APOGAMOUS FERNS. EVOLUTION OF THE SEPARATE SPECIES 



fornia and communicated to me by the kindness of Mr Alston of the British Museum. 

 In this case the internal facts are easily ascertained, but their interpretation is hampered 

 by the absence of any other information re- 

 garding related species. The form of the leaf 

 of the specimen used is shown in Fig. 191, 

 and the chromosomes in a root, an eight- 

 celled sporangium and a sixteen-celled sporan- 

 gium, are shown in Figs. 192 and 193. The 

 chromosome number is approximately (and 

 perhaps exactly) 87, and pairing in the 

 sixteen-celled sporangia falls into approxi- 

 mately equal numbers of pairs and univalents. 

 Both this pairing and the chromosome number 

 itself are strongly suggestive of another triploid, 

 although acceptance of this interpretation 

 requires some independent evidence that a 

 monoploid complement of 29 exists in this 

 group. If it does the pairing could be regarded 

 as that of a backcross between an allotetraploid 

 and one of its diploid parents, one at least of 

 which must have reproduced sexually. The 

 search for sexual species of Pellaea or related 

 genera * with the required chromosome num- 

 bers may therefore be recommended to local 

 botanists to whom these plants may be acces- 

 sible in the wild. 



Returning now to Dryopteris we have D. re- 

 mota A.Br, and D. Borreri Newm. still to 

 consider. With regard to D. remota much 

 has already been said in Chapter 5, and the 

 chief thing to recall is the hope that it will 

 shortly be synthesized. Whether or not the 

 parentage deduced for it on p. 79 is correct, 

 however, the evidence from the sixteen-celled 

 sporangia shows clearly that, as in D. atrata, 

 there is almost complete lack of homology 

 between the chromosomes of the two com- 

 ponent species. As Fig. 194^ displays, the 

 almost complete failure of pairing of all the 



chromosomes in the sixteen-celled sporangia of this species, in marked contrast to the 

 regularity of pairing in the eight-celled sporangia, is as characteristic a feature of the Irish 

 specimen, as it appears, from Dopp's description (1932), to be of continental material. 



* The existence of « = 29 in the related genus Pteris has, of course, actually been demonstrated in 

 this chapter by finding that diploid Pteris cretica has 2n = 58 (p. 174 above). 



185 



Fig. igi. Pellaea atropurpurea (L.) Link, live 

 frond of a young plant gVown in cultivation. 

 Natural size. 



