Field observations on the cyanogenesis polymorphism in Trifolium repens

Abstract

We surveyed a population of Trifolium repens L. polymorphic for cyanogenesis to determine whether either of two biotic factors were concerned as selective forces affecting the balance between the cyanogenic and acyanogenic forms. We found no difference between the distributions of cyanogenesis scores for uninfected leaves and for leaves infected by the fungus Cymadothea trifolii Wolf. Acyanogenic plants, however, were more heavily eaten than cyanogenic ones, the principle herbivores being the slugs Arion ater L. and Agriolimax reticulatus Muller. Selective eating of isolated plants in a herbaceous border was due both to more acyanogenic leaves being touched and to more being eaten per a cyanogenic leaf. For plants embedded in the lawn selective eating was due mainly to the latter cause. Possible explanation for the differences in degree of selective eating between lawn plants and border plants and between this survey and choice experiments are discussed.