The floral biology and seed production of Nymphoides peltata (GMEL.) O. Kuntze (Menyanthaceae)

A study has been made on the prefloral, floral and postfloral biology of Nymphoides peltata (Gmel.) O. Kuntze in a former river bed of the Waal and in two concrete tanks situated in the grounds of the Catholic University of Nijmegen (The Netherlands). The structure of the flowering stem and the flowering pattern have been described. The plant grows in such a way that a continuous field of flowers is maintained over a long period of the season. In the development from flower bud to mature fruit, eight stages are distinguished. The structure of the flower is described. The flowers of Nymphoides peltata fall within category ‘B’ of the system of H. Muller who divided the entomophilous flowers in seven categories, as they are not strongly specialized to cater for the visiting of only one insect group, while the nectar is totally hidden. The u.v. pattern can be considered a specialized feature of the N. peltata flower within the genus Nymphoides, being the same as in N. geminata (R. Br.) O. Kuntze (also a yellow flowering species), as a primitive feature the heterostytyl combined with a weak incompatibility system. Nymphoides peltata is a day-flowering species, the flowers withering within one day. Features of the flower which attract anthophilois insects are: a high flower-frequency and long flowering period within the season; the yellow coloured corolla with a characteristic u.v. pattern and an average diameter of 3.54 cm; a height, on average, 3.27 cm above the water surface; and a weak, sweet odour. The existence of higher temperatures round the flower than exist in the surrounding area perhapps plays no role as an attractive features, as the difference is small. The flowers have to offer nectar (sheltered by staminodes of 0.5–0.6 cm in height, pollen and perhaps some stigmmatic exudate. Predators are attracted by the insects present. The list of flower-visiting creatures contains 44 species, of which 43 are species of Hexapoda and 1 is a species of Aranea. The possible reasons for flower-visiting of the insects and their possible role in pollination are discussed. With respect to pollination, species of Apidae, Syrphidae and Ephydridae seem to be most important. Species of Apidae and some species of Syrphidae can reachh the nectar by using their long tongues (longer than 5–6 mm). Most species of Syrphids and all Ephydrid species use pollen and liquid substances, but their tongues are too short to reach the nectar, which contains fructose and glucose in equal amounts. Apidae and Syrphidae fly from flower to flower while the Ephydridae show an irregular pattern of flower visiting; they are very important as they are present in large numbers. In contrast to the Apidae and Syrphidae, the Ephydridae also occur very commonly on the floating leaves of Nymphoides peltata. Of the 44 species mentioned, 30 species were seen only on the flowers while 14 species were also commonly found on the floating leaves. The first category is attracted most strongly by the characteristics features of the flower, while the second category is more strongly attracted to the nymphaeid system. Each flower visitor can cause pollination, and in the case of short-styled flowers also self-pollination, by which small capsules are produced with a low number of seeds (generally below 10–15). The release of developed seeds occurs 32–60 days after the day of anthesis of the flowers. Both in the field and the concrete tanks where both floral forms occur, distinct differences in the fruit and seed production were observed. The number of fruits and the number of developed seeds per m2 was much higher in the concrete tanks due to the development of much more flowers than in the Oude Waal, but the average number of seeds per fruit was here much lower and the percentage of fruits with a number of seeds below 15, was higher which indicates that the flowers in the small isolated concrete tanks were not so efficiently pollinated as in the field, possibly because some important Ephydrid species such as Notiphila brunnipes R.-D. were not present in the concrete tanks. The seeds of Nymphoides peltata produced in the Oude Waal numbered more than 3000 per m2, which initially floating on the water surface, and then become dispersed over the water by wind, and by water birds. When the frost period begins, nearly all seeds have sunk to the river bottom.