HYBRIDIZATION 



J. H. PAUIJ-, PII. D., LECTL'KKK IX liOTAXV, TOROXTO UXIVERSITV. 



A PLANT hybrid is the product of a 

 cross between two plants that do not 

 belong to the same variety. It may be a 

 cross between two varieties, between a 

 variety and the parent species, between two 

 species, or between two genera. It may re- 

 semble either of the parents, or it may re- 

 semble the male parent in some respects and 

 the female in others, or it may be interme- 

 diate between the two, or it may resemble 

 neither. 



Can the nature of the cross be foretold ? 

 On what are the characters of the hybrid 

 dependent? What will be the characters 

 of its progeny and can they be determined 

 beforehand ? These are pertinent ques- 

 tions for the plant breeder. 



Unfortunately our knowledge of the laws 

 of hybridization is meagre. Not long since 

 I read a short article in one of the Ontario 

 trade journals in which a prominent florist 

 advised crossing everything that could be 

 got to cross, on the basis that some valuable 

 results were likely to be attained. This was 

 an unwitting testimony to the fact that that 

 florist knew of no guiding principles — to 

 him the production of a valuable race of 

 plants b}- hybridizing was a matter of mer- 

 esit chance. The day has not yet dawnfed 

 when we may select two plants of different 

 kinds and predict the characters of their off- 

 spring, but we can do this with a few, and it 

 is of these that I wish to speak. Species 

 is a group of like plants that possesses some 

 one or more characters that are not common 

 to its ancestral species. A variety is a 

 group of like plants in which some character 

 of the species from which it was derived re- 

 mains dormant, or in which there is the re- 

 appearance of an ancestral character that 

 was dormant in the parent species. These 

 definitions are due to the Dutch botanist. 

 DeVries. 



Practicallv the first contribution to our 



knowledge of the laws of hybridization was 

 made by an Austrian priest, Gregor Johann 

 Mendel. He studied the crosses between a 

 species and its variety and succeeded in dis- 

 covering the law governing hybrids of this 

 kind. 



The law, stated in brief, affirms that when 

 a species and its variety are crossed the pairs 

 of differentiating characters become disso- 

 ciated in the hybrid, and re-arrange them- 

 selves in the offspring into as many combi- 

 nations as is possible in ratios that are de- 

 terminable by the theory of probability. 

 There are no intermediate forms or new 

 qualities produced. 



In order to understand the workings of 

 the law let us select a specific example, say 

 a cross between a 'blue-flowered species and 

 its white-flowered variety, all other charac- 

 ters in the plants crossed being the same. 

 The results are not affected by the direction 

 ui which the cross is made. The only re- 

 striction placed on the experiment is that 

 there must be no subsequent crossing, the 

 progeny must be kept isolated from other 

 plants with which they might hybridize. 



All of the flowers of the first generation 

 after the crosses are blue. Blue then is a 

 dominant character. Indeed, the species 

 character is almost uniformly dominant. In 

 the second generation there is a splitting up : 

 25 of the offspring out of every 100 bear 

 white flowers and 75 blue. The white flow- 

 ers are of the pure variety type, and breed as 

 true as if there had never been a cross ; 25 

 of the 75 are likewise of pure type, of the 

 species kind, and they breed true. The re- 

 maining 50 are hybrids, their flowers being 

 all blue because this quality is a dominant. 

 In this generation then one-half of the plants 

 have reverted — an equal number to each 

 parent type, and but one-half are hybrids. 

 The latter break up in exactly the same man- 

 ner in the succeeding generation, namely in 



* Extract from an address before the Natural Hislorj- Section of the Canadian Institute. 



226 



