MEWS IS 211 



It has been known for a long time that the (Enotheras differ 

 from other plants on hybridisation ; the specific liybrids are very 

 constant throughout all succeeding generations. Chromosomal 

 linkage can be correlated with much of the genetical behaviour of 

 these plants. The constancy of interspecific hybrids, the produc- 

 tion of twin hybrids and the differences resulting from reciprocal 

 crossing, can be correlated with chromosome behaviour. The 

 occurrence of chromosome linkage, resulting as it does in linkage 

 between the genes of a number of chromosomes, modifies 

 normal Mendelian behaviour. It enables hybrids to breed true 

 and behave in the same way as wild species. Indeed, there 

 is little obvious difference between known hybrids and wild 

 species. For the present, the origin of the rings of chromosomes 

 still remains a controversial point and only further breeding can 

 lead to a true understanding of the problem relevant to chromo- 

 some linkage. 



Gates and Sheffield have investigated five generations of 

 hybrids from CEnothera (biennis X ruhricalyx) X ammophila and 

 CE. ammophila X {biennis X rubricalyx) and their cytological 

 peculiarities. The chromosome linkages appear to be a means of 

 explaining the genetical behaviour observed in these forms. The 

 reciprocal F^ hybrids are very different. They are patroclinous, 

 CE. (bietinis x ruhricalyx) x ammophila especially showing the 

 peculiar leaning stems and bent stem tips of CE. a7nmophila. The 

 rubricalyx character — an excess of anthocyanin in various parts — 

 is dominant in all crosses. The F^ generation of CE. [biennis X 

 rubricalyx) X ammophila contained two types which differed in 

 having light and dark green leaves. In Fg, two families were 

 erect, uniform, and resembled rubricalyx, while the third family 

 segregated into two types : A, which agreed with the dark green 

 Fj plants, except in the presence of erect stems ; and B, having 

 light green leaves and bent stems. In Fg, F4 and F5 these types 

 bred true in essentials. The F^ generation of CE. ammophila X 

 (biennis X rubricalyx) contained a single type (B^) which was 

 erect, but possessed the grey-green leaves of CE. ammophila. In 

 F2, three families bred true and two segregated into types A and B, 

 which resembled the original parents of the cross. In later 



