Summary.In the young ovule of Welwitschia mirabilis the nucellar apex is dome shaped and starch begins to accumulate near the female gametophyte. With the degeneration of the cells of the nucellar apex, a pollen chamber is formed, which contains the micropylar fluid. Starch storage increases considerably in the upper part of the nucellus. Pollen drop emission is not a rhythmic process, and pollination does not produce the rapid withdrawal of droplets. The micropylar drop consists almost entirely of sugars, uronic acids and a very small amount of free amino acids and enzymes. The mechanism of micropylar drop secretion and its probable role in the process of pollination is discussed.

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The present paper examines some biological and ultrastructural aspects of fertilization and early development of the embryo in Cytitzrrs hypocistis, a parasitic plant belonging to the Ru//lesiaceae. The probable functions of a mucilaginous substance contained i n the ovary and embedding the numerous pollen tubes coming from the style are discussed.I t mas ascertained that pollen tubes pass through the micropyle and enter a synergid pushing, thcir way through the nucellar cells that show swollen walls owing to a probable enzymatic action whose function is to facilitate pollen tube penetration. I t was hypothesized that the secretion of such enzymes is attributable to the numerous pollen present in the ovary or entering the mycropyle.Since, in a11 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.