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INTRODUCTION TO CYTOLOGY 



(like the mother) or patromorphic (Uke the father), according to the 

 derivation of their genetic factors ;^^ hence two main processes may be 

 distinguished: pseudogamy and androgenesis. 



A. Pseudogamy. — The development of metromorphic offspring 

 induced by pollination, but without complete syngamy. The possible 

 explanations are several: 



1. Offspring with the reduced chromosome number may arise by (a) 

 reduced parthenogenesis, the male gamete not entering the egg; or 

 (6) gynogenesis, the male gamete entering the egg, but playing no further 

 role; or (c) reduced apogamy. 



Fig. 228. — A-C, gynogenesis in Solanum nigrum. A, embryo sac, with curved male 

 nucleus near egg nucleus. B, male nuclear matter degenerating in egg. C, young embryo, 

 with male matter still evident in upper cell. (After C. Jdrgensen, 1928.) D, E, apospory 

 in angiosperms. Z), nucellar cell {Ap) beginning development of aposporous embryo sac 

 in Hieracium flagellare. E, aposporous embryo sac (Ap) outgrowing a normal one (e) 

 in H. excellens. (After Rosenberg, 1930.) 



2. Offspring with the unreduced chromosome number may arise by (a) 

 reduced parthenogenesis followed by chromosome doubling; or (6) unre- 

 duced parthenogenesis; or (c) unreduced apogamy; or (d) sporophytic 

 budding. 



Examples of some of the above modes of behavior are the following: 

 Datura stramonium 9 X D. ferox cf yielded a metromorphic monoploid 

 plant, in all probability as a result of reduced parthenogenesis (explana- 

 tion, la) (Belling and Blakeslee, 1927). Solanum nigrum 9 X S. 

 luteum cT yielded a few plants with the maternal gametic chromosome 

 number, and cytological study showed that the male nucleus degenerates 

 after entering the egg (explanation, lb) (Fig. 228, A to C) (J0rgensen, 

 1928). Fragaria vesca 9 X F. chiloensis cf gave metromorphic offspring 

 with the maternal zygotic chromosome number (probable explanation, 



" Millardet used the term "false hybrids" for actual hybrids which were strongly 

 metroclinous or patroclinous, but Renner (1929) and Kuhn (19306) employ it only 

 for individuals arising apomictically. We follow Kuhn in using the terms pseudo- 

 gamy (Focke, 1881) and androgenesis (Kuhn). 



