SUMMARYThe distribution of the white leaf mark polymorphism of Trjfoliuns repens L. was studied in a lowland pasture in North Wales and in a series of 1 m' quadrats elsewhere in Britain. Each quadrat was subdivided into 10 x 10 cm quadrats to allow the study of pattern. The number of morphs found in a 10 cm square was in the range b5-60 with a mean value of 416. In a 5-year-old seeded ley the mean was 275. The populations consist of a large number of morphs intimately mixed. The frequency distribution of morphs per quadrat differs strikingly from the frequency distribution of species suggesting that some selective force operates to prevent any morphs from dominating the sward even on a local scale. The frequency of morphs bearing leaf marks decreased with increasing grass length suggesting a relationship with grazing. There is a seasonal pattern in the expression of the leaf mark phenotypes and the floristic diversity of such a pasture is therefore represented at three levels, (a) the species present, (b) the genotypes present within each species and (c) the various phenotypic expressions of these genotypes.

SUMMARYIt was suggested that the leaf mark polymorphism of Trfolium repens might be important in visual selection of diet by sheep. Sheep with rumen fistulae were allowed to graze in restricted areas of permanent grassland and the morph frequency in the rumen was compared with that on offer in the pasture. There was clear preference by the sheep for unmarked leaves and some evidence that suggests apostatic selection between the various forms of mark.