Cytinus hypocistis L. embryogenesis: Some biological and ultrastructural aspects of fertilization and embryo development

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Abstract
The present paper examines some biological and ultrastructural aspects of fertilization and early development of the embryo in Cytinus hypocistis, a parasitic plant belonging to the Rafflesiaceae. The probable functions of a mucilaginous substance contained in the ovary and embedding the numerous pollen tubes coming from the style are discussed. It was ascertained that pollen tubes pass through the micropyle and enter a synergid pushing, their way through the nucellar cells that show swollen walls owing to a probable enzymatic action whose function is to facilitate pollen tube penetration. It was hypothesized that the secretion of such enzymes is attributable to the numerous pollen present in the ovary or entering the mycropyle. Since, in all the ovules observed, synergid degeneration was never found before the arrival of the pollen tube, this degeneration was interpreted as being caused by the material disharged by the pollen tube, rather than being an essential prerequisite for pollen tube penetration into the synergid. Pollen tube content was observed to be made up of an intensely electron-dense substance surrounding many lipidic globules and numerous polysaccharide vesicles that fuse with the pollen tube wall, clearly contributing to its growth. The sequence of the first divisions of the developing embryo was followed and the extreme reduction of the embryo is confirmed. In Cytinus hypocistis starch is totally absent from all the sells belonging to the female gamethophyte as well as to those belonging to the embryo, but lipidic globules are very frequent; it is therefore supposed that these bodies constitute good material for the nutrition of the zygote and early embryo.