Leaf gigantism is an example of marked morphological variation associated with abrupt environmental gradients of increasing coastal exposure. This study characterises the morphology and anatomy of leaf gigantism in four species across two habitats on the coastal headlands of the Tasman Peninsula, Tasmania, Australia. In addition, the genetic basis and adaptive significance of leaf gigantism are examined. Leaf gigantism was characterised in Leptospermum scoparium, Acacia verticillata and Allocasuarina monilifera by greater thickness and succulence, and by greater thickness and increased support tissue in Allocasuarina crassa. Glasshouse-grown seedlings of each species derived from exposed and inland field sites showed that leaf gigantism has both genetic and environmental components. Leaf succulence and a slower growth rate were shown to be heritable in seedlings from the exposed site of L. scoparium and A. verticillata, indicating genetic differentiation. In the reciprocal translocation trial, the higher degree of stress tolerance (as measured by chlorophyll florescence) exhibited by seedlings of L. scoparium and A. verticillata from the exposed site demonstrated the adaptive significance of leaf gigantism in these species. The ecological and evolutionary implications of leaf gigantism on the Tasman Peninsula are discussed.