EFFECTS OF EXPOSING GERM-CELLS TO RAYS OF RADIUM 253 



*' Oenothera^'' * amnwpkila into characters of O. biennis. But where a 

 seed-mutant of biennis, from carefully guarded seed, produced by a 

 pedigreed plant, gave a bud-sport bearing the characters of the an- 

 cestral type or true biennis, the significance of the bud-sporting as 

 pointing to the hybrid nature of the plant seems to be excluded. 



However, in the cases of the morphologically asymmetrical plant 

 and the plant with two diverse shoot-systems we know that, in the 

 first case the sperm-cell and, in the second the egg-cell were sub- 

 jected to a treatment (exposure to radium rays) which has the power 

 to affect marked change in the chromatin, as is shown in Chapter 

 XVII. We further know that such exposure has, in another experi- 

 ment been followed by the appearance of a plant similar to a type 

 that has previously appeared by spontaneous mutation. 



If one of the altered gametes takes part in an act of fertilization, 

 the resulting zygote, as already suggested (p. 245), is fundamentally 

 of hybrid nature ; as truly so, indeed, as when, without experimental 

 treatment, a female gamete of one elementary form is fertilized by a 

 male gamete of another. Whether the characters, represented poten- 

 tially in the gametes by certain factors, have previously found ex- 

 pression in a zygote in the direct ancestral line is immaterial, and, 

 for the purpose in hand, of wholly secondary importance. A zygote 

 is a hybrid, not because the parents of the fusing gametes are differ- 

 ent, but because the gametes themselves differ. A mature plant of 

 a mutant of O. biennis has been known to bud-sport into the parental 

 ancestral form. If this mutation had taken place in one of the sperm- 

 cells of a pollen-grain of this mutant, instead of in the primordium 

 of a bud, and if subsequently a "true" biennis egg had been ferti- 

 lized by that sperm-cell, the resulting zygote would, in reality, not 

 have been a hybrid f at all, though its parents were distinct elementary 

 species. Such a zygote might, however, be classed as a " crypto- 

 hybrid " of Tschermak.^* At any rate we see that the two cases of 

 the bud-sporting of Oenothera ammophila into a true biennis branch, 

 and the production, by a seed-mutant of Onagra biennis, of a bud- 

 sport bearing the ancestral characters, are capable of the same in- 

 terpretation, viz., that the sporting plant in each instance was, in 



*This species {Oetiotkera am7nophila Focke, Abh. Nat. Ver. Bremen i8 : 182. 

 1904) is closely related to Onagra biennis, but does not appear to have received a 

 binomial name in Onagra, and it does not seem advisable to rename it here. 



tin the customary sense of the term. 



