468 PLANT GROWTH AND PLANT COMMUNITIES 



the tip for peas) at which the cells are fully extended. Thus the great 

 increase of protein and the greatest respiration are not associated with 

 cells that divide and multiply their self-duplicating structures as much 

 as might have been supposed. If, however, respiration is measured per 

 unit of protein in the cell, the protein present in the youngest cells 

 (0.0 to 0.4 mm.) is found to be more eflFective in maintaining respira- 

 tion than that in the older ones. Clearly there is initially a rapid phase 

 of growth in cell volume, accompanied by a rapid increase in both 

 protein nitrogen and respiration, followed by a protracted phase of 

 slower development with respect to all three parameters (volume, 

 respiration per cell, and protein-nitrogen content). It is data such as 

 these that have prompted the view that relatively non-vacuolated, 

 densely cytoplasmic cells, which divide more frequently, are not as 

 active centers of metabolic activity as those cells to which they give 

 rise by the onset of enlargement and the retardation of division. 



The case of the potato tuber is instructive, because it can be shown 

 that the ability of cells in thin disks to produce, concomitantly, a re- 

 crudescence of growth by cell division, of protein synthesis, and of 

 respiration may be altered by prior storage at 1° C. when, although 

 the respiration may be increased thereby, the effective energy coupling 

 to growth by cell division and to protein synthesis is disturbed. In this 

 respect the temperature treatment resembles the use of 2,4-dinitro- 

 phenol, because the energy of the carbohydrate oxidation runs to waste 

 and is equally unavailable for protein synthesis, cell division, and pro- 

 longed bromide accumulation ( Steward et al., 1943). 



The studies of enucleated cells and systems (see Brachet, 1957, 

 pp. 308-351 ) agree that cellular respiration ( oxygen respiration ) may 

 continue for relatively long periods in the absence of the nucleus. This 

 agrees with views that assign the oxidative function to other organelles 

 {e.g., mitochondria). However, studies with P'^- upon nucleated and 

 enucleated portions of amoebae indicate that the nucleated portion 

 maintains its ATP better than the enucleated one. Studies upon protein 

 synthesis and incorporation of labeled amino acids into the protein of 

 nucleated and enucleated amoeba (Brachet, p. 325) show that the 

 activity of the nucleated fragments is maintained, whereas that of the 

 enucleated one is not. In Acetabularia the physiological activity of the 

 enucleated portion remains high and relatively unaffected by lack of 

 the nucleus for a relatively long time, with respect to both respiration 

 and photosynthesis. While the fact of independent RNA and protein 

 synthesis in enucleated Acetobularia seems definite, it is equally true 

 that eventually the lack of the nucleus causes protein synthesis to be 

 arrested. 



In general, then, all the evidence favors the participation of the 

 DNA of the nucleus in the primary formation of the proteins of the 



