THE GENUS DRTOPTERIS IN BRITAIN 



which was noticed in Dryopteris Borreri (Chapter 4) as indicative of obhgate apogamy. 

 In the Beech Fern its significance is no less and a chromosome count in a root, as 

 indicated by Figs. 70^ and 71^, also shows about 90 chromosomes. The meiotic 

 process, for all its apparent regularity, is, therefore, ineffectual as a means of changing 

 the nuclear content, and both morphological generations possess the same number 

 of chromosomes. By sowing the spores it is not difficult to demonstrate that the 

 resulting prothalli are indeed apogamous as anticipated. 



t 





^ Phe^opf-en's 



^^ (Bazch F(zrn) ^"\-90 



t 



«•« "* 



i ' ^*M 





|fV ***^*' Sfjl/^ 



- ♦ \ %t<^ 



Fig. 70. Explanatory diagram to Figs. 6ga and yie. 



The detection of obligate apogamy yet once more is a matter of some interest. We 

 have met it so far in D. Borreri and D. remota, but that there can be any close relation- 

 ship between either of these and the Beech Fern is quite out of the question, and the 

 polyphyletic origin of apogamy must be accepted as irrefutable. With regard to the 

 possible parentage or phyletic affinities of the Beech Fern we are wholly in doubt. 

 The chromosome number is so unlike that of any other known species of either Dryopteris 

 or Thelypteris that pending further information it seems better to suspend judgement by 

 placing the Beech Fern in a separate genus Phegopteris as has sometimes been done, 

 rather than to prejudge the position by following Christensen in uniting it with 

 Thelypteris. Its name under this treatment should then be Phegopteris polypodioides Fee. 



A similar procedure may also be advised for the remaining two species. The Oak 

 Fern and the Limestone Polypody are much alike cytologically and have a normal 



83 6-2 



