CHAPTER 9 

 APOMIXIS 1 



Apomixis maybe defined (Winkler, 1908, 1934) as the substitution 

 for sexual reproduction (amphimixis) of an asexual process which 

 does not involve any nuclear fusion. For the sake of convenience 

 it may be subdivided into four classes. In the first, or nonrecurrent 

 apomixis, the megaspore mother cell undergoes the usual meiotic 

 divisions and a haploid embryo sac is formed. The new embryo 

 may then arise either from the egg (haploid parthenogenesis) or 

 from some other cell of the gametophyte (haploid apogamy). 

 Since the plants produced by this method contain only a single set 

 of chromosomes, they are usually sterile, and the process is not 

 repeated from one generation to another. 



In the second or recurrent type of apomixis, the embryo sac may 

 arise either from a cell of the archesporium (generative apospory) 2 

 or from some other part of the nucellus (somatic apospory) . There 

 is no reduction in the number of chromosomes, and all the nuclei 

 of the embryo sac are diploid. The embryo may arise either from 

 the egg (diploid parthenogenesis) or from some other cell of the 

 gametophyte (diploid apogamy). 



In the third type, whatever the method by which the embryo sac 

 is formed and whether it is haploid or diploid, the embryos do not 

 arise from the cells of the gametophyte but from those of the nu- 

 cellus or the integument. This is called adventive embryony or 

 sporophytic budding. 3 Here we have no alternation of generations, 

 as the diploid tissues of the parent sporophyte directly give rise to 

 the new embryo. 



In the fourth type the flowers are replaced by bulbils or other 

 vegetative propagules which frequently germinate while still on 



1 For a more detailed treatment of apomixis, see Gustafsson (1946, 1947 a, b). 



2 Some authors (see Gustafsson, 1946) call it "diplospory." 



3 Winkler (1934) regards this as only a special form of vegetative propagation, 

 but, as Gustafsson (1946) points out, the morphological and physiological char- 

 acters of adventive embryos do not support this view. 



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