298 REPRODUCTION 



other allelomorphic pair, and consequently segregate in various 

 ways when self-fertilised. 



From the above example it is seen that certain characters 

 existing in two separate varieties of plants may be combined in 

 one variety, and this is not an isolated case. Many others have 

 been worked out experimentally. 



(vi) The Mendelian conception of distinct unit characters 

 which are capable of being inherited independently of each 

 other has given precision to our views of the nature of 

 heredity and the constitution of pure breeds and hybrids or 

 crosses. 



A pure-bred individual is one which has developed from the 

 union of male and female cells containing similar elements or 

 characters, while a hybrid or cross-bred organism has arisen 

 from sex cells conveying different allelomorphic elements. A 

 plant may be pure bred in respect of one character, and yet be 

 cross-bred in regard to another character. 



This h3^othesis of the distinctness of hereditary characters 

 greatly assists the efforts of the plant breeder, inasmuch as it 

 indicates the line along which crossing must take place to effect 

 a desired combination in one plant of characters only met with 

 in separate varieties, and makes his selection among the 

 offspring of crosses to obtain the wished-for result, simpler and 

 more direct than heretofore. 



(vii) It has been long known among hybridists that certain 

 cross-bred varieties of plants which exhibit characters different 

 from either of the two parents cannot be fixed. On self- 

 fertilisation the new character is not met with in all the offspring, 

 there being many individuals (rogues) which have to be 

 discarded. No amount of selection or self-fertilisation is found 

 to fix the new type. 



These hybrid forms are generally merely heterozygotes, and 

 on Mendel's hypothesis ought to break up into 25 per cent. 



