nybom: vegetatively propagated species 293 



MacKey: In the discussion as to preference of chronic or acute radia- 

 tion, I think it is very dangerous to generalize. Species with different 

 speeds in cell division and different patterns of diplontic selection are 

 bound to respond very differently to the two kinds of treatment. 



Hough: In reference to the use of chronic irradiation in gamma field 

 versus acute irradiation, we have obtained useful mutations in peaches 

 from both chronic irradiation of whole trees and acute irradiation of 

 buds. We think that acute irradiation of buds and acute and semi- 

 chronic irradiation of dormant or potted trees will be most useful in 

 the future. 



Gamma irradiation appears to be quite valuable in irradiating some 

 Prunus species in which microsporogenesis occurs in midwinter and in 

 which the buds can be maintained only on the tree for the remainder 

 of the dormant season. Thus, for irradiation during microsporogenesis 

 in these Prunus species, whole-tree gamma irradiation is required. 



Singleton: Concerning chronic versus acute radiation it has also been 

 our experience that a Co 60 source in a gamma field is useful in provid- 

 ing gamma radiation to a wide variety of plants brought near the 

 source for short periods of time similar to the way Nybom uses his 

 gamma field. The gamma field at the Blandy Experimental Farm has 

 been used since 1957 to give semi-acute doses of radiation in a 24-hour 

 period. This machine is an economical radiation source. The machine 

 is on loan to us by the Atomic Energy Commission. Total cost of the 

 gamma field, 1 acre in extent, exclusive of the radiation machine, was 

 $5,554. 



Strauss: It seems that the terms micromutation and macromutation 

 have been used also by you in an operational field sense only. Should not 

 these terms be defined more carefully in terms of actual gene changes? 



Nybom: I have used the terms in the same sense as Doctor Gaul, just 

 for the sake of convenience only. In higher plants we still know very 

 little about the actual genetical changes underlying the phenotypical 

 effects observed. 



Dermen: In the research of the African Violet are the mutations in 

 the control or irradiated plants from adventitious buds? 



Nybom: According to the paper of Doctor Sparrow, quoted in the list 

 of references, the new plantlets are derived from single cells of the leaf 



