308 



ASCORBIC ACID 



per unit of dry matter in the region of expansion was an outstanding feature 

 of the observations. The dry substance in the region of elongation contained 

 60 % more ascorbic acid than that of the embryonic region where nuclear 

 activity is high and 30 % more than the region of maturation where cellulose 

 accumulation appears to be the dominant activity. The data suggest that 

 the increase in ascorbic acid is associated with an increase in the cytoplasm 

 and/or cell wall. The parallelism between ascorbic acid and surface area is 

 probably not fortuitous, and there may even be a true growth-controlling 

 relationship between them. 



2500 







14 



; 4 6 8 10 12 

 Age of cultures in days 



Fig. 2. Effect of 3.5 mg. per liter of indoleacetic acid on ascorbic acid oxidase 

 activity of tobacco pith sections cultured in vitro. Curves based on oxygen consump- 

 tion by homogenates on ascorbate. (By E. H. Newcomb.^"*) 



The concept of ascorbic acid as having a possible controlling effect on 

 cell enlargement has received strong support from an interesting study by 

 Newcomb'"^ on the effects of auxin on the growth of cultured pith cells of 

 tobacco stems. He found that the auxin caused a striking increase in the 

 ascorbic acid oxidase acti\'ity of the tissue. This increase was followed by 

 cell enlargement, gains both in fresh and dry weight, some respiratory in- 

 crease, but no cell divisions. By differential centrifugation the ascorbic 

 acid oxidase activity was found to ])e localized in the cell wall fraction; 

 hence it was suggested that the enzyme occurs in the cytoplasm intimately 

 associated with the wall where it may be causally related to growth. Figure 

 2 shows the quantitative results of these experiments. Newcomb suggested 



