Biology XII:2 Sexual rep in flowering plants(2 Gynoecium and Format

Gynoecium and Formation of Female Gametophyte The gynoecium represents the female reproductive part of a flower. It may be mono-carpellary (one pistil) or multi-carpellary (many pistils). In multi-carpellary, the pistils may be fused in one (syncarpous) or free (apocarpous). Each pistil consists of: Stigma − Receives the pollen grains Style − Elongated, slender part below the stigma Ovary − Bulged basal part containing the placenta, which is located inside the ovarian locule (cavity) The placenta contains the megasporangia or ovules. Megasporangium The ovule is attached to the placenta by the funicle. The junction of the ovule and the funicle is called hilum. Each ovule has one or two protective layers, called integuments, which cover the rest of the ovule, except for a small opening called micropyle. The chalaza lying on the opposite side of the micropyle end represents the basal part of the ovule. Nucellus is present within the integuments and contains reserved food. The embryo sac or female gametophyte is located within the nucellus. Megasporogenesis The megaspore mother cell (MMC) gets converted into megaspores by the process of megasporogenesis. The MMC is large and contains a dense cytoplasm and a prominent nucleus. It undergoes meiosis to produce four megaspores. Female Gametophyte In most flowering plants, only one megaspore is functional while the other three degenerate. The single functional megaspore develops into the female gametophyte. This kind of development is called monosporic development. The nucleus of the functional megaspore divides mitotically to form 2 nuclei, which move towards the opposite ends, forming a 2-nucleate embryo sac. Two more mitotic divisions ensue, leading to the formation of 4-nucleate and 8-nucleate embryo sacs. After the 8-nucleate stage, the cell walls are laid down and the typical female gametophyte (embryo sac) gets organised. Six of the 8-nuclei get surrounded by the cell wall and the remaining two, called polar nuclei, are situated below the egg apparatus in the large central cell. Three of the six cells are placed at the micropylar end and constitute the egg apparatus (2 synergids + 1 egg cell). The synergids have special thickenings at the micropylar end. These are together called the filiform apparatus. It helps in leading the pollen tubes into the synergids. Three cells are at the chalazal end, and are called antipodal cells. A typical angiosperm female gametophyte is 7-celled and 8-nucleated at maturity.