Studies With C^i-Iabeled Ethylene 775 



system which produces ethylene is probably the mitochondria and 

 that when the solute concentration of the cell declines, as during 

 water soaking, these particles become swollen and the swelling dam- 

 ages this particular enzyme system. High solute concentrations, such 

 as would occur after plasmolysis, do not adversely affect the enzyme 



system. 



Dr. Hitchcock: First, I would like to comment on the ethylene 

 molecule and then on some of the responses induced by ethylene. 

 Many growth regulator structures have been drawn on the blackboard, 

 but I think this is one of the simplest for a growth regulator (CHo:CHo 

 drawn). Carbon monoxide with the simple structure C:0 also has 

 growth regulating properties. Indirect evidence for the translocation 

 of ethylene, also carbon monoxide, throughout plants was obtained a 

 number of years ago. When one leaf on a tomato plant was sealed in 

 a flask containing ethylene or carbon monoxide, all leaves showed an 

 epinastic response. Hall's work with radioactive ethylene now gives 

 direct proof of the translocation of ethylene throughout the plant. 

 Hall's finding that the stem contains only trace amounts of ethylene as 

 compared to large amounts in the leaves appears to explain why there 

 is no stem bending — only epinasty of leaves on plants exposed to 

 ethylene. Ethylene induces many responses characteristic of growth 

 regulators — epinasty of leaves involving cell elongation in the petiole 

 or in the midrib, differences in flowering and the ripening, develop- 

 ment and coloration of fruit, inhibition of nutational movements, 

 and interference with the correlative movements of leaves and leaflets 

 of plants such as Mimosa piidica. The last two are anaesthetic effects. 

 Thus ethylene is unquestionably a growth regulator, but there is dis- 

 agreement as to whether ethylene is a plant hormone. 



Dr. Nitsch: I'd like to ask Dr. Hitchcock if the response to CO is 

 observed in the light or only in darkness? 



Dr. Hitchcock: I don't recall whether we had ever run tests in 

 darkness alone as against light, so I can't answer that question. 



Dr. Galston: I would like to ask several questions of Dr. Hall. First, 

 do you know whether the entrance of ethylene into the leaf is through 

 the stomata? For instance, if you permit a leaf to wilt completely, will 

 the ethylene still enter? 



Dr. Hall: We have not conducted experiments as you describe 

 but ethylene is absorbed by the petiole. That is, if you remove the 

 blade ethylene is still absorbed by the debladed petiole. Ethylene also 

 enters through the stem or by the roots. So I would judge that it en- 

 ters in other ways in addition to entering through the stomata. 



Dr. Galston: I was struck by the fact that the ethylene appeared 

 not to enter the very youngest leaf very effectively. I wonder whether 



