STRUCTURAL AND DEVELOPMENTAL DIVERSITY IN PODOSTEMACEAE (RIVER-WEEDS)

The structural and developmental diversity of the Podostemaceae is rem arkable. A comparison of 19 genera (of 48 genera), and 27 species (of 270 species) is presented, covering both subfamilies, the Podostemoide ae (= Pod.) and Tristichoideae (= Trist.). Assuming the Podostemaceae to be derived from a typical cormophytic plant with the classical root -shoot (CRS) model, then evolutionary dynamics of vegetative structure s in this family include: formation of green crustaceous structures as a result of dorsiventral flattening of roots, stems or both, reductio n and loss of root caps, occurrence of sticking rhizoids and often dis k-like holdfasts, and heterotopy of organ formation (from exogenous to endogenous bud formation). Some examples of structural dynamics are u nique to certain taxa, e.g. 90 degrees switch of the dorsiventrality p lane in leaves of Marathrum, Mourera, and Oserya (Pod.); positional co rrelation of dithecous leaves (i.e. leaves with two sheaths) and dicho tomous shoot branching in Podostemum and genera mentioned above (Pod.) ; occurrence of unique leaf-shoot mosaics (called ramuli) in Tristicha and allies (Trist.). Thus, the structural categories of typical flowe ring plants are transcended in the Podostemaceae due to developmental changes and saltational evolution. An overview of reproductive traits in Podostemaceae is given, focussing on characters that are useful in traditional systematics: shape, texture, and dehiscence of spathella ( Pod.); presence and shape of cupule (Trist.). Starting with neotropica l taxa showing radial flower symmetry (e.g. Rhyncholacis, Marathrum pr o parte), most Podostemoideae can be derived by reduction of the numbe r of stamens and basal fusion of the two remaining stamens. Most podos temads are wind-pollinated or autogamous. A few neotropical genera suc h as Mourera (Pod.) and Weddellina (Trist.) show polyandry, probably a s an adaptation to insect-pollination. Other taxonomically used charac ters such as pollen (dyads in many Pod.), ovules, seeds, and capsules (ribs, dehiscence) are also demonstrated and discussed. (C) 1997 Elsev ier Science B.V.