330 D. J. Osborne and M. Hallawny 



with a radioactive butyl ester of 2,4-D labeled with C^* in the car- 

 boxyl group showed that radioactivity was initially concentrated be- 

 low the treated area with only traces of activity in the remainder of 

 the blade. In attached leaves, activity was eventually confined to the 

 areas below the applied drops and corresponded to the areas of tis- 

 sue which remained green. It had therefore been possible to arrest 

 senescence in a specific group of cells by a relatively high dose of 

 2,4-D and to maintain these cells within an area of similar cells 

 containing little or none of the acid or related C^^-containing com- 

 pounds. 



This method of applying 2,4-D offers a means of studying the 

 effects of relatively high auxin concentrations upon the general 

 metabolism of groups of cells in situ, and for studying differential 

 senescence within a single leaf blade. 



The present communication is concerned with some of the changes 

 which occur in the carbon and nitrogen fractions in the leaf blades 

 of Euonymus japonica following such local applications of the butyl 

 ester of 2,4-D. 



MATERIAL AND METHODS 



The experiments were carried out during the summer months of 

 1959 on well-developed Euonymus bushes growing out of doors. Only 

 leaves from the second year wood were used. 



One spot of 2,4-D butyl ester in ethanol (26 /i.g//xl) was applied by 

 micropipette to the lamina on one side of the main vein of the adax- 

 ial surface of the blade, so that each leaf received 50 /xg. of the ester. 

 Each drop spreads over an area approximately 1 to 1.5 cm. in diameter, 

 and the outer margins of the spread were finely marked round Avith a 

 nonwater soluble white ink. Control leaves receiving ethanol only 

 were marked in a similar way. 



Within six days the leaf tissue bordering on a treated spot became 

 visibly lighter green and later became progressively more yellow imtil 

 the twelfth day from treatment, when the 2,4-D-treated area appeared 

 as a fresh green spot in an otherwise yellowing blade. At intervals of 

 1, 3.5, 6, 12, and 13 days from the initial treatment, the respiration 

 of different portions of the blade was measured and the distribution 

 of nitrogen within these parts was determined. The estimations were 

 carried out on 1 cm. leaf discs cut from the blade, antl each sample 

 comprised six discs. From each 2,4-D-treated leaf, one disc was cut 

 from within the area delimited by the spread of the spot, and a sec- 

 ond from the untreated and opposite half of the leaf. One disc was 

 cut from each control leaf. 



