APOGAMOUS FERNS. EVOLUTION OF THE SEPARATE SPECIES 



technical deficiency in the earlier workers. As determined by me (Figs. 187-190) the 

 chromosome number of Drjopteris atrata is indistinguishable from that of Cyrtomium 

 and is of the order of 120 on the imperfect evidence of sections and exactly 123 when 

 squash methods are available. 



Chromosome pairing in the sixteen-celled sporangia of Dryopteris atrata is shown in 

 ^^ Fig. 189 a. Unlike the case of Cyrtomium, pairs 



jJHfc are few and multivalents apparently absent; the 



"^^^ resemblance is therefore with Dryopteris remota 



jtft^^ ^»^ described by Dopp. Since from the chromosome 



^^^j^P-^^^^ - t***L number and systematic position of the species it 



^ ^■^■1^ "^ may safely be assumed to be another triploid, 



the absence of homology among the chromosomes 



^'J^^ 



D. o/ra/-^ 3/7 = c. /20 



Fig. 188. Explanatoiy diagram to 

 Fig. 187a. X 3000. 



Fig. 187. Dryopteris atrata (Wallich) Ching. Mitotic chromosome counts from section:, a, A root, 

 showing a good metaphase plate surrounded by heavily staining cytoplasmic inclusions, x 1000. 

 For explanatory diagram see Fig. 188. b. A dividing cell in a young sporangium, x 1500. 



present would seem in this case, for the first time in this chapter, to rule out the possi- 

 bility of really close affinity between the two parents. Autopolyploidy followed by minor 

 evolutionary changes cannot be wholly excluded from either Pteris cretica or Cyrtomium, 

 but in Dryopteris atrata there is no reason of any kind to bring it to mind. The species 

 here seems to be a triploid hybrid, either formed directly or by descent from some other 

 polyploid in which there is virtually no affinity between the chromosomes of the 

 hybridising parents. In this case it is therefore necessary to accept an interspecific and 

 not an intervarietal cross in its immediate ancestry. [For another similar case see foot- 

 note on Asplenium monanthes L. added in proof on p. 195.] 



Phegopteris, the Beech Fern, has already been dealt with at some length in Chapter 5, 

 and there is nothing to add to that description. The sixteen-celled sporangia in this 



183 



