APOGAMOUS FERNS. THE GENERAL PHENOMENON 



should change the eight-celled archesporium into the sixteen-celled is imperfect. 

 Metaphase starts in each of the eight archesporial cells in the usual way, the split chromo- 

 somes taking their places on the spindle (Fig. 167 a) ; but there the division ends. There 

 is no anaphase separation of half-chromosomes and cleavage of the cytoplasm is also 

 omitted. The mass of split chromosomes remains in the centre of the cell losing some- 

 thing of its regular outline (Fig. 167^) and then reverts to the resting state (Fig. 167 c). 

 As may be seen by comparison of Fig. 166 b with Fig. 167 c before and after this abnormal 

 division, the number of cells present remains unaltered, four of the eight being generally 

 contained in any one median section no matter what the plane of cutting. The nuclei 





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Fig. 166. Young sporangia of C>r;omm?n/a/ca<Mm (L.f.) Presl from sections, x 1000. Showing two stages 

 in the formation of the eight-celled archesporium, (a) the younger, (b) the older. 



of the later stage are, however, distinctly larger and their shape at first less regular. 

 In due course they become mother cells (Fig. iGyd). Meiosis is then exceedingly regular 

 (Fig. 168), every chromosome pairing with its sister half, and thirty-two large well- 

 filled spores result. An accurate chromosome count made during meiosis will, however, 

 at once show what has occurred. The number of pairs which present themselves at 

 metaphase of the first meiotic division or at diakinesis is not half that of the single 

 chromosomes in a root or other somatic cell of the parent plant but identical with it. A 

 demonstration of this has already been given for Phegopteris (Figs. 69 and 70, Chapter 5), 

 and it is one of the ways in which apogamy can be detected before the spores are sown. 

 The explanation is that the abnormal premeiotic mitosis has momentarily doubled 

 the chromosomes present and therefore meiosis merely restores the condition to 



164 



