102 CHAPTEB VI 



When mature the rows are harvested separately, the good 

 and bad ears separated and weighed. If in both series an ear 

 is high yielding, it may be taken for granted, with reasonable 

 certainty, that the parent ear was of a productive strain. 

 The highest yielding ears are then taken and planted in an 

 isolated field, and from this field seed is obtained for the 

 general crop the succeeding year. 



Cross-breeding takes place in the breeding plot ; fresh 

 blood being introduced from various sources into the ear-row 

 test each year, and, while the breeding may be narrowed 

 down by the introduction of ears from other breeders into 

 the ear-row test of the sixth and subsequent years, a suffi- 

 ciently heterozygotic condition is established to counteract any 

 possibility of a loss of vigour due to the in-brceding of norm- 

 ally cross-fertilised plants. The ear-to-row series need not be 

 isolated, but the fields used to increase the seed from the 

 productive ears should be kept apart from other maize. The 

 ear-to-row test should be continued, each year ears being 

 selected from other reliable breeders of the same variety in 

 the same locality. As a means of isolating high yielding 

 strains the ear-to-row test is very much discredited by some, 

 chiefly because it is said that the results are masked by the 

 xenial effect caused by crossing. Nevertheless, in varieties 

 where no continued selection has been practised, this method 

 will undoubtedly assist in procuring some of the most pro- 

 ductive strains. 



Shull's Pure-Line Method in Maize-Breeding. — Be- 

 fore devising the above method, Shull's previous experiments 

 had led him to conclude, firstly, " that in an ordinary field of 

 maize the individuals are generally very complex hybrids ; 

 secondly, that the deterioration which takes place as a result 

 of self-fertilisation is due to the gradual reduction of the 

 strain to a homozygous condition, and, thirdly, that the object, 

 of the maize-breeder should not be to find the best pure- 

 line, but to find and maintain the best hybrid combination." 



The process may be considered under two heads : (i) Find- 

 ing the best pure-lines, and (ii) the practical use of the 

 pure-lines in the production of seed maize. 



(i) In finding the best pure-lines it will be necessary to 

 make as many self-fertilisations as practicable, and to continue 

 these year after year until the homozygous state is nearly or 

 quite attained. Then all possible crosses are to be made 

 among these different pure strains, and the Fl plants coming 



