Abstract
The Podostemaceae are aquatic angiosperms of the tropics and subtropics that have remarkably specialized morphology and are adapted to rheophytic habitats that extreme torrential floods. The subfamily Podostemoideae is devoid of typical shoot apical meristem and form an aggregate of leaves arising from the root. By contrast, the basal subfamily Tristichoideae has a shoot-like structure called “ramulus”. The ramulus branches several times. The new ramulus is initiated as extra-axillary buds of immediately older ramulus. After forming ramulus branches, the main ramulus become determinate in growth, associated with the loss of the apical meristem. This manner of branching system is the sympodial branching which is often found in other angiosperm shoots. The sympodial branching pattern in Tristichoideae resembles that in the subfamily Weddellinoideae. The “ramulus” of the Tristichoideae and the “leaf” Podostemoideae share an organogenetic pattern in which new organs initiate at the base of organs, irrespective of differences in the kinds of organs. Additionally, Podostemaceae has dimorphic chloroplasts in the epidermis which have not been reported in any other aquatic angiosperms. This suggests that the dimorphism represents adaptation to the unique habits of Podostemaceae.
