Littorella uniflora (L.) Ascherson is a small, perennial,
amphibious rhizophyte of rosette life-form which is
common along the margins of lakes, tarns and reservoirs
where water-level fluctuations are often rapid and
unpredictable. The majority of plants are continuously
submersed and reproduce vegetatively, but a small
proportion become completely emersed for variable lengths
of time, when flowering and seed set occur. To find
out how L. uniflora adjusts to sudden emersion we studied
the plants at a reservoir where water level falls each
spring and remains low throughout the summer; L. uniflora
adjusted very quickly showing a degree of phenotypic
plasticity not expected in a ‘stress tolerator’, including
the production of a new set of terrestrial leaves with
reduced lacunal volume and increased stomatal density, a
rapid increase in leaf growth rate, and flowering within
3–4 wk. Comparison of terrestrial L. uniflora with aquatic
plants growing permanently submersed in lake and tarn
habitats showed that three to fourfold more carbon (C) and
nitrogen (N) was incorporated into above-ground
biomass by emersed plants. However, ramet production in the
aquatic environment appeared to be more costly,
in terms of C and N invested, than terrestrial flower and
seed production. The combination of continuous,
submersed vegetative spread with the capacity for a high
degree of phenotypic plasticity allowing some flower and
seed production to occur during brief periods of emersion
seems to account for the success of this plant in the amphibious niche.