the sap and the 

 less in the surrounding medium. 



Prom this brief consideration it is clear that the turgor of the 

 cell can be changed in different ways. First, the quantitative 

 relations of the osmotic substances within and without the cell can 

 change, by the concentration outside or inside becoming increased 

 or decreased. If, e.g., substances in solution be added gradually to 

 the surrounding medium, water will be drawn out constantly from 

 the interior, and the turgor will decrease. This phenomenon has been 

 termed, with little appropriateness, plasmolysis. Further, the 

 turgor can likewise be changed by the wall of the primordial utricle 

 from some cause becoming permeable to the substances in solution 

 in the cell-sap. Then an equalisation by diffusion must take place, 

 and the tension of the cell-wall must disappear. Finally, a change in 

 turgor will take place when the tension of the primordial utricle in- 

 creases or decreases because of active changes in its protoplasm. If, 

 e.g., the protoplasm contracts, the contraction will partially or wholly 

 overcome the osmotic pressure opposing it, and the result will be 

 that a corresponding quantity of water minus the osmotic sub- 

 stances will be pressed out from the sap through the primordial 

 utricle. When the contraction of the primordial utricle ceases, 

 the osmotic substances of the sap will attract more molecules of 

 water, and the turgor will again increase. 



The result of diminishing the turgor must in all cases be the 

 same. The primordial utricle, which previously was stretched 

 from within outward by the tension, will shrink together, and its 



