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Nine populations of Centaurium were studied at Freshfield and Ainsdale, near Southport, Lancashire, England. The two species C. erythraea Rafin. and C. littorale (D. Turner) Gilmour flowered during the same time and were found growing sympatrically in some populations. Cytological studies revealed that both species had approximately the same range of variation in their chromosome number; C. erythaea had n = 7,8,9 and 10 while C. littorale had n = 8,9, 10 and 11. These chromosome numbers are new records for these species. Breeding experiments showed that both the species were self-compatible and largely autogamous but were capable of cross-pollination. Large populations of hybrids were seen at Freshfield. These hybrids were morphologically distinct and almost exactly intermediate between the two species. The hybrids were analysed by the hybrid index method of Anderson (1936) and inspite of being fertile did not show any great variability among themselves and probably represent an F1 generation. Although C. erythaea and C. littorale were sympatric in several different kinds of habitat it was only in Pinus nigra plantations that the hybrids became established.
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