220 Mineral Nutrition of Plants 



Permeability of roots 



A very important aspect of internal control is posed by the per- 

 meability of roots. The rate and levels of accumulation of inorganic 

 solutes are governed physico-chemically by two prime factors, the 

 gradient of energy intensity of the substances accumulated, between the 

 internal and external phases of the system, and the permeability of the 

 cells or tissues. These two factors have been discussed at some length 



3200 



2800 



400 



INITIAL COHC. 

 OF SUGAR 



a. 

 < 



o 



20 



10 



< 

 o 

 3 

 •A 



o 



100 '50 



PERIOD IN HOURS 



200 



Figure i8. Time-course of respiration (C0 2 evolved) and sugar 

 tion in abscised barley roots. 



deple- 



in recent publications (g, n). Permeability is obviously particularly 

 concerned with phase boundaries. The permeability is related to the 

 specific protoplasmic organization characteristic of the species and 

 altered, within limits imposed by genetic factors, by the environment. 

 Some of the features of surface limitations have been outlined earlier. 

 A lowering of temperature reduces the rate of metabolism, but it will 

 also increase viscosity. Permeability may be more readily separated from 

 metabolic effects on accumulation, by studies in which the external 

 medium is generally more concentrated than that within the tissues, 

 so that simple diffusion could account for the influx without neces- 



