Sexual Reproduction in Flowering PlantsMind Map

A typical flower has four whorls: calyx, corolla, androecium and gynoecium. For sexual reproduction, androecium and gynoecium are essential. Each stamen has a filament and anther; a typical anther is bilobed, dithecous and tetrasporangiate. Each microsporangium has epidermis, endothecium, middle layers and tapetum surrounding sporogenous tissue. The pistil consists of stigma, style and ovary. Inside the ovary, ovules develop on placenta. A typical ovule has funicle, hilum, integuments, micropyle, nucellus and embryo sac. NCERT frequently asks anther wall layers, tapetum function, ovule parts, micropyle position and the difference between male and female reproductive organs.

Male gametophyte development begins in microsporangia, where diploid pollen mother cells undergo meiosis to form haploid microspores in tetrads. Each microspore matures into a pollen grain with exine, intine, vegetative cell and generative cell. The generative cell divides to form two male gametes. Female gametophyte development begins in the ovule, where a diploid megaspore mother cell undergoes meiosis to form four megaspores, usually only one being functional. This functional megaspore undergoes three mitotic divisions to form an 8-nucleate embryo sac, which organizes into 7 cells: egg apparatus, central cell and antipodals. Pollen germination on stigma produces a pollen tube that carries male gametes toward the embryo sac.

Pollination is transfer of pollen grains from anther to stigma. It may occur within the same flower as autogamy, between flowers of the same plant as geitonogamy, or between different plants as xenogamy. Pollination agents include wind, water and animals such as insects, birds and bats. Plants have evolved adaptations for each agent: wind-pollinated flowers produce abundant light pollen, water-pollinated plants may release pollen into water, and insect-pollinated flowers are often colourful, fragrant and nectar-rich. To avoid inbreeding and promote genetic variation, plants use outbreeding devices such as dichogamy, herkogamy, self-incompatibility and dioecy. Pollen-pistil interaction determines compatibility, and artificial hybridisation uses emasculation and bagging to produce desired crosses.

After compatible pollination, pollen grain germinates on the stigma and the pollen tube grows through the style toward the ovule. It usually enters the embryo sac through the micropyle and one synergid, guided by filiform apparatus. The pollen tube releases two male gametes. One male gamete fuses with the egg cell to form a diploid zygote; this is syngamy. The other male gamete fuses with the two polar nuclei or secondary nucleus in the central cell to form the triploid primary endosperm nucleus; this is triple fusion. Because two fusion events occur in the same embryo sac, it is called double fertilisation. It is unique to angiosperms and coordinates embryo and endosperm development.

Post-fertilisation events begin after double fertilisation. The primary endosperm nucleus divides to form endosperm, a nutritive tissue for the developing embryo. In nuclear endosperm, nuclei divide freely before cell wall formation, as seen in coconut water. The zygote develops into an embryo through proembryo stages, forming radicle, plumule, hypocotyl, epicotyl and cotyledons. In dicots, the embryo has two cotyledons; in monocots, one cotyledon called scutellum is present. The ovule becomes seed, integuments form seed coat, and ovary becomes fruit. Seeds may be albuminous or non-albuminous depending on retained endosperm. Seed dispersal by wind, water, animals or bursting helps reduce competition and spread the species.

Apomixis is a special method in which seeds are formed without fertilisation, so it resembles asexual reproduction through seeds. It is important because apomictic seeds can preserve the exact genotype of the parent, including hybrid vigour, generation after generation. Apomixis may occur through development of an unreduced embryo sac, egg development without fertilisation, or embryo formation from nucellar or integumentary cells. Polyembryony means the presence of more than one embryo in a seed. It may occur due to cleavage of the zygote, development of more than one embryo sac, or adventive embryos from nucellus. Citrus and mango are important examples. In agriculture, apomixis is valuable because farmers could reuse hybrid seeds without losing desirable traits.