The growth of the rhizomatous perennial Acaena magellanica was studied in a variety of communities on South Georgia (54-55⚬ S, 36-38⚬ W). Maximum leaf area per shoot and leaf number per shoot were attained considerably later in the growing season than peak levels of chlorophyll per shoot. The proportional allotment of dry matter to different parts of the shoot was affected by competition and the duration of the growing season. At a snowbed site there were severe reductions in shoot dry weight and leaf number. In extensive, pure stands (with high leaf area index) short stems were produced with long leaves and the reverse was true in pioneer situations at low altitude. Regardless of flowering shoot size proportional dry matter allotment to the fruiting head remained constant at 36-39%, adjustments in the absolute weight resulting from changes in both fruit number and fruit weight. Dry weight data suggest that the final development of the fruits was made possible by translocation from the vegetative shoots. The maximum size of the flowering shoot is limited by initiation of a terminal inflorescence after the production of six or seven leaves. This is not the case in A. tenera, which has axillary inflorescences, but is hampered, when in competition with A. magellanica, by its low growth rate and branching pattern. Reasons are suggested to explain the ecological separation of the two species on South Georgia, and the success of A. magellanica in southern tundra sites. A schematic model is presented linking the growth of pioneer plants with that of established pure stands.