PLANT CELL GROWTH AND NUTRITION 



463 



Figure 4. Starch disappearance and 

 cell division in cells near a cut surface 

 of potato-tuber tissue exposed to moist 

 air. 



In the second phase of investigation, certain features in the cells, 

 other than the activated respiration, could be identified with the ability 

 to accumulate the salts. These properties could be identified with 

 ability of the surface cells of the disk to grow, as shown by their ability 

 to divide when in moist air (Figure 4). Treatments that inactivated 

 this type of activity (as, for example, storage of potato tubers at P C; 

 Steward et ah, 1943 ) simultaneously inactivated the salt accumulation, 

 although it simultaneously increased the output of carbon dioxide. 

 Thus the property to be causally linked to the accumulation of ions by 

 the tissue was certainly not the carbon dioxide production alone. Un- 

 less the accentuated oxygen respiration, which was so characteristic of 

 many accumulating systems, resulted in metabolic energy that could 

 be directed into the constructive processes recognizable as growth, the 

 salt accumulation would not occur or continue. The most significant 

 property of the accumulating cells that are engaged in growth is their 

 ability to synthesize new protein from their store of soluble nitrogenous 

 reserves. In many hitherto unsuspected ways, it was found that the 

 ability to accumulate ions was regulated according as the cells were or 

 were not able to synthesize protein. Thus, paradoxically, one energy- 

 requiring process ( salt accumulation ) that increased the free energy of 

 the system became linked to another (protein synthesis), probably 

 because both came to a common focus at a point at which metabolic 

 energy was canalized into useful work. 



It is not necessary here to follow the course of events through the 

 investigations of other plant systems ( such as Valonia species ) or the 



