BOTANY WITH THE BASICS OF HYDROBOTANY (AQUATIC PLANTS OF UKRAINE) - B.Ye. Yakubenko - 2011

VIII. BASICS OF PLANT MORPHOLOGY AND ANATOMY

Features of Reproduction in Angiosperms

The Female Gametophyte (Embryo Sac)

The female gametophyte of angiosperms develops from a megaspore and is known as the embryo sac. In the majority (80%) of flowering plants, the female gametophyte originates from a haploid mononuclear megaspore, whose Nucleus undergoes mitosis three times. As a result of the first nuclear division of the megaspore, two nuclei are formed, which migrate to opposite poles. Subsequently, each of these nuclei undergoes two more mitotic divisions, resulting in The formation of four primary haploid nuclei at each pole. This marks the eight-nucleate stage of female gametophyte development.

Next, one primary haploid nucleus from each polar quartet migrates toward the central part of the embryo sac; here they fuse to form the diploid secondary Nucleus of the central Cell of the embryo sac, or they come into close proximity to form a dikaryon—a pair of primary haploid nuclei. Following this, Cells of the embryo sac form around the remaining haploid nuclei. Near the micropylar pole, three cells of the egg apparatus (the egg cell and two synergids) are formed, while at the opposite, chalazal pole, three cells—the antipodals—differentiate. The central cell is located between these two groups of cells (Fig. 85). This type of female gametophyte and embryo sac formation is termed monosporic. In the monosporic type of female gametophyte development, cell walls are formed after the First and Second meiotic divisions during megasporogenesis.

In addition to the monosporic type, bisporic and tetrasporic types of embryo sacs also exist. A bisporic embryo sac develops from a binucleate megaspore. During the formation of such a megaspore, cell walls appear after the first meiotic division, but do not form after the second, resulting in two binucleate cells. One of these cells develops into the female gametophyte. To form an eight-nucleate embryo sac, the binucleate megaspore undergoes mitotic division twice, rather than three times as in the monosporic type.

A tetrasporic embryo sac develops from a tetranucleate megaspore, whose nuclei undergo mitosis only once. During megasporogenesis in this case, Meiosis is not accompanied by the formation of cell walls, leaving the megaspore tetranucleate.