Role of Auxins in Control of Leaf Senescence 335 



sample leaves were removed from the branch and subjected to the 

 usual processes of autoradiography. 



Reference to the autoradiogram photographs reproduced in Fig- 

 ure 3 indicate that there was an appreciable fixation of Ci^02 by all the 

 leaves. Two and one-half days after treatment with 2,4-D some of the 

 leaves showed considerable darkening near the boundary of the 

 treated area indicating either an accumulation or a greater retention -^ 

 of C^"* material. This is clearly visible (Figme 3, top) in several of 

 the leaf veins leading to the treated spot. The center of the spot ap- 

 pears paler than the surrounding leaf after 2.5 days, possibly as a re- 

 sult of the more rapid loss of C^* as C^^Os due to the relatively higher 

 rate of respiration in these areas. By 6.5 days and later, there is a 

 marked concentration of C^^ in all the 2,4-D-treated spots. 



Evidence for the Accumulation of Nitrogen Compounds 



It should be noted that control nitrogen values for attached leaves 

 (Figure 2) show daily variation. This variability from occasion to 

 occasion in outdoor control plants is not surprising and has been 

 discussed by Steward et al. (6) in considerable detail. He has shown 

 that a wide variety of environmental conditions can produce marked 

 effects upon the nitrogen composition of plants. Although the data in 

 Figure 2 show an increase in both total and protein nitrogen in the 

 2,4-D-treated spots, a more critical investigation was made using de- 

 tached leaves so that the majority of the original nitrogen should 

 remain within the leaf and none could be transported away from the 

 blade to other parts of the plant. 



The detached Euonymus leaves were kept in a relatively damp 

 atmosphere in the following way. The cut end of each petiole was 

 placed in a small tube containing 0.5 ml. of distilled water. To prevent 

 drying out the base of each tube was embedded in 3 per cent agar in 

 the bottom of a larger specimen tube which was plugged with cot- 

 ton wool. The leaves were treated with a spot of ethanol containing 

 50 |Ltg. 2,4-D or with ethanol only, and were stored out of doors away 

 from direct sunlight. Nineteen days later, the 2,4-D-treated spots were 

 still green, but the remainder of the blade was yellowing. Respiration 

 and nitrogen determinations were made on discs cut from the leaves, 

 and the results are listed in Table 1. It is seen that there is again a / 

 higher rate of oxygen consumption in the 2,4-D-treated tissues than ' 

 in the untreated parts of the leaf, although both tissues are respiring 

 faster than the controls. There is a loss of both total and alcohol- 

 insoluble nitrogen in the untreated and yellowing areas of the 2,4-D- 

 treated leaves and a statistically significant increase in the total nitro- ly' 



