CONCLUSIONS 



(3) Allopolyploidy, i.e. polyploidy imposed upon hybridity, is surprisingly common, 

 and although, in the Pteridophyta, the best authenticated examples such as the Male 

 Fern or Polystichum aculeatum have all so far been found among ferns, it is most unlikely 

 that the phenomenon is confined to these but rather that full demonstration in the 

 microphyllous groups is more difficult. In the Polypodiaceae, however, as we have seen, 

 the evidence from the British flora is sufficiently advanced to permit, for the first time, 

 of a statistical estimate of frequency. Within the 50 per cent of polyploids which 

 characterize the whole fern flora (Table 9) we have diagnosed or given reasons to sus- 

 pect a hybrid origin in at least eleven cases, i.e. D. Filix-mas, Polystichum aculeatum, 

 perhaps the ancestor of the D. spinulosa or D. dilatata complexes, perhaps D. Villarsii, 

 at least one and possibly more species in each of Asplenium, Cystopteris and Polypodium, 

 and perhaps Woodsia alpina, Phegopteris and Dryopteris Borreri. This number is almost 

 certainly an underestimate but it amounts to about a quarter of the total flora. 



(4) Aneuploidy is also very conspicuous though less frequent. As in the Cruciferae it 

 tends to characterize the relation between genera or groups of genera rather than be- 

 tween species. A partial exception is at first sight suggested by Isoetes and Lycopodium 

 (see Chapter 15), though the true meaning of the facts here may merely be that the 

 greater antiquity of the Lycopods relative to the ferns has resulted in a shift in the 

 phyletic value of the systematic units named by taxonomists, so that a species in, for 

 example, Lycopodium, is really the equivalent of a genus among ferns. As in the Cruci- 

 ferae the numerical order of chromosome numbers involved in effective aneuploid 

 changes is far lower than those involved in polyploidy, though the actual numbers are 

 considerably above those of the Cruciferae. The commonest monoploid numbers among 

 ferns fall (Chapters 4-1 1) between 29 and 41, as opposed to 6 to 1 1 in the dicotyledonous 

 family, again perhaps the result of antiquity. 



(5) Genie mutations must also be important, perhaps indeed of primary importance, 

 since they are presumably involved as a principal factor in the formation of many, if not 

 all, of the residual half or three-quarters of total species in which allopolyploidy is not 

 involved. It may, indeed, be suspected that a most essential clue to the causal aspects 

 of evolutionary mechanism must lie in the analysis of the nature of interspecific diflfer- 

 ences between species in which no gross cytological differences can be detected. In the 

 Pteridophyta this subject is at present a closed book, and it is perhaps the largest topic 

 in which the study of this group at present lags greatly behind what has been learnt 

 from some of the more favourable Flowering Plants. 



(6) Another subject on which our knowledge is deficient is the phyletic significance 

 of chromosome shape. In the course of this book spiral structure of chromosomes has 

 been demonstrated incidentally in Hymenophyllum, Equisetum, Psilotum, Todea and 

 Leptopteris in addition to Osmunda in which it was already known, thereby increasing the 

 comparative interest of the Pteridophyta as cytological objects. The peculiarities of 

 texture encountered in the chromosomes of many of the microphyllous groups which 

 result in their very unusual shapes is, however, a new phenomenon, and one which well 

 deserves further study from the point of view of pure cytology. 



Changes of chromosome shape are less easily observed in this group than in many 

 others, though the genus Hymenophyllum would be admirably suited to their study if more 



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