OSMOTIC PRESSURE AS A FACTOR 



167 



The water-imbibing force of any body is well known to depend 

 upon two conditions, the nature of the imbibing material and its 

 ivater content. For any gi\-en material, the greater its water con- 

 tent the less is its attraction for water, so, that the entrance of the 

 liquid into the imbibing mass is concomitant with a rapid fall in 

 the magnitude of the force that causes entrance. We may safely 

 assume that nearl}^ dry protoplasm (such as occurs in seeds and 

 spores, for example) possesses an enormous attraction for water. 

 As this liquid is absorbed, however, the attractive force rapidly 

 falls off in magnitude, and in the ordinary active plant cell the 

 resistance to water loss must be comparatively small. Neverthe- 

 less, we may be sure that the imbibition force here under discus- 

 sion is, in every static case, in equilibrium with the osmotic pres- 

 sure of the vacuole; were it otherwise water would either enter 

 or leave the protoplasm, and this process would continue until 

 the attainment of the equilibrium which is here assumed. 



Reference has heretofore been made only to the naked cell, 

 where the peripheral bounding surface, which separates the cell 

 from its environment, is the external film of the protoplasm itself. 

 Immediately surrounding most plant cells is, however, the cell wall 

 which is conveniently considered as part of the cell. Beyond 

 the cell wall lies what we term the cell environment, this being 

 usually made up of other cells (in which case a single wall becomes 

 common to two cells) . Less frequently the cell environment con- 

 sists of objects which are not so clearly related to protoplasmic 

 processes as is the cell wall. I shall consider next the free, walled 

 cell, such as that of a single-celled alga, immersed in a weak solu- 

 tion. 



If such a free cell is turgid and in osmotic equilibrium, the wall 

 is more or less deformed by the vacuolar pressure, and, being 

 elastic, it exerts an inward pressure upon the protoplasm which 

 just balances the stretching force applied from within. The 

 wall is, for all practical purposes, a solid; hence we do not need to 

 consider its surfaces as essentially different from its general mass, 

 and its extensibility and elasticity are functions of its cohesion. 



There are two other forces resident in the wall, however, both 

 tending to resist the stretching force from within. As in the case 



