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SUMMARY

In the vicinity of Sheffield, natural populations of Arrhenatherum elatius exhibit continuous variation in tiller and leaf number, leaf length and tiller angle. In the associated range of morphologies, the extremes are represented by erect and prostrate plants. Field and laboratory investigations suggest that this polymorphism is genetically controlled and may be attributed to the conflicting selection pressures imposed by competition for light in tall herbaceous vegetation and by continuous defoliation in pastures.