MYERS! DISCUSSION OF SESSION III 301 



crop, agents that will increase the frequency of mutation can be a 

 powerful tool. This is particularly true with the ornamentals in which 

 deviations from existing varieties have immediate appeal and in 

 which vigor and reproductive capacity are not characters of prime 

 importance. 



Two points made by Doctor Nybom, in addition to the docu- 

 mentation of varietal improvements resulting from induced muta- 

 tions, seem to deserve special mention. One of these is his suggestion 

 that mutation induction may be expected in the vegetatively repro- 

 duced crops to pave the way for production of new commercially 

 important varieties only if generally "good" varieties are used in 

 mutation induction. The other point is that what appear to be 

 mutations can be produced in periclinal chimeras by tissue 

 alterations. 



Doctor Gaul has cited cases of favorable mutations induced in 

 seed-propagated crops and of varieties released which owed their 

 improvements apparently to induced mutations. From these reports 

 one could conclude that induced mutations are a valuable addition 

 to the natural variability that exists in the crop species. In fact, one 

 is tempted to say that these reports are "too good to be true". This is 

 not a valid reason for failure to accept the documented results of 

 research. It does seem appropriate, however, to point out some pre- 

 cautions that must be used both in conducting mutation experiments 

 and in evaluating reported successes. 



The first precaution is the need for adequate controls. The 

 varieties of self-pollinated crops are not, contrary to a commonly held 

 opinion, pure lines. Rather they are mixtures of pure lines and, 

 although they may appear to be uniform, they exhibit variability in 

 physiological character and often, under neAV environments, in mor- 

 phological characters as well. It is not sufficient proof of mutation- 

 induced variability merely to have selections from a treated variety 

 which differ from the variety in physiological characteristics. The 

 treated population must be, in fact, a pure line or an untreated por- 

 tion of the population must be subjected to selection pressure 

 comparable to that used in the treated material. 



The nature of varieties of self-pollinated crops, namely, that 

 they are mixtures of pure lines, is probably in some cases responsible 

 for their broad adaptation in years and locations. When single lines 



