THE AMERICAN NATURALIST [Vol. XLVIII 



A A (or aa) individuals = 2s ( o n ) |+ s (p„) + 1/2* (r«), 

 ^la (or oA) individuals = 2s (q n ) + s(l/2p„) + l/2s(r„). 

 The first of the above expressions multiplied by 2 gives the 

 total homozygotes, and the second multiplied by 2 gives the total 

 heterozygotes. 



Raymond Pearl 



PARALLEL MUTATIONS IN CENOTHERA BIENNIS L. 



In the summer of 1912 I cultivated pure strains of 0. biennis 

 L. and of the O. biennis cruciata de Vr. of our Dutch dunes, as 

 well as of their hybrids, made with the purpose of studying the 

 behavior of the mfr/ato-charaeter in crosses. In one of these 

 cultures I unexpectedly obtained two mutants, which because of 

 their similarity to corresponding variants derived from O. 

 Lamarckiana have been called 0. biennis nanella and 0. biennis 

 semi-gigas. The first mutant. (). biennis uandla. occurred in the 

 second generation of the cross 0. biennis X 0. biennis cruciata 

 and differed from 0. biennis in all those points which separate 

 0. Lamarckiana nanella from 0. Lamarckiana. The other vari- 

 ant, 0. "biennis s< m i-gigas. appeared in the second generation of 

 the reciprocal cross, O. biennis cruciata X 0. biennis, suggesting 

 immediately by its much more vigorous habit and especially by 

 the larger size of its buds and flowers the differences between 0. 

 Lamarckiana and 0. (jitjas. A count of its diploid number of 

 chromosomes proved it to deserve the name semi-gigas, 21 

 chromosomes being shown by nuclear plate-stages in the meris- 

 tematic tissue of young buds. From these facts, showing that 

 O. biennis is in a mutating condition, I drew the conclusion that 

 the phenomenon of mutation in the genus (Knotbera is older than 

 the species 0. Lamarckiana — (). bioinis generally being consid- 

 ered to be an older species than 0. Lamarckiana — and further, 

 that the mutations in this group can not be the result of hybridi- 

 zation, as was assumed by some authors at that time — nobody 

 doubting of the purity and constancy of (). biennis. As a mat- 

 ter of fact, both of my mutants have been derived from crosses 

 between <). biennis and O. biennis cruciata. But I laid 

 emphasis on the fact that 0. biennis and 0. biennis cruciai 

 exactly the same germinal constitution except for the facto 

 determine the shape of the petals, 0. biennis cruciata bein< 



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