CHAPTER 17 



CONCLUSIONS 



The facts are now before us and though minor conclusions have been drawn as they 

 arose in chapter after chapter it is perhaps worth the attempt to stand back a little to 

 survey the whole before ending. The survey will be partial and incomplete as are the 

 facts to be summarized but within the multiplicity of details which have at times 

 perhaps seemed over elaborate a kernel of continuity can nevertheless be extracted 

 which may be worth looking for. Before doing so, however, it will repay us to forget 

 for a moment the microphyllous and ancient groups which have occupied the last few 

 chapters and to think again of the more modern ferns dealt with earlier in the book, in 

 order to consider a little more closely the British fern flora as a sample of the world's 

 vegetation. 



As we have seen, there are some 47 species of leptosporangiate ferns in the British 

 Isles, which were analysed in detail in Chapters 4-8. Polyploidy and hybridization 

 were met with so abundantly that we were at once able to conclude that the Fili- 

 cales have indeed utilized the same types of evolutionary mechanism as the Flowering 

 Plants. As the narrative unfolded in species after species, another impression was also 

 conveyed, namely that many of the polyploid changes encountered seemed to be of 

 relatively recent date and sometimes only partly fulfilled. Time after time, e.g. in 

 Polypodium, Cystopteris, Asplenium, Dryopteris, we were forced to discriminate, often with 

 difficulty, between populations in various grades of polyploidy and others, of lower 

 chromosome number, still present in the same geographical area. In many cases the 

 lineal relationship between low-numbered and high-numbered forms could be clearly 

 demonstrated though, in some, the low-numbered forms were either imperfectly known 

 or missing. Throughout, the general impression was gained as of a wave (or epidemic) 

 of polyploidy which has affected the flora as a whole and which has recently resulted 

 in the partial replacement of low-numbered species by their higher-numbered de- 

 scendants, a process which is perhaps still continuing. This confronts us at once with 

 the question as to whether the British fern flora is to be regarded as a fair and repre- 

 sentative sample of the ferns of the world or whether it is in fact only typical of the 

 vegetation of this part of Europe. 



This is where a statistical comparison with the Flowering Plants is perhaps of 

 importance. As was foreshadowed by Tischler (1935) and more recently worked out 

 in greater detail by Love and Love (1943 and 1948) there exists a statistically significant 

 correlation between latitude and frequency of polyploidy if comparisons are made 

 within the rather limited area of Western Europe.* From this, Love and Love conclude 

 that polyploidy as such is an adaptation to cold. 



* The very fragmentary information quoted by these authors from Sicily and Timbuctoo must here 

 be discounted as too unreUable to bear the weight of statistical comparison. 



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