THE GENUS DRTOPTERIS IN BRITAIN 



spores are commonly shrivelled, though it is not known whether a few might be viable. 

 The general appearance of mixed populations suggests strongly that this is probably 

 the case, and it is much to be desired that breeding work should be undertaken to 

 test this point, preferably after resynthesis of the hybrid from known parents. 



The hybrid between D. spinulosa and D. cristata is better known than the one last 

 mentioned perhaps, paradoxically, in part because of its greater rarity, its frequency of 

 occurrence being necessarily hmited by that of Z). cristata, the scarcer of its two parents. 

 The morphological differences between D. cristata and D. spinulosa are greater than those 

 between the latter species and D. dilatata, so that it is not surprising to find that the 

 intermediate characters of the hybrid are sufficient to endow it with a marked indivi- 

 duality of its own. This led at an early stage to its recognition in the field as a distinct 

 morphological type to which the name D. uliginosa (Newman) Druce has been given. 



Meiosis in D. uliginosa is shown in Fig. 570, and the close similarity between this and 

 the preceding hybrid is very evident. 



The interpretation of the spinulosa complex on the basis of all this information would 

 therefore appear to be that we have here an old polyploid ' coenospecies ' * which 

 now contains three ' ecospecies ' * adapted to different degrees of waterlogged soil. 

 These have spread over a much larger geographical area than that so far colonized by 

 the relatively young Male Fern, and D. spinulosa, D. dilatata and D. cristata are wide- 

 spread in North America as well as in Europe and Asia, whereas the Male Fern itself 

 seems to be a characteristic occupant of the Old World only. The three ecospecies are, 

 however, still sufficiently akin to hybridize when they meet and such hybrids, though 

 highly sterile, nevertheless show sufficient proportion of pairing chromosomes to 

 demonstrate a fairly high degree of chromosome homology to be present among the 

 three ecospecies. Whether these themselves arose suddenly or by gradual accumulation 

 of mutational differences is, however, entirely unknown. 



This is as far as the study of the D. spinulosa complex would have gone but for the 

 evidence of 'Z). remota\ to which problematical plant we may now turn. D. remota 

 is a putative species hybrid, thought to combine the characters of D. spinulosa and 

 D. Filix-mas. It is listed in the British Flora (e.g. Babington, 1922) on the basis of one 

 plant which was once found (by Huddard in 1867) in Brathay Wood on the shore of 

 Lake Windermere, and thereafter exterminated in the wild state though maintained and 

 multiplied by vegetative means m cultivation. It was much prized by the amateur fern 



* We can scarcely do better here than to quote the definitions of terms given by Clausen, Keck and 

 Hiesey (1945), for the apphcation to experimental taxonomy of the ecological concepts originally intro- 

 duced by Turesson. The chief of these are : 



'(i) Ecotype (Turesson, 1922a, 19226): all the members of a species that are fitted to survive in a 

 particular kind of environment within the total range of the species. 



(2) Ecospecies (Turesson, 19226, 1929): all the ecotypes genetically so related that they are able to 

 exchange genes freely without loss of fertility or vigour in the offspring. 



(3) Coenospecies (Turesson, 19226, 1929): all the ecospecies so related that they may exchange genes 

 among themselves to a limited extent through hybridization.' 



It will probably already be apparent that while knowledge of the Pteridophyta is only rarely sufficiently 

 advanced to deal in ecotypes, both of the other concepts will be found from time to time to coincide with 

 the taxonomic species. 



71 



