The submerged leaves of Ludwigia arcuata are much narrower than the terrest
rial leaves. Such heterophyllous changes in leaf shape have been observed i
n other aquatic angiosperms, such as Callitriche heterophylla, Hippuris vul
garis and Ranunculus flabellaris, but the cause of the formation of heterop
hyllous leaves in L. arcuata seems to be quite different. In contrast to ot
her species, in which the shapes of epidermal cells have been changed, the
change of leaf shape in L. arcuata was found to be due to changes in the nu
mbers of epidermal cells aligned in transverse sections. The susceptibility
of leaves to changes in shape above and below the water is dependent on th
e developmental stages: leaves younger than the fourth leaf responded to a
change in environment, while leaves older than the fifth leaf did not. Trea
tment with ACC (1-aminocyclopropane-1-carboxylic acid), a precursor to ethy
lene, induced the formation of submerged-type leaves on terrestrial shoots,
implying that ethylene might be the endogenous factor responsible for the
change in leaf shape. The results are discussed with reference to the signi
ficance acclimation plants to environmental changes.