Summary Silene dioica (L.) Clairv. was studied in several wild populations near Swansea and in an experimental population to determine the extent of differences between the sexes in floral morphology, nectar production, and insect visits to flowers, and the effects of these differences on seed set. Male flowers are larger than female flowers and are borne more densely over a longer period. Male flowers have longer and more frequently white corona scales which may mimic the projecting white styles of female flowers. Male and female flowers were observed to secrete similar quantities of nectar sugar on the first morning of opening, but female flowers secreted more than males subsequently, and the total volumes of nectar secreted by female flowers were considerably greater. Male plants varied more than females in the total quantity of nectar sugar secreted per flower. Nectar concentration was nearly always greater in male flowers than in female flowers. Bumblebees, particularly Bombus hortorum (L.), B. pascuorum (Scop.) and B. terrestris (L.), were the most important visitors to the. flowers of S. dioica in the study sites, with some honeybees [Apis mellifera (L.)], butterflies (mainly Pieridae) and hoverflies (Syrphidae). Some visitors, including those that were most numerous at some sites, made most or all of their visits illegitimately. In the field in 1980 many visitors were observed to discriminate in favour of male flowers, which were much more numerous than female flowers. In the experimental population in 1981, many visitors discriminated in favour of female flowers, which were more numerous than in the field populations but were still substantially outnumbered by male flowers. Some individual bumblebees showed a change in preferences during the day, disciminating in favour of female flowers only during the afternoon, when the nectar resources available from female flowers were proportionately greater. The total numbers of visits that were observed to female and male flowers in the experimental population corresponded closely with the total quantities of nectar sugar available from each sex. The importance of the different insect visitors and of their discrimination between sexes in pollination is discussed. Seed set did not appear to be reduced by discrimination or insufficiency of pollinator visits in closely spaced plants, but was reduced in plants isolated by distances of 15 m or greater.
SUMMARY Observations are presented on: (a) the numbers of insects foraging on Centaurea scabiosa and Centaurea nigra per available capitulum of each species; (b) the numbers of insect visits to each capitulum of C. scabiosa and C. nigra in 30 min intervals; (c) the numbers of insect visits to rayed capitula compared with ray less capitula; (d) the number of successful pollinations. The study was carried out on the Devil's Ditch, Cambridgeshire where C. nigra is rayless, and on Hod Hill, Dorset where C. nigra is rayed, with a few results from a garden trial set out in the Cambridge Botanic Garden. In the counts of insects in each area of the Devil's Ditch the numbers recorded feeding on each Centaurea species per capitulum were approximately proportional to the nectar reward of each species. On Hod Hill the number of insect visits to C. nigra plants from which rays had been removed was half to three‐quarters of the number of visits to rayed capitula. This is very similar to the difference on the Devil's Ditch between the number of visits to C. scabiosa (rayed) and C. nigra (rayless), and this difference may therefore be a result simply of the possession of ray florets in C. scabiosa but not in C. nigra. The difference between the two species on Hod Hill was greater (C. scabiosa receiving two to three times as many visits as C. nigra) although C. nigra possesses ray florets and therefore resembles C. scabiosa. Some bees were observed visiting C. scabiosa selectively on Hod Hill but none did so on the Devil's Ditch. On the Devil's Ditch both species had similar numbers of styles successfully fertilized on nearly all dates. Hod Hill on most dates a similar number of styles was fertilized in each species, but on a few dates C. scabiosa had more successful fertilizations than C. nigra. It is concluded that competition for pollinators is not a strong selective factor in Centaurea on the Devil's Ditch but may be a factor promoting the later flowering and the possession of ray florets in C. nigra on Hod Hill. Competition for pollinators may have been more important as a selection pressure in the past when less suitable habitat was available.
No abstract
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