POLYPODIUM VULGARE 



named by Hulten) and 'somewhere in Asia'. From these refuge areas under more equable 

 conditions our present taxonomic species have spread again, though in most cases the 

 areas occupied have not yet Hnked up with each other. An exception is, however, to be 

 found for the tetraploid of Europe which may perhaps be a fairly recent species and 

 which has certainly spread very vigorously over the heavily glaciated territories of 

 our continent. Where this new tetraploid has encountered the remnant of the older 

 population still persisting in the south it has hybridized with it, and the newest species, 

 the hexaploid, is the result. 



What are the nearest ancestors of the tetraploid or where in Europe or Asia it can have 

 arisen can only be elucidated by further inquiry. We can likewise hope to investigate 



i^ar. occ/c/en/-c7/e n ^ 37 



Fig. 143. Gnxoxao^ovn&'i, oi Folypodium vulgare Fig. 144. Explanatory diagram to 



var. occidentale from the Rocky Mountains. Fig. 143. x 1500. 



X 1000. 



the nature of the genetical differences which at present separate the various known 

 diploid species from each other. The present state of knowledge is thus clearly not the 

 end but the beginning of a problem which, if it can be unravelled, should illuminate the 

 migrations of floras, their origins and developments, not only in Europe but over the 

 whole of the northern hemisphere in the time lapse of a geological period. This hope is 

 also inherent in the problem of Cystopteris, but the additional complexities of that 

 genus may in the end impede progress irremediably. The relative simplicity of the 

 situation in Polypodium is likely to be its greatest asset, and it is no exaggeration to 

 say that even in the present state of knowledge Polypodium has turned out to be by far 

 the most interesting member of the British fern flora which we have so far encountered. 



SUMMARY 



Summarizing this chapter, it is perhaps sufficient to say that morphological and 

 cytological reasons have been given for recognizing three separate species of Polypodium 

 in Europe and at least two from the P. vulgare complex in America. A preliminary 

 discussion of geographical distribution is included. Some information regarding the 

 mutual relationships of the European species has been supplied from wild hybrids. 



141 



