UNREDUCED EGG CELLS 451 



4. MEIOSIS IN RELATION TO PARTHENOGENESIS 



(i) Meiosis in Functional Cells in Parthenogenesis. Wliere the cell 

 which would normally undergo meiosis still enters into the life cj^cle 

 (as do the oocyte, the embryo-sac mother-cell or the spore mother- 

 cell in diploid parthenogenesis) it is clear that meiosis must be 

 suppressed, so far as it effects a numerical reduction of the chromo- 

 somes. The ways in which this happens may for convenience be 

 provisionally grouped in five classes, although gradations are com- 

 monly observed between some of them (cf. Ch. X). 



The first two and the fourth of these types of behaviour correspond 

 closely with the characteristic abnormalities of meiosis found in the 

 absence of paired chromosomes (Fig. 127). The third and the 

 fifth, on the other hand, are found only in conjunction with 

 parthenogenesis and cannot be derived directly from the behaviour 

 known in hybrids and autopolyploids. Their origin we shall 

 consider later. 



Side by side with these regular abnormalities others occur, 

 particularly in the spore mother-cells of aposporous plants, but 

 also sometimes in those reproducing facultatively by diploid 

 parthenogenesis. The irregularities consist in the characteristic 

 results of failure of pairing in hybrids — irregular distribution of the 

 chromosomes at either the first or the second division, and formation 

 of numerous daughter-nuclei (Artemisia nitida, Nephrodium hirtipes). 

 Diploid parthenogenesis is out of the question in the cells affected, 

 just as it would be following normal reduction, which is occasionally 

 to be observed following more regular pairing in certain plants 

 that are obligatorily apomictic {Artemisia nitida ; Ochna serrulata). 

 Regular reduction with complete pairing is always to be found in 

 those that are facultatively sexual [e.g., Elatostema sessile and 

 Allium odonim, Modilewski, 1931). Plants that are apogamous or 

 aposporous and not parthenogenetic {Hieracium pilosella, Citrus, 

 probably Rosa canina) are nearly always facultatively sexual and 

 therefore have, as a rule, regular meiosis. The Canince roses, for 

 example, have a meiosis which is not normal but which is regular 

 in effect [v. infra). 



The great range of behaviour that is possible in certain apomictic 



15—2 



