472 PLANT GROWTH AND PLANT COMMUNITIES 



machines (with their much more intricate cytoplasmic structure than 

 was previously recognized) to the actual events involved in salt ac- 

 cumlation in vivo. Merely to compare actively growing and potentially 

 dividing cells with their quiescent, inactive counterparts (see Figure 

 8) is to contrast a working factory in which every operation is going 

 apace in a coordinated way with the same shop with the fires banked 

 down and the machinery stilled. 



The problems of actively metabolizing cells are acutely problems 

 of organization, and the need is to understand how the several parts 

 work together to form a harmonious whole. For example, it seems 

 hardly likely that light will be thrown upon the problem of salt ac- 

 cumlation by applications of enzyme kinetics which are strictly appro- 

 priate only to homogeneous systems— especially when this form of 

 analysis is even applied to whole organs such as roots and there is no 

 attempt to locate the postulated sites of ion-absorbing activity in the 

 root. In these circumstances any resemblance between the processes of 

 ion intake in vivo and the relations of a substrate and an enzyme seem 

 to be coincidental. 



The crucial question seems to be this. In order to grow, cells must 

 absorb; to absorb, they must also grow. The cells that can grow use 

 their energy of metabolism in many ways— to synthesize protein and to 

 maintain the cellular machinery in a variety of ways. Provided the 

 metabolic machinery is "in gear" and canalized toward the construc- 

 tive use of respiratory energy in growth, some of it gets incorporated 

 into the phosphorylated compounds that can apply the energy at those 

 focal points in the cell at which work is being done. 



Cells that cannot synthesize protein may not accumulate ions de 

 novo. Cells that grow predominantly by division absorb and accumu- 

 late ions by different devices from those that grow predominantly by 

 cell enlargement. Cells which, by virtue of the cell-division factors in 

 their medium, have their activities predominantly directed toward cell 

 division, tend to absorb ions to a relatively low degree of "accumula- 

 tion," and this type of absorption is found to be dependent upon stoi- 

 chiometrical binding at sites that are being multiplied. This absorption 

 in rapidly proliferating cells reaches a fairly stable "acumulation ratio," 

 and the cells' concentration of the ion bears a linear relationship to the 

 external concentration. Thus the degree of accumulation of an "indi- 

 cator ion" (e.g., Cs^^") is relatively unaffected by the presence of a 

 large excess (100,000 times) of the carrier cesium ions, which are in- 

 distinguishable by the cells from the radiocesium ions ( see first section 

 of Table V). These data (extracted from a review by Steward and 

 Millar, 1954 ) can be interpreted on the basis that the concentration of 

 cesium in these proliferating cells varies as the first power of the ex- 

 ternal concentration. The situation is very different for carrot cultures 



