Chapter 1: Sexual Reproduction in Flowering Plants | Summary

The stamen consists of a filament and a bilobed anther. A typical angiosperm anther is dithecous (two-lobed, four microsporangia). The microsporangium wall has four layers: epidermis, endothecium, middle layers, and tapetum. The tapetum is the innermost layer and nourishes developing pollen grains. The sporogenous tissue in the centre undergoes meiosis (microsporogenesis) to produce microspore tetrads, which mature into pollen grains.

Pollen grains have a two-layered wall — the outer exine (made of sporopollenin, highly resistant) and the inner intine (cellulose and pectin). Germ pores are apertures on the exine through which pollen tubes emerge. Pollen grains are shed either at the 2-celled stage (vegetative + generative cell) in over 60% of angiosperms, or at the 3-celled stage in the remaining species. The pistil consists of stigma, style, and ovary. Inside the ovary, ovules are attached to the placenta. Each ovule has integuments, nucellus, micropyle, chalaza, funicle, and hilum. The megaspore mother cell (MMC) undergoes meiosis (megasporogenesis) to produce four megaspores, of which only one is functional. This megaspore undergoes three successive mitotic divisions to form an 8-nucleate, 7-celled embryo sac — with a 3-celled egg apparatus (2 synergids + 1 egg cell), 3 antipodals, and a central cell with 2 polar nuclei.

Pollination is the transfer of pollen grains from the anther to the stigma. There are three types: autogamy (within the same flower), geitonogamy (between flowers of the same plant), and xenogamy (between flowers of different plants). Only xenogamy results in the transfer of genetically different pollen. Cleistogamous flowers are invariably autogamous, while chasmogamous flowers may be cross-pollinated.

Pollination can be achieved by abiotic agents (wind — anemophily; water — hydrophily) or biotic agents (insects, birds, bats, etc.). Wind-pollinated flowers produce large amounts of light, non-sticky pollen and have well-exposed stamens. Water-pollinated plants (e.g., Vallisneria, Hydrilla) are rare. Animal-pollinated flowers are typically large, colourful, and nectar-rich. Outbreeding devices such as protandry, herkogamy, self-incompatibility, and dioecy prevent self-pollination and inbreeding depression. Pollen-pistil interaction ensures that only compatible pollen germinates and the pollen tube grows through the style to deliver male gametes to the embryo sac. Artificial hybridisation techniques — emasculation and bagging — are used in crop improvement programmes.

After reaching the embryo sac, the pollen tube enters one of the synergids and releases two male gametes. One male gamete fuses with the egg cell (syngamy) to form the diploid zygote (2n). The other male gamete fuses with the two polar nuclei (triple fusion) to form the triploid primary endosperm nucleus (PEN, 3n). Since two fusions occur — syngamy and triple fusion — the process is called double fertilisation, a unique feature of angiosperms.

The central cell after triple fusion is called the primary endosperm cell (PEC), which develops into the endosperm. The zygote develops into the embryo. Endosperm development always precedes embryo development, providing nutrition to the developing embryo.

After fertilisation, the PEN divides repeatedly to form the triploid endosperm tissue. Endosperm development begins as free-nuclear (e.g., coconut water) and later becomes cellular (e.g., coconut kernel). The endosperm is either completely consumed during embryo development (non-albuminous seeds: pea, groundnut) or persists in the mature seed (albuminous seeds: wheat, maize, castor). The zygote develops into the embryo through proembryo, globular, heart-shaped, and mature stages. A dicot embryo has two cotyledons, epicotyl, hypocotyl, plumule, and radicle. A monocot embryo has one cotyledon (scutellum), coleoptile, coleorrhiza, epicotyl, and hypocotyl.

The ovule develops into the seed and the ovary develops into the fruit. Seeds may be albuminous or non-albuminous. Perisperm is the residual persistent nucellus found in seeds of black pepper and beet. The pericarp is the wall of the fruit derived from the ovary wall. False fruits (e.g., apple, strawberry) are formed when the thalamus also contributes to fruit formation. Parthenocarpic fruits (e.g., banana) develop without fertilisation. Seeds offer advantages: independence from water for reproduction, food reserves for seedling nourishment, dispersal to new habitats, protection by the seed coat, and genetic variation.

Apomixis is the production of seeds without fertilisation — a form of asexual reproduction that mimics sexual reproduction. It occurs in some Asteraceae, grasses, and species like Citrus and Mango. In some species, a diploid egg cell (unreduced) develops directly into the embryo. In others, nucellar cells surrounding the embryo sac divide and develop into embryos (nucellar polyembryony). Apomicts maintain hybrid characters without segregation, making them highly valuable in agriculture.

Polyembryony is the occurrence of more than one embryo in a seed. It is commonly seen in Citrus and Mango. In hybrid seed production, converting hybrids into apomicts would allow farmers to reuse seeds year after year without loss of hybrid vigour, eliminating the high cost of buying fresh hybrid seeds annually.

Sexual reproduction in flowering plants is a highly coordinated process involving the development of male and female gametophytes, pollination, double fertilisation, and post-fertilisation development of seeds and fruits. The unique phenomenon of double fertilisation — syngamy forming the zygote and triple fusion forming the triploid endosperm — distinguishes angiosperms from all other plants. Specialised adaptations for pollination, outbreeding devices, and strategies like apomixis and polyembryony highlight the remarkable evolutionary diversity of flowering plants.

For CBSE board examinations, this chapter carries significant weightage. Students must master the structure of microsporangium and embryo sac, the process of microsporogenesis and megasporogenesis, types and agents of pollination, the events of double fertilisation, embryo and seed development in dicots and monocots, and the significance of apomixis and polyembryony. Questions may be diagram-based, application-based, or analytical, covering all sections of the chapter.