5. Female gametogenesis and early seed formation in plants

The formation of reproductive organs is a defining characteristic of sexual reproduction in flowering plants. Unlike animals, plants do not establish a germline early in development. In flowering species, specific somatic cells present in sexual reproductive organs divide by meiosis and differentiate haploid precursors (named spores) that undergo several mitotic divisions before giving rise to functional reproductive cells (gametes). It is therefore considered that the life cycle of flowering plants is composed of two distinct phases or “generations”: the diploid phase that is phenotypically predominant and represented by all vegetative organs (roots, stems, flowers, etc...), and the ephemeral haploid phase represented by a limited number of cells that differentiate deep within male or female reproductive organs. Another distinctive feature of flowering plants refers to the unique sexual event of double fertilization in which two functional female gametes fuse with two sperm cells and generate two different products: the embryo and the endosperm. The correct development of these two tissues coordinated with the development of the maternal seed coat allows the formation of a viable seed.