Facing the furious fifties: The contractile stem of the subantarctic megaherb Pleurophyllum hookeri

On the cool, moist, extremely windy subantarctic islands, vegetation is treeless and megaherbs, tussock grasses, and cushion plants are the dominant plant growth forms. In world terms the subantarctic environment is an unusual one, and megaherbs are unusual plants which have some unusual anatomical features. On Macquarie Island the megaherb Pleurophyllum hookeri has a vertical contractile stem, largely underground, which maintains the plant's rosette of large leaves close to the ground surface. Stem contraction is associated with reduction in cell volume and increase in wall thickness of pith cells, and greater contraction of inner cortical than outer cortical cells. The network of vascular tissue adjusts to the dimensions of the contracted stem without disruption but the outer surface of the stem becomes wrinkled. We believe this to be the first record and description of permanent contraction of extensive stem tissue in an angiosperm.