CONTROL OF LEAF GROWTH 173 



leaves serve as a source of auxin. Thus, under the above experimental 

 conditions, auxin was never limiting, and auxin effects could not be 

 determined in this manner. The thought occurred to me at that time, 

 although we were unable to run the experiments, that a possible 

 approach to this problem lay in reducing the level of native auxin in 

 the leaf. Attempts to do this with anti-auxins were not effective (un- 

 published results). 



Gordon showed sometime ago that x-ray would knock out the 

 enzyme responsible for the conversion of indoleacetaldehyde to in- 

 doleacetic acid (Gordon, 1955, 1956). There is no proof that such 

 a system operates in etiolated leaves, but this approach seemed worth 

 while, so we set about irradiating plants with x-ray and determining 

 the response. The method of treatment consisted of growing the 

 plants for about 6 days, exposing them to x-ray irradiation for the 

 required time, and returning them to the darkroom for an additional 

 60 hr growth before discs were removed. From this point on, the 

 treatment was as outlined earlier. It is obvious from Table VI that 

 x-irradiation results in a marked inhibition of expansion. 



Table VI. X-Ray and Light Versus Leaf Disc Expansion" 



" Increase in diameter, mm. 



It was next desirable to find if a portion of this loss could be over- 

 come by the addition of auxin. In the particular case illustrated here, 

 indoleacetic acid was used, but naphthalene acetic acid works equally 

 well. The 175 r exposure data have been used in Table VII, but the 

 same effect prevails at 25 and 100 r. All concentrations of auxin 

 caused inhibition of growth in the non-x-rayed discs, whereas all 

 concentrations of auxin promoted well above the x-ray controls, and 

 the higher concentrations of auxin promoted well above the non-x-ray 

 controls. Thus, whatever the nature of the action of x-irradiation, 

 auxin effectively replaces the removed factor. This evidence, although 



