136 



ELIZABETH W. DAVIES 



related to ecolog\0 can be correlated with this fact. The isolated populations throughout 

 the British Isles show minor differences, which are retained in cultivation; the most 

 striking examples of such isolated populations are some of the forms which are found in 

 Ireland and the Baltic Islands of Gotland and Oland, for these plants are almost worthy 

 of recognition as distinct taxa. This continued isolation is prjDbably the reason for the 

 existence of a form on Shapwick peat moor, Somerset, sufficiently different to be considered 

 a variety (C. serotina var. cyperoides) . Further results of this isolation are possibly to be 

 seen in the origin of C. scandinavica and C. viridula Michx. (Davies 1953 d). These two 

 plants, which are rather similar to C. serotina but have different and more limited geo- 

 graphical distributions, have only recently been considered distinct species. They may 

 have been evolved within comparatively recent times owing to geographic separation. 



6. Conclusion and SltvOviary 



A c>1;ogenetical surv^ey of the C. flava aggregate (C. flava L., C. lepidocarpa Tausch, 

 C. demissa Hornem., C. serotina Merat, C. scandinavica E. W. Davies) is made, and the 

 origin and evolution of the species discussed. 



These five members of section Extensae and C. mairii are found to form a short 

 aneuploid series, with their haploid numbers ranging from n = 30 to n — 35. 



The interspecific hybrids between the members of this group, with the exception of 

 C. demissa, were found to be rather uncommon. This seems to be due to the different 

 and specialised ecological requirements of the species, topographic separation and the 

 late flow^ering season of C. serotina. Hybrids were examined and synthesised whenever 

 possible, and in every case the pollen showed at least 20 per cent fertility; this increased 

 when the parent species had the same or nearly similar chromosome numbers. 



Thus these five closely related species, which are capable of considerable gene-flow 

 from one to another, and produce natural and artificial hybrids v,"ith between 20 and 

 30 per cent good pollen, are regarded as ecospecies and the group as a whole as one 

 coenospecies. 



C. flava seems to be a relict and stable plant with an isolated and scattered distribu- 

 tion. C. lepidocarpa, with its three subspecies, namely subsp. lepidocarpa and subsp. 

 scotica, which form a distconinuous topocline in the British Isles, and subsp. jemtlandica, 

 so far only known from Scandinavia, is more widespread. 



The abundant C. demissa appears to be a variable plant, but the range of variation 

 is continuous, and within the limits of one adaptable species. In contrast, C. serotina, 

 with its local and fragmentary distribution, has a number of forms which, owing to long 

 isolation, seem to give rise to varieties, such as var. cyperoides, and eventually species ; 

 such is probably the origin of C. scandinavica and C. viridula. 



7. Acknowledgments 



I have pleasure in recording my gratitude to Professor T. G. Tutin for his advice 

 and encouragement throughout this work. I should also like to express my thanks to 

 the Research Board, University College, Leicester, for the award of a scholarship during 

 the period 1950-53, and for giving considerable financial help towards my field studies 

 during this period. 



REFERENCES 



ANDERSON, E., 1949, Introgressive hybridization. New York. 



BAKER, H. G., 1948, Stages in invasion and replacement demonstrated by species of Melandrium, /. EcoL, 

 36, 96-110. 



, 1951, Hybridization and natural gene-flow between higher plants, Biol. Rev., 26. 



CLAUSEN, J.. KECK, D. D. & HIESEY, W. M.. 1939, The concept of species based on experiment, 

 Amer. J. Bot., 26, 103-106. 



