1907T GATES— HYBRIDIZATION OF OENOTHERA MUTANTS 7 



breeding true- Ostenfeld^ first showed the presence of apogamy 

 in the genus by castration experiments. Murbeck^ has since found 

 that the egg in these cases develops the embrj^o parthcnogcnetically. 

 The absence of fertilization in some cases is thus a factor at least 

 in the peculiar form of hybridization, , Ostenfeld^ has recently 

 extended these studies to a considerable number of species. Among 

 other species he has shown by castration experiments, confirmed by 

 the cytological investigations of Rosenberg, '° that if. excellens is 

 partly parthenogenetic and partly develops normally fertiHzed eggs; 

 its pollen is also sterile. Rosenberg found in this form the interest- 

 ing condition which he calls apospory, where a cell of the nucellus 

 enlarges and obHterates the normally formed tetrad of megaspores. 



mor 



without fertihzation. More rarely apogamous embryo sacs are 

 formed, as in Taraxacum. Usually, however, normal tetrad forma- 



tilization. 



reduction of chromosomes, takes place, followed by fer- 

 Part of the plants produced in a cross are then pure H. 

 excellens produced from apogamous or aposporous cmbr}'0 sacs arid 

 part are real hybrids. 



The similarity of this form of hybrid to the O. lalaXO. Lamarck- 

 iana cross suggests the presence of apogamy in O. lata also, though 

 it is highly improbable that this can explain all the phenomena of 

 hybridization concerned. In the latter cross only the two parent 

 types are produced and the O. Lamarckiana breeds true, while in the 

 Hieracium cross several types are produced intermediate between the 



H 



determi 



8 



character of later generations, and the number of plants he has 



OsTENFELD, C. H., Zur Kenntnis der Apogamie in der Gattung Hieracium. 



Ber. Deut^rh.Bot. Gesells. 22:376-381. 1904. 



-, Weitere Beitrage zur Kenntnis der Fruchtentwickelung bei der Gattung 



Hieracium. Idem 537-541. 



9 MuRBECK, S., Parthenogenese bei den Gattungen Taraxacum und Hieracium. 

 Bot. Notiser 1904:285-296. 



Rosenberg, O., Ueber die Embryobildung in der Gattung Hieracium. Ber. 



10 



Deustch. Bot. Gesells. 24:157-161. pi. il. 1906. 



"" , Experimental and cytological studies in the Hieracia. II. Cytological 



studies on the apogamy in Hieracium. Bot, Tidssk. 28: 143-1 7°- P^^- ^~^- ^9<=>7- 



