OSMOTIC PRESSURE AS A FACTOR 



171 



rounded by a solution, and let us assume an equilibrium therein 

 of all the various forces heretofore dealt with. We have seen that 

 an increase in the concentration of the environmental solution 

 must result in decreased turgidity and in exit of water from the 

 protoplasm and vacuole, provided only that the assumed increase 

 in concentration be due to solutes to which the protoplasm is not 

 as permeable, at this time, as it is to water. This environmental 

 change afTects the cell by increasing the effective osmotic pressure 

 acting inwardlj^ from the wall upon the protoplasm. That it 

 may also alter the imbibing properties of the wall is not ruled out, 

 but such alteration would probably be of relatively' little impor- 

 tance in most cases. As a result of increased osmotic pressure in 

 the wall, we have a readjustment of the wall itself; it becomes 

 less stretched and its particles rearrange themselves. This may 

 well be a cause for alteration in rate or mode of action of those 

 coagulative or precipitating processes by which wall growth is 

 carried out. Furthermore, water is removed from the protoplasm 

 and from the vacuole. This means that the nature of the surface 

 films between vacuole and protoplasm and between protoplasm 

 and wall may be more or less profoundly^ altered, from which 

 may arise, as may be conceived, permeability changes and many 

 changes in the numerous protoplasmic processes which we group 

 together as metaboUsm. The removal of water from the proto- 

 plasm probably not only alters its surface films but also changes 

 its state of suspension, thus accelerating or retarding various 

 processes which may be dependent upon the surface relations 

 existing between the numerous phases of the mass. A glance 

 at the results of recent studies upon enzyme action suggests 

 many possibilities in this connection. A decrease in the effective 

 osmotic pressure of the wall may be supposed to institute similar 

 alterations, in the opposite direction. 



It is to be remembered that osmotic changes in the environment 

 have not been distinguished, in many cases as yet, from chemical 

 changes, so that little definite knowledge may be brought forward 

 in this connection. Nevertheless we are pretty^ sure that an in- 

 crease in en^nronmental osmotic pressure results, sometimes at 

 least, in altered cell growth, thickening of wall, formation of pig- 

 ments, and the like. 



