CHAPTER XI 



THE BREAKDOWN OF THE GENETIC SYSTEM : 



CONTROLLED 



Classification — Apomixis and the Life Cycle — Parthenogenesis — The Haplo- 

 Diploid System — Apogamy — Apospory — Adventitious Apomixis — Meiosis in 

 Functional Cells — In Non - Functional Cells — Pseudogamy — Conditions 

 Determining Apomixis — Chromosome Numbers of Apomictic Plants. 



And Earth . . . bare also the fruitless deep without the sweet union of 

 love. 



Hesiod, Theogony, 131-132. 



I. DEFINITION AND CLASSIFICATION* 

 Having considered the breakdown of reproduction inductively 

 from the great variety of unregulated examples arising chiefly in 

 experiment, we can now turn to the natural and regulated types of 

 breakdown which are included under the description of apomixis. 



Apomixis may be defined (following Winkler, 1908) as a system 

 of reproduction having the external character of sexual reproduction 

 but omitting one or both of its essential cellular processes. These 

 are meiosis and fertilisation. Clearly if only one of these is omitted 

 the irregularity cannot become a habit, and examples of such non- 

 recurrent abnormalities will be specially considered. The abnor- 

 mality is recurrent where both processes are omitted from the life 

 cycle, and this happens in species of all the principal groups except 

 the Vertebrata, Echinoderma, Bryophyta and Gymnospermse 

 (cf. Ernst, 1918 ; Winkler, 1921 ; Prell, 1923 ; Ankel, 1929 ; 

 Rosenberg, 1930). 



Where the specialised reproductive bodies, spore and egg-cell, 

 continue to function the abnormality is known as parthenogenesis. 

 An important distinction can then be made between a type of 



* The usage of terms in this chapter is that found most convenient in 

 approaching the matter from a genetic point of view. Confusion in the 

 terminology is due to writers approaching it from different points of view. 



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