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Summary

Centaurea nigra s.l. forms a very variable complex which has received extensive taxonomic treatment. From analysis of previous work it is shown that populations growing on chalk and limestone in association with C. scabiosa flower later than those in the more typical habitat of neutral and acid grassland. The rayed form of the capitula is dominant in these populations, whereas elsewhere unrayed capitula predominate. From observations on the relative numbers of visiting insects, competition for pollinators with C. scabiosa is proposed as a major selective factor influencing these changes.