MYERS: DISCUSSION OF SESSION III 299 



natural elimination or by chance. Even so, it is not surprising that 

 so much variability exists. Even with the greatly accelerated incidence 

 of mutations induced by mutagenic agents, the plant breeder may, 

 with his limited time and facilities, be unable by controlled mutagene- 

 sis to exceed or even to equal the work of nature. 



Faced with this fact and realizing that mutations provide the 

 "life blood" of plant breeding, it seems appropriate to sound again 

 the alarm regarding the danger of losing our natural sources of germ 

 plasm. In a sense, the plant breeder is his own worst enemy. Improved 

 varieties developed by him replace the natural stands, farmer's seed 

 lots, and land races that are the reservoirs of the accumulated spon- 

 taneous mutations. It is fortunate for the future of plant breeding 

 that the eight regions of origin, as defined by Vavilov, are in the 

 less-developed areas of the world. But this situation is changing 

 rapidly. Some of the great gene centers may have already been 

 destroyed by the advancing science of plant breeding (4). So-called 

 "World Collections", valuable as they are, represent only a fraction 

 of the world's germ plasm. The "Germ Plasm Bank" at Fort Collins, 

 Colorado, is only a beginning of the facilities required if the best of 

 the world's germ plasm is to be preserved. Plant breeders throughout 

 the world must increase their efforts to assemble and save the store- 

 house of variability which has accumulated over so many centuries. 



This discussant believes that in many crop plants, natural vari- 

 ation can be found for almost any characteristic that might be sought 

 in the crop. Obviously, there are limitations. One would probably 

 not expect to find a genotype in wheat capable of symbiosis with 

 Rhizobium bacteria. One of the interesting stories of extensive search 

 for and the eventual finding of mutations required is that of the work 

 done to convert the Lupinus spp. from wild plants to cultivated. This 

 may be found in the publications of Von Sengbusch and his German 

 associates which are reviewed by Hackbarth and Troll (3). As one 

 example, Von Sengbusch and his staff examined more than one and 

 one-half million plants searching for low alkaloid content. A total of 

 six such plants were found. 



In search for genetic variability in one crop, the plant breeder 

 can use as a guide the range of variability found in related species 

 and genera. This is in accordance with the Law of Homologous Series, 

 proposed by Vavilov (5). It is interesting that Doctor Quinby has 



