EMBRYOLOGICAL AND FLORAL STUDIES IN WEDDELLINA SQUAMULOSA TUL. (PODOSTEMACEAE, TRISTICHOIDEAE)

The embryological developmental pattern of the aquatic Podostemaceae i s marked by characteristic deviations from the usual pattern of angios perms. Reduced types of embryo sac development and a nucellar plasmodi um are of particular interest. The present study of Weddellina squamul osa Tul. deals with the development of the male and female gametophyte , the nucellar plasmodium, embryogenesis and seed structure. A reinves tigation of embryo sac development reveals a five-nucleate 'Apinagia t ype' (form B1 after Battaglia, 1987) of embryo sac. The embryo sec is monosporic in origin. The description of a bisporic embryo sac develop ment ('Podostemum' type) in Weddellina squamulosa (Chiarugi, 1933) is refuted. The developmental pattern of the nucellar plasmodium represen ts a third made not otherwise found in Podostemaceae. Its generation b egins from the central row of the nucellus with the outer layer kept i ntact. The cells of the outer layer disintegrate later. Thus, the nuce llar plasmodium seems to be established only by the outer nucellar lay er. Embryogenesis conforms to those of other Podostemaceae. Microspore and pollen development show no deviations from the usual angiospermou s pattern. Pollen is a monad and shed in the two-celled stage. Seed se t of 100% is possibly due to autogamy during anthesis. The structure o f the gynoecium in Weddellina squamulosa differs from other Podostemac eae. The bicarpellate ovary consists of four sections. The main part, above an ascidiate region, is the syncarpous zone with an axile placen ta, superimposed on this is a paracarpous region that lacks a septum. The uppermost section develops an additional 'apical septum', a mode s tructurally connected with anacrostyly. This brings about a slender (c ompound) style (instead of stylodes as in other Podostemaceae) and a g lobular stigma. New data favour a re-establishment of the former subfa mily Weddellinoideae. (C) 1997 Elsevier Science B.V.