6] FOUNDATIONS OF MOD KRN DISTRIBUTIONS 1 79 



groups of plants and, in the Angiosperms, are particularly plentiful 

 among perennial herbs. They are often more vigorous than their 

 diploid (that is, with body-cells having double the number of 

 chromosomes basic to the species or group and characteristic of the 

 reproductive cells) relatives even of the same species, and, in addition 

 to anatomical differences such as larger cells and pollen grains, may 

 show morphological deviations such as usually larger flowers and 

 coarser stems. Nevertheless it is customary, unless these differences 

 of form are striking, to keep related diploids and polyploids in the 

 same (major) species. Polyploids also exhibit a greater tendency 

 to adopt vegetative or asexual means of reproduction than related 

 diploids. Even more important from our point of view is the fact 

 that they may show very different ecological preferences and geo- 

 graphical distributions from diploids, though no definite rules can 

 safely be formulated to govern their behaviour in this respect. 



It has been widely contended that polyploids are more hardy 

 and consequently more northerly (in the northern hemisphere) and 

 high-alpine in distribution than the diploids from which they have 

 been derived. About this there is, however, no unanimity of 

 opinion — largely because there are numerous exceptions to what still 

 appears to be a distinct tendency. Suggestions that polyploids are 

 unusually prevalent in hot and dry regions and that they favour 

 coastal rather than inland areas, seem to be based on less factual 

 evidence and, indeed, to be without adequate foundation when the 

 situation is viewed on a sufficiently broad basis. There does, how- 

 ever, appear to be some tendency for polyploids, especially when 

 they have arisen through hybridization (allopolyploids), to have a 

 wider geographic range than diploids : thus of 100 examples 

 assembled by Professor G. L. Stebbins as recounted by him in his 

 book Variation and Evolution in Plants, 60 polyploids were more 

 widely and 33 less widely distributed than their diploid relatives. 

 It is thought that the proportions of polyploids showing wider 

 distributions would be higher if the examples were limited to closely 

 related pairs of entities, such as polyploids and their more immediately 

 ancestral diploid progenitors. There are also indications that poly- 

 ploids may be more prevalent in regions that were glaciated in the 

 Pleistocene than in those which were not, and in the peripheral 

 areas or near the ecological boundaries rather than towards the 

 centres of distribution of particular plant groups. This tendency 

 evidently goes hand in hand with variation, and results from the 

 fact that polyploids have changed reaction norms. As Professor 



