Embryological and floral studies in Weddellina squamulosa Tul. (Podostemaceae, Tristichoideae)

Abstract

The embryological developmental pattern of the aquatic Podostemaceae is marked by characteristic deviations from the usual pattern of angiosperms. Reduced types of embryo sac development and a nucellar plasmodium are of particular interest. The present study of Weddellina squamulosa Tul. deals with the development of the male and female gametophyte, the nucellar plasmodium, embryogenesis and seed structure. A reinvestigation of embryo sac development reveals a five-nucleate ‘Apinagia type’ (form B1 after Battaglia, 1987) of embryo sac. The embryo sac is monosporic in origin. The description of a bisporic embryo sac development (‘Podostemum’ type) in Weddellina squamulosa (Chiarugi, 1933) is refuted.The developmental pattern of the nucellar plasmodium represents a third mode not otherwise found in Podostemaceae. Its generation begins from the central row of the nucellus with the outer layer kept intact. The cells of the outer layer disintegrate later. Thus, the nucellar plasmodium seems to be established only by the outer nucellar layer.Embryogenesis conforms to those of other Podostemaceae. Microspore and pollen development show no deviations from the usual angiospermous pattern. Pollen is a monad and shed in the two-celled stage. Seed set of 100% is possibly due to autogamy during anthesis.The structure of the gynoecium in Weddellina squamulosa differs from other Podostemaceae. The bicarpellate ovary consists of four sections. The main part, above an ascidiate region, is the syncarpous zone with an axile placenta, superimposed on this is a paracarpous region that lacks a septum. The uppermost section develops an additional ‘apical septum’, a mode structurally connected with anacrostyly. This brings about a slender (compound) style (instead of stylodes as in other Podostemaceae) and a globular stigma. New data favour a re-establishment of the former subfamily Weddellinoideae.