CHAPTER 5 

 THE GENUS DRTOPTERIS* IN BRITAIN 



Proceeding from the single species, or, as it has turned out species complex, to the 

 wider background of the genus, the other British members of Dryopteris may next claim 

 our attention. But here the diversity of forms is such, even in the limited flora of one 

 small island, that a simple narrative will at times have to give place to something 

 resembling a catalogue in which the logical coherence is supplied only by the fact that 

 each of the species listed has, until quite recently, been regarded as sufficiently akin to 

 all the others to be classified with them. 



At first sight the dozen or so British species of Dryopteris (sensu lato) fall into two rather 

 distinct groups. On the one hand there are about eight species of the 'Buckler Fern' 

 type with kidney-shaped indusium, formerly placed in the genus Lastrea (or sometimes 

 Mephrodium or Aspidium). On the other hand, there are the three species with naked 

 sori and small creeping rhizomes popularly known as the Oak Fern, the Beech Fern, 

 and the Limestone Polypody. These are very different in habit from the British species 

 of ' Lastrea ' and were all formerly placed in the composite genus Poly podium owing to the 

 absence of an indusium to their sori. The inadequacy of this negative feature as the 

 basis of a genus has, however, been generally recognized for some time, and though 

 it is not alv/ays easy to determine the affinities of non-indusiate species with certainty, 

 it was Bower's (1935) opinion that the nearest indusiate relatives to the three species in 

 question were in Dryopteris. 



That this simple description of the genus is an incomplete picture has, however, been 

 shown with surprising clarity by the cytological results. And since it will be necessary to 

 call attention on several occasions to the need for taxonomic revision, not of species 

 only but also of the genus, it may be well in the first instance to disregard all previous 

 impressions and deal with each species categorically in the order that its chromosomes 

 suggest. 



Dryopteris aemula (Ait.) O. Kuntze {Nephrodium foenisecii Lowe) makes an appropriate 

 starting point. This very characteristic and beautiful species (Fig. 50) has a markedly 

 Atlantic distribution, being far more abundant in Ireland than in England, where it 

 is commonest in the west and south-west. I have investigated plants from Cornwall 

 and from Ireland with identical results. 



Meiosis in Dryopteris aemula as seen in a section is reproduced in Fig. 7 1 b, but the 

 details of the chromosome count are better displayed in a 'squash' preparation as may 

 be seen in Figs. 51 and 53a. The chromosome number in both is exactly n = /\.i. This 



* The classification adopted at the beginning of this chapter is that of Druce's Comital Flora (1932), 

 which follows Christensen's Index Filicum. Recommendations on generic subdivisions to suit the cytology 

 will be found at the end, and it is perhaps only fair to state that some of these have already been 

 introduced on other grounds by systematists (cf. Clapham's List 1946). 



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