THE GENUS DRTOPTERIS IN BRITAIN 



to prove in my earlier papers on Dryopteris, that large genus of ferns may be divided 

 into a number of well-defined subgenera, and in the first part of the present monograph 

 I have referred the tropical American species . . .to ten subgenera. Since that part was 

 published I have examined about 3000 specimens of species with more divided leaves, 

 and these later studies have confirmed that my classification is a natural one. I have 

 no doubt that those species which are grouped together within the same subgenus 

 belong together genetically, while, on the other hand, they are very remotely 

 related to species belonging to other subgenera. I am convinced that most, if not 

 all, of the defined subgenera really are good genera such as genera are commonly 



understood.' 



A further stage in the subdivision of Dryopteris on these lines is contained in Ver- 

 doorn's Manual of Pteridology published in 1938, in which Christensen himself separates 

 a much-reduced genus Dryopteris, with D. Filix-mas as its type species, into a separate 

 tribe from the genus Thelypteris with the Marsh Fern ( T. palustris) as its type species. 

 With the Marsh Fern is placed the Mountain Fern under the name T. Oreopteris, and 

 also the Beech Fern, which will be dealt with below. In Christensen's opinion (Ver- 

 doorn's Manual, p. 543) the old genus contains 'at least two phyletic lines which 

 have arrived at, or perhaps preserved the same soral condition, but as to anatomical 

 structure. . .have followed different lines'. A still more extreme view is expressed by 

 Holttum, whose Revised Classification of the Leptosporangiate Ferns (1947) appeared during 

 the writing of this chapter. In this he remarks that 'it seemed to me that too little stress 

 had been laid on the differences of the Thelypteris group of genera from the Dryopteris 

 group'. In Holttum's view Thelypteris is likely to have been descended from a Cya- 

 theaceous or Gleicheniaceous ancestor, as Bower believed to be the case with the whole 

 'genus', but Dryopteris in the narrow sense he would relate to Dennstaedtia. Copeland's 

 views (1947) are not dissimilar though his nomenclature is somewhat dififerent. 



These expressions of opinion are perhaps enough to warn us against wasting time at 

 this stage in premature discussions as to what nuclear condition can have been primitive 

 in 'Dryopteris' as a whole, and we need only record the fact that the Marsh Fern and 

 the Mountain Fern are as diflTerent from all the species previously considered in their 

 chromosome numbers as they appear to be in their other characters. 



Christensen's mention of the Beech Fern as a possible relative of Thelypteris rather 

 than of Dryopteris in the narrow sense may serve to introduce the last three species with 

 which this chapter will be concerned. These are, as explained at the beginning of the 

 chapter, the Beech Fern, the Oak Fern and the Limestone Polypody. So confused is 

 the Latin nomenclature of these very distinct and well-known species that for once the 

 English names may be preferred. The naked sorus, which they all share, has not 

 confused the specific identity of any of them, but it has led to considerable doubt 

 regarding the generic affinities which are by no means yet resolved. The old solution, 

 to put them all in the genus Polypodium along with the common Polypody itself 

 {P. vulgar e), has by common consent now been abandoned. To assimilate them all into 

 Dryopteris is, however, also unsatisfactory now that the polyphyletic nature of that 

 •genus' has been made clear. The other akernatives can perhaps better be discussed 

 after the cytology has been examined. 



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MFC 



