654 P. W. Brian and H. G. Hemming 



Withrow, ed., AAAS, \Vashington, D.C., pp. 225-242. 1957). Actually, 

 not one, but possibly three to four compounds appear, or at least in- 

 crease in the tips of bean seedlings after the application of gibberellic 

 acid. (Nitsch, J. P. - XVth Interntl. Hort. Cong., Nice. 1958; Nitsch, 

 J. P., and Nitsch, C.-Bul. Soc. Fran^aise Physiol. Veg. 5:20-23. (1959.) 



Dr. Galsion: I would like to say that, like Dr. Brian, we have re- 

 cently been working almost exclusively with the green sections that 

 we started with 10 years ago, and we feel much as Dr. Brian does 

 that they have a great many advantages for this kind of work. VV^e 

 completely agree with him that sugar replaces part but not all of the 

 enhancing effect of light uj^on growth. We agree also that the extra 

 promotive effect of light is a high-intensity reaction. \Xq also feel 

 that the previous photoperiod to which the plant has been exposed 

 has a very marked effect on the response of these excised green sections 

 to gibberellin. 



There is one respect in which our results differ from those of Dr. 

 Brian. We do not ever find that the sections fail to respond to gib- 

 berellin in the absence of auxin. We always get a small but signifi- 

 cant increase in growth with gibberellin alone, and this effect is 

 greater, the greater the previous photoperiod to which the plants 

 had been exposed prior to excision of the sections. 



Dr. Brian and I have also used a somewhat similar approach in 

 attempting to inliibit photosynthesis. I would like to mention one 

 other compound that we have used which behaves rather like DON 

 and azaserine. This is a compound called fenuron whicli belongs to 

 the urea herbicide group. It is 3-(phenyl)-3,3-dimethylurea. The p- 

 chlorophenyl analogue (CMV or monuron) has been widely used in 

 the inhibition of the Hill reaction. We find that with this compound 

 we can almost completely inhibit photosynthetic activity of green sec- 

 tions but not affect their ability to grow heterotrophically in the pres- 

 ence of sugar. W^e have found to our surprise that fenuron acts as an 

 auxin or auxin synergist in the dark in the presence of sugar. This is a 

 rather surprising eftect for this type of compound. Of course, Drs. 

 Steward and Shantz have reported the isolation of 1,3-diphenylurea 

 from coconut milk and have shown tliat this has some growth-promot- 

 ing ability in tissue culture. I would guess, then, that we are dealing 

 here with a new class of auxins. 



Mr. Barlow: In your starved sections, is it a (jucstion of actually 

 leaching in water, or have you for instance kept them on damp filter 

 paper? Do you also get the same effect of starvation by simj^ly keep- 

 ing the whole plant in darkness before cutting the section out? 

 Apropos of that, how does the growth of the section compare with 



