APOGAMOUS FERNS. EVOLUTION OF THE SEPARATE SPECIES 



of the species over most of its range. Local populations of diploids * have, however, 

 been met with both in Britain and in Switzerland, the only two countries in which a 

 range of material has been studied. Tetraploids and pentaploids have so far only been 

 encountered in England and Ireland, though they will probably be detected elsewhere 

 if search is made for them. They differ from the diploids and triploids in being single 

 individuals and not local populations, and all those so far met with have suggested by 

 their appearance that they are hybrids between D. Borreri and D. Filix-mas. Such an 

 origin is in agreement with their observed chromosome numbers, since a cross between 

 D. Filix-mas and diploid D. Borreri would be tetraploid and between/). Filix-mas and 

 triploid D. Borreri would be pentaploid. That hybrids of this constitution can be syn- 

 thesized readily has already been shown by Dopp (1939), and the experiment has 

 been repeated by myself more recently though the resulting plants are still too young to 

 provide full evidence. 





:n** 



Fig. 200. Enlarged detail of chromosome pairing in sixteen-celled sporangia of Dryopieris Borreri Newm. 

 X 1000. a. Diploid with only univalents, b. Pentaploid with numerous pairs. 



Samples of comparable pinnae from diploid, triploid, tetraploid and pentaploid 

 specimens of the various types referred to above are contained in Fig. 198. The close 

 resemblance of diploid and triploid will perhaps be recognized. It is perhaps remark- 

 able that no tetraploid or hexaploid populations with the pure D. Borreri morphology 

 have yet been encountered, although a mechanism for their production (i.e. from four- 

 celled sporangia, cf. Chapter 10) undoubtedly exists. The reason for this is uncertain, 

 and further search may still perhaps reveal them. 



Chromosome pairing in the sixteen-celled sporangia of one example of each of the 

 four grades of polyploidy is shown in Fig. 199 and in greater detail in Fig. 200. In all 

 cases pairing is irregular though it is not equally so throughout. Pairs are numerous in 

 tetraploid and pentaploid, though not so complete even in the tetraploid as to suggest 



* For anyone interested in acquiring material of such local diploid populations four, which are well 

 known to me, may be listed as: (i) The Kentmere Valley near Kendal, Westmorland, both wood and 

 scree populations. (2) A wood near Bangor, north Wales. (3) A wood near Dartmouth, Devon. 

 (4) The neighbourhood of Dublin, a very golden yellow type. (Note that the tawny appearance of the 

 Dublin diploids is not diagnostic of the diploid state in other places.) 



