APOGAMOUS FERNS. EVOLUTION OF THE SEPARATE SPECIES 



since polyploidy as such is by no means confined to apogamous ferns; but that triploidy 

 may perhaps in some way predispose to the type of unbalance required is nevertheless 

 perhaps indicated by the remarkable preponderance of triploids among the species 

 analysed. No reason for this can at present be advanced, but the fact gives emphasis to 

 the general conclusion that, at least as regards the ferns, hybridization is (as was first 

 suggested by Ernst in another connexion) a cause and not merely the occasion for the 

 manifestation of apogamy, whatever the physiological mechanism may be. 



One last conclusion is perhaps worthy of comment. The whole phenomenon of 

 apogamy of the type under discussion is a complicated departure from the normal which 

 is nevertheless repeated with almost monotonous identity of detail in every case which 

 has arisen. The somatic organization and normal development of sexual ferns is pre- 

 sumably such as to lend itself rather easily to this particular innovation, but whatever 

 may be the facts regarding this, a more striking example of parallel evolution would be 

 hard to find. This is of some importance, since we have already had occasion in 

 Chapter 6 to comment on the evidence for parallel evolution in the developmental 

 changes affecting soral structure and the form of the indusium. We seem forced to 

 conclude that wherever (for reasons known or unknown) certain types of change can 

 occur more easily than others, they will make their appearance repeatedly as long as 

 the structural conditions facilitating them prevail. This is perhaps the reason for the 

 taxonomists' perennial difficulties in tracing phylogeny and why, in the well-known 

 phrase, a phyletic 'tree' so often resembles less a trunk with branches than a bundle 

 of sticks. 



SUMMARY 



The following list summarizes the basic facts for each species mentioned in the chapter, 

 together with the country of origin of the actual material used: 



Grade of 

 polyploidy 



Diploid 

 Triploid 



Tetraploid 



Pentaploid 

 Incertae sedis 



Species 



Pteris cretica L. 

 Dryopteris Borreri Newm. 



Chromosome 

 no. 



58 

 82 



Pteris cretica L. var. albolineata c. 90 



Hook. (probably 87) 



Dryopteris Borreri Newm. 123 



D. remota (A.Br.) Hayek 123 



D. atrata (Wall.) Ching 123 



Cyrtomium falcatum (L.f.) Presl 123 



C. Fortunei ].Sva. 123 



C. caryotideum (Wall.) Presl 123 



Asplenium monanthes L.* 108 



{?)Pellaea atropurpurea (L.) Link 87 



Pteris cretica L. 



Country of origin 



Italy 



Britain and Switzerland 



Ceylon and Hort. 



Britain, Switzerland, Ger- 

 many, Norway 

 Britain, central Europe 

 Hort. (probably China) 

 Hort. 



Hort. and China 

 Uganda 

 Madeira 

 California 



Hort. and Uganda 

 Britain 

 Britain 



Britain, Sweden 

 * Information added in proof on new material from Madeira. 



c. 120 

 (probably 116) 

 Dryopteris Borreri (secondary 1 64 



hybrids) 

 Dryopteris Borreri (secondary 205 



hybrids) 

 Phegopteris polypodioides Fee 90 



Derivation 



Probably hybrid 

 Certainly hybrid 





Probably triploid 

 Double the diploid 



Hybrid with D. 

 Filix-mas 



13-2 



