Patterns of growth of clonal plants vary widely among and within species. The combination of developmental constraints and phenotypic plasticity interact, producing modified clone structures under different conditions. The resulting physical structure of clones influences resource exploitation, competitive interactions, and reproduction, among other processes. Iris lacustris produces a rhizome that generates a distinctive swollen apical region during each year of growth. This record of growth, flowering and branching can be used to reconstruct the growth of the clone over a period of 20 or more years. The structure of I. lacustris clones was examined to determine whether rhizome elongation, flowering and branching frequency vary across habitats in ways that might influence relative vegetative and sexual reproduction in this threatened species. I hypothesized that flowering frequency and associated branching would be greater in a sunny habitat than in a shady habitat, leading to a greater density of flowers and ramets in sunny sites. In two contrasting habitats supporting I. lacustris, annual growth of rhizomes was greater and flowers and branches were produced much more frequently in the sunny site than in the shady site. Flower development and branch formation were developmentally linked in both sites. Branch angle and survival of new branches were similar in the sunny and shady sites. The main structural effects of habitat differences, therefore, were on rhizome elongation and flower initiation, with concomitant lateral shoot formation, rather than on survival of new shoots. Under shady conditions, I. lacustris exhibited reduced sexual and asexual reproduction due to reduced initiation of flowering and vegetative shoots.

Key words: architecture, clonal, development, growth, Iris lacustris, reproduction