VX)LUME XLIV 



NUMBER 6 



Botanical 



■ 



Gazette 



DECEMBER 1907 



ON TWIN HYBRIDS 



Hugo de Vries 



In the group Onagra of the evening primroses the hybrids between 

 different species are, as a rule, constant and uniform through the 

 succeeding generations. In this respect they comply with the com- 

 mon rule for the majority of the characters of specific hybrids. Only, 

 on account of the scarcity of regressive marks in this group, the 

 phenomenon of constancy is allowed to show itself pure and complete. 

 An exception is afforded by Oenothera hrevistylis^ the character of 

 which splits up according to the formulae of Mendel.^ 



As an example of a constant hybrid race I quote the O. muricafaX 

 biennis. It has been found uniform through at least four generations. 

 In the second, the one in w^hich Mendelian marks are seen to split, 

 I cultivated over 80 flowering plants and over 100 rosettes, but no 

 differences could be detected. Last summer (1907) I cultivated, 

 on neighboring beds, 35 plants of the first and 18 of the fourth genera- 

 tion. Both groups produced a number of flowering stems, but, apart 

 from the ordinary fluctuating variabihty, the characters were exactly 

 the same in all the specimens. They had the flowers almost hkc those 

 of O. biennis^ but dense and richly flowered spikes like the O. muricata. 



The types of the species used for this experiment were the forms 

 which occur everywhere on waste places throughout Europe, having 

 been introduced, the first from Virginia and the second from Canada, 

 about three centuries and one century ago respectively. They are 

 probably the types on which Lixxaeus based his descriptions of the 

 species. In the United States, however, these Linnean species con- 



' Die Mutationstheorie 2:429. 



401 



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bi' 



