POLTPODIUM VULGARE 



summer of 1948 I was able to discuss the world distribution of Polypodium with 

 Professor Hulten of Stockholm, who alone has adequate information regarding large 

 tracts of central and eastern Asia, and he has kindly given permission to publish the 

 gist of his verbal communication. This is that P. vulgare sens. lat. is to be found right 

 round the northern hemisphere in a total area which is not now continuous though it 

 may formerly have been so. The most conspicuous gaps are east of Lake Baikal in 

 central Russia, the whole of central China, central North America and the whole of 



Fig. 142. Characteristic paraphyses diagnostic oi Polypodium virginianum, after Martens (1943) 



Greenland. The primary cause of disruption of this kind is believed to be glaciation, 

 and the present distribution of diploid populations in Europe is in agreement with this. 

 If now the cytological facts so far obtained are superimposed on this general distribu- 

 tion we may interpret it as meaning that an ancient diploid stock must have spread 

 round the world, breaking up as it went into ecospecies. During periods of extermina- 

 tion some of these ecospecies survived in major refuge areas of which we may discern 

 at least four, namely, south Europe, eastern North America, western North America 

 (including perhaps ' Beringia ' as the unglaciated region of the Behring Straits has been 



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