| Summary: | A survey of Lotus corniculatus populations in the Jostedal valley of S. W. Norway has revealed that plants with cyanogenic leaves are rare in the south of the valley, but predominate in the harsh conditions which are found further north. A similar cline occurs for cyanogenesis in the petals and scoring of plants for both leaf and petal cyanogenesis has confirmed that there is a relationship between the expression of cyanogenesis in these two parts of the plant. The colour of the keel petals, a third polymorphic character, also has a clinal distribution in the Jostedalen, but its expression is not associated with cyanogenesis. Factors which might be responsible for the cline in cyanogenesis have been investigated, and we conclude that variation in edaphic conditions droughting, temperature and invertebrate herbivory are unlikely to be important. Norway lemmings (Lemmus lemmus) were present at a very high density in one of the areas where L. corniculatus is mainly cyanogenic and differential feeding by this mammal may constitute a strong selective pressure. Further field studies are required to test this hypothesis.
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