IV. OSMOTIC PRESSURE AND RELATED FORCES AS 

 ENVIRONMENTAL FACTORS 



BURTON EDWARD LIVINGSTON 



The Johns Hopkins University, Baltimore, Md. 



In every vacuolated cell that is in osmotic equilibrium, the 

 outwardly directed pressure of the vacuole must be opposed by an 

 equal force directed inwardly. The latter force is a resultant of 

 several component forces, which arise in various ways. If the 

 cell is not in osmotic equilibrium the vacuole must enlarge or 

 contract more or less rapidly, this change tending always to equa- 

 lize the outwardly and inwardly directed forces, until equilibrium 

 is established. 



"WTiile the origin and magnitude of vacuolar pressures have been 

 the subject of many publications, I am not aware that the forces 

 directed against these have ever received anything like adequate 

 treatment. This is somewhat surprising, when we consider that 

 all phenomena that can be related to the former must be equally 

 related to the latter; for, all movements of osmotic adjustment are 

 directly conditioned by the difference in magnitude between these 

 two factors. 



It is my purpose in this paper to examine the forces opposed 

 to vacuolar osmotic pressure, and to consider the conditions which 

 seem to control these in ordinary plant cells. Our study will 

 also bring us into touch with the physical factors which appear to 

 control turgidit}^ and water-content in non-vacuolated cells. 



The forces which oppose the vacuolar osmotic pressure of a 

 naked cell belong to several different physical categories. There 

 are first of all, (1, 2) the two forces of surface tension, of the inner 

 and outer surfaces of protoplasm. Both of these surface films, 

 tending to contract, exert a slight (but probably not nearly always 

 negligible) inward pressure upon the vacuole. 



(3) Between the two surface layers just mentioned lies the pro- 



165 



THE PLANT WORLD, VOL. 16, xo. 6, June, 1913 



