460 PLANT GROWTH AND PLANT COMMUNITIES 



The nature of ion accumulation in cells. This being so, it is sur- 

 prising how recent are the views concerning active transport of sokites 

 and ions across the boundary surfaces of cells. The early insight of 

 Hoagland, prompted by his first studies upon Nitella ( 1923 to 1929, see 

 Steward and Sutcli£Fe for references), ushered in an era in which the 

 rigid adherence to simple equilibrium criteria, such as the Donnan 

 equilibrium, was seriously questioned. It became possible to see cells 

 as working machines which converted energy through metabolism to 

 forms which could be directed to the initial accumulation of ions, or 

 solutes, in cells and to their subsequent maintenance there against a 

 concentration gradient which causes them to emerge from the cell 

 when the source of that energy supply is withdrawn or diverted. In- 

 deed, cells appear in true physico-chemical equilibrium with their en- 

 vironment only when they are dead! In this respect life may be seen as 

 the maintenance of essentially non-equilibrium states. 



Hoagland saw in the effects of light upon the accumulation of ions 

 in Nitella ( especially of bromide ion, which was first thought to enter 

 the cells in large part by exchange for chloride— see Figure 2) the evi- 

 dence that this was the ultimate source of energy for this essentially 

 dynamic process. However, he also maintained that this energy was 

 mediated for the accumulation per se through the metabolism of the 

 cell. This truth was indeed self-evident. However, it later became ap- 

 parent (see Steward and SutcliflFe, 1959, p. 325) that light must also 

 have played a role in determining the growth of some of the cells that 



CI cone, in cell sap 

 o * 



Br cone, in cell sap 

 External solution 5 meq KBr 



Figure 2. Effect of time on 

 bromide and chloride concen- 

 tration of Nitella clavata sap. 

 (From Hoagland et al, 1926.) 



