PROBLEMS OF PRACTICAL BREEDER 83 



been carried out on an enormous scale by Nilsson and 

 his assistants. It is interesting to note that Nilsson 

 discovered the pure line method of breeding for him- 

 self, before the conception of a pure line was known to 

 scientific workers. From the ordinary varieties of the 

 country, which consisted of mixtures of numerous types 

 of very various productiveness and quality, he has 

 isolated the best pure varieties, and these have proved 

 highly valuable. 



It is interesting to note that many of our standard 

 varieties of cereals have been produced from a single 

 ear found growing in some unexpected place. The old 

 agriculturist thus unconsciously produced a pure line, 

 and if it happened to be one of the better types from 

 the common mixed varieties, it proved valuable and 

 was preserved. Thorough systematic search for the 

 best pure lines is a promising method of improvement 

 of such of our economic plants as are continually self- 

 fertilised. 



The discovery of Shull and East regarding the increase 

 of vigour on crossing will no doubt have important 

 applications in the improvement of maize, which can 

 be easily crossed or self-pollinated. The method which 

 has been proposed is to use two distinct types, which 

 may or may not be actual pure lines, as parents; to 

 cross them systematically, and to use the cross-bred 

 seed for the production of the commercial crop, a fresh 

 supply being produced each year. This principle would 

 not be applicable with regard to many species of plants, 

 as the type of floral arrangement which obtains in 

 maize is rare. 



There are other types of plants in which new theories 

 of heredity fail to suggest any improvement on old 

 methods of breeding. Such, for instance, are those 

 plants which are propagated vegetatively by sets, 

 grafts, cuttings, or the like. In this case the problem 

 of the improver is simply to discover valuable new types. 

 No fixing process is necessary, since, as long as the new 

 type is reproduced vegetatively it will, generally speak- 

 ing, remain constant, whether it be pure or hybrid. 



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