This thesis is concerned with the role of submergence in the ecology of the perennial Ranunculus repens and the annual R. sceleratus, both of which are common in seasonally submerged habitats. Evidence is presented for genetic differentiation between R. repens populations with respect to tactical changes in allocation and growth form in response to submergence. R. repens plants maintain or increase their dry weight allocation to stolons in response to submergence independent of site of origin. A decrease in flowering in submerged R. repens plants was related to a change in the positive correlation between total dry weight and the probability of flowering in these plants. The dry weight per unit length of petioles decreased in response to submergence in plants from all sites but changes in the dry weight per unit length of stolons depended on the site of origin of the plants. Plants from all sites showed plasticity in growth form with respect to stolon internode lengths but the direction of change in response to submergence depended on site. At low nutrient levels R. sceleratus plants showed increased dry weight allocation to vegetative organs in response to submergence but unchanged or reduced allocation to sexual reproductive structures. However, all changes in allocation in response to submergence disappeared at higher nutrient levels. Reductions in the number of flowers per plant in response to submergence were independent of nutrient status but plants at high nutrient levels showed no change in the number of seeds per plant due to a larger number of seeds per seed head. This may be related to the association of this species with nutrient rich habitats. A demographic analysis of R. sceleratus showed that these plants can exhibit a wide range of life history tactics. There was a trend for a longer pre-flowering period to result in lower survivorship but greater seed output. Seed germination in this species is promoted by diurnally fluctuating temperatures in both the light and dark but the promotion is greatest in the former.