MEGASPOROGENESIS IN GYMNOSPERMS: ASPECTS OF TRE CITOPLASM AND TRE CELL WALLS

From the numerous studies carried out, over the years, using mostIy the optic microscope (OM), on the megasporogenesis of living Gymnosperms s. it is well known that one or more sporogenous cells function as megaspore mother cells (MMC). Meiosis brings about the formation of three or four megaspores. Only in Gnetum and Welwitschia is the product of meiosis a tetranucleate coenomegaspore. Thus, in these two genera the female gametophyte has a tetramegasporic ori gin; in Stangeria paradoxa it is perhaps bimegasporic, since it is presumed that only the upper cell of the dyad undergoes both meiotic divisions. However, normally the female gametophyte develops from a single megaspore, i.e. the chalazal one, while the other megaspores degenerate and are utilized in the nutrition of the young gametophyte. It has been suggested that the megaspores which are formed from a meiotic event are all potentially equivalent and that the chalazal megaspore develops more rapidly that the others because of its proximity to the vascular terminations of the ovule. Nonetheless, much research, subsequent to the hypothesis here mentioned, indicates that the direction of trophic fIow would be effective even prior to meiosis, inducing a greater metabolic potentiality in the chalazal cytoplasma of the megasporocyte as compared to the micropylar region. The polarization of the MMC is, in many cases, expressed by a chalazal concentration of the greater part of the mitochondria amI/or plastids. The polarized distribution of these organelles is maintained during meiosis. Thus it follows that the chalazal megaspore has a greater development potentiality, which is manifested almost immediately after meiosis, favoured, undoubtedly, by the proximity of the food supply. The influence of the direction of the trophic fIow appears atypical in Taxus, where all the products of a meiotic act can show an equal development capacity, even for quite a long periodo The" causes of such behaviour may be fol'md in some peculiar morpho-physiological aspects of the nucellus as yet not identified. Another topic worthy of particular note in the megasporogenesis of Gymnosperms is the behaviour of the cell walls. In some cases the formation of callose wall deposits during meiosis, which are then destined to disappear rapidly after meiosis, has been pointed out. It has been suggested that the callose is involved in separating the micropylar megasporesof reduced functionality from the more active chalazal megaspore. The presence of tickenings, which are added to the originaI wall of the megasporocyte, is a generai feature. The thickenings are maintained or accentuated around the enti re dyad and tetrad or tryad. These facts suggest a tendency of the cytoplasm which is the seat of meiosis to isolate itself entirely from the surrounding cells by depositing wall materia! In some cases this material is certainly different from the callose. Undoubtedly definitive knowledge of the "nature, function and possible evolution of the wall thickenings observed requires further ultrastructural and cytochemical research. MEGASPOROGENESIS IN GYMNOSPERMS: ASPECTS ETC. 165