CHAPTER XXIV 

 APOMIXIS AND RELATED PHENOMENA 



The many known kinds of reproduction in organisms are ordinarily 

 grouped in two main categories, sexual and asexual. In the latter cate- 

 gory are several modes which seem clearly to represent aberrations or 

 derivatives of the sexual process in that they involve the structures 

 commonly concerned in sexual reproduction. Often there is no sexual 

 fusion whatsoever, in which case the reproductive process is referred 

 to as apomixis. This is in contradistinction to amphimixis, in which 

 the new individual arises from the fusion product of two gametes. In 

 many organisms apomixis recurs regularly in successive life cycles owing 

 to other compensatory processes. In other instances it may occur occa- 

 sionally without leading to the establishment of apomictic races. 



In this chapter attention will be devoted chiefly to apomixis in its 

 two principal forms, parthenogenesis and apogamy. The first of these 

 occurs in both plants and animals, while the second, as well as a related 

 condition known as apospory, occurs in plants only.i 



PLANTS 



The life cycle in vascular plants, bryophytes, and certain thallophytes 

 is characterized by the alternation of two generations differing in chromo- 

 some number. This is not, however, true of all cases. Many plants are 

 now known in which the life cycle shows the two generations as usual 

 but without any changes in the number of chromosomes. This must be 

 borne in mind in reading the following pages. 



Apomixis. — No sexual fusion occurs. 



A. Parthenogenesis. — The development of a sporophyte from a 

 female gamete (or sometimes a male gamete?) without syngamy. 



1 The classification employed here for apomixis in plants follows, in the main, that 

 drawn up by Winkler (1908, 1920) with certain modifications. The classification for 

 animals resembles that of Prell (19236). To aid the student in interpreting the con- 

 fused terminology in the literature, the terms of other authors are included in brackets 

 under each heading. It is because of complications introduced by heteroploidy that 

 we substitute the words "reduced" and "unreduced" for the conventional terms 

 "haploid" and "diploid" ordinarily used in such classifications. For discussions 

 of the phenomena treated in this chapter, some of them with classifications of cases, 

 see Winkler (1908, 1920), Strasburger (19096), Ernst (1918), Renner (1916), M. 

 Hartmann (1909), Prell (19236), Vines (1911), P. Hertwig (1920a), Tischler (1921- 

 1922), Ankel (1927), Rosenberg (1930), and Darhngton (1932a). 



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