SHOOT ARCHITECTURE AND BRANCHING PATTERN IN PERENNIAL HYDATELLACEAE (NYMPHAEALES)

New data are presented on shoot architecture and branching pattern in one of the two perennial species of Hydatellaceae (Nymphaeales), Trithuria filamentosa, in the first detailed comparative study that covers both development and anatomy. Perennial rhizomes of T. filamentosa bear vegetative leaves and leafless stalks of reproductive units. Rhizome growth is sympodial. Each shoot produces four to nine vegetative leaves and in its distal part one or two lateral shoots and one to three reproductive units. Lateral shoots develop in the axils of vegetative leaves. The two first leaves of each lateral shoot lie in a transversal plane, with a divergence angle close to 180 degrees; they are interpreted as prophylls. From the third leaf onward, divergence angles are close to the Fibonacci pattern. Stalks bearing reproductive units are interpreted as the uppermost lateral extra-axillary organs that continue the shoot phyllotaxic spiral; in this interpretation, the mode of insertion of reproductive units in T. filamentosa is similar to the mode of flower insertion in Nymphaea. An alternative but less plausible interpretation assumes presence of both lateral extra-axillary and terminal reproductive units.