SUMMARYEcological studies on the cyanogenic form of Lotus corniculatus L. on the high dunes of West Holland do not support the hypothesis that grazing by rabbits is an important factor in the maintenance of the dine.

SUMMARYGenotype x environment interactions associated with temperature were shown to occur in populations of L. corniculatus. The effects were related to the genetic structure of the populations. We were able to show, however, that cold temperature does not act directly as a selective agent on the polymorphism of cyanogenesis in L. corniculatus.

Detailed ecological studies of some maritime populations of Lotus corniculatus L. have shown that the distribution of the cyanogenic form of this plant is directly related to the distribution and density of molluscs which graze selectively the acyanogenic form. This work, on a genetic polymorphism, is interpreted as giving direct evidence of chemical defence in natural populations.

SUMMARYExperiments on plants and animals from Porthdafarch, Anglesey, were designed to determine the effects of salt (both as salt spray and saline soil water), of grazing, and of trampling on cyanogenic and acyanogenic plants of Lotus corniculatus L. Two species of snails, Helicella itala (L.) and Cochlicella acuta (Muller), were found to be differential and selective herbivores of acyanogenie plants of L. corniculatus. The extrapolation of the experimental results to the field situation, previously reported, is discussed.

SUMMARYThe environmental factors influencing a steep morph-ratio dine in a maritime population of Lotus corniculatus have been studied in detail. The frequency distribution of cyanogenic plants in the dine has remained stable for 16 years, plants very near the sea being predominantly acyanogenic, whereas 200 m inland 70 per cent of the plants are cyanogenic. Analyses of the biotic, edaphic and microclimatic environment of this population showed that an exposure gradient (wind and windborne salt) and the distribution of known selective herbivores were the only factors which were consistently associated with the dine.On the basis of these results and the hypothesis that cyanogenesis is a protection against herbivores it was predicted that other sites along the coast, which showed similar environmental variation, should also show a similar distribution of selective herbivores and of cyanogenic plants. It was confirmed that at sites exposed to wind and windborne salt the selective herbivores were rare and the frequency of cyanogenic plants was low. At sites which were less exposed, the numbers of selective herbivores and the frequency of cyanogenic plants were both higher.