FIGURE 1.

Sexual reproduction in flowering plants. The process of sexual reproduction begins with sporogenesis where spore mother cells (SMCs) differentiate in the floral organs of adult plants. The female SMC, also called megaspore mother cell (MMC) differentiates from a subepidermal nucellar cell within the ovule primordium, the MMC then undergoes meiosis to produce four haploid spores while only one survives to form the functional megaspore (FM). In the stamen primordium, one subepidermal cell enlarges to from the archesporial cell (AC). The archesporial cell then divides to form one primary sporogenous cell (PS) on the inner side and one primary parietal cell (PP) toward the outside. The primary parietal cell divides periclinally and anticlinally to generate the anther wall that is composed of epidermis (E), endothecium (En), the middle layer (ML), and the tapetum (T), while the primary sporogenous cell divides to give rise to the male SMCs, also called the pollen mother cells (PMCs). Each PMC then undergoes meiosis to form four haploid microspores (MS). During gametogenesis, the FM undergoes three rounds of mitosis and cellularization to generate the female gametophyte that harbors two gametes: the egg cell and the central cell, accompanied with three antipodals and two synergids. While for the male side, each microspore undergoes an asymmetric division to give rise to a larger vegetative cell and a smaller generative cell within the bicellular pollen grain. The generative cell divides further to produce the gametes: two sperm cells. During double fertilization, the egg cell is fertilized by one sperm to form the zygote that will give rise to the embryo, while the central cell fuses with the other sperm to generate the triploid endosperm. Original drawings were made after microscopy pictures (female sporogenesis) or inspired from Zhang et al. (2011) (male sporogenesis).