Sexual Reproduction in Flowering PlantsNEET Botany · Class 12 · NCERT Chapter 1

Sexual Reproduction in Flowering Plants is a Very High Weightage chapter with 5 to 7 questions in most NEET exams. Questions focus on microsporogenesis and pollen grain structure (sporopollenin, exine and intine, 2-celled vs 3-celled), megasporogenesis and the 7-celled 8-nucleate embryo sac (Polygonum type), pollination types and agents, double fertilisation and triple fusion, endosperm types, embryo development, apomixis, and polyembryony. Memorise structures and the precise sequence of events for the highest score.

The mature embryo sac of the Polygonum type (the typical angiosperm embryo sac) has 8 nuclei but only 7 cells. The 8 nuclei are: 1 egg + 2 synergid + 3 antipodal + 2 polar nuclei (the polar nuclei are inside the central cell). However, the 2 polar nuclei share a single central cell, so the total number of cells is 7 (1 egg + 2 synergids + 3 antipodals + 1 central cell with 2 polar nuclei) = 7 cells, 8 nuclei. This is why the structure is called 7-celled, 8-nucleate. The development requires 3 mitotic divisions of the functional megaspore.

Double fertilisation is a unique feature of angiosperms (flowering plants), discovered by S.G. Nawaschin in 1898. Two fertilisation events happen simultaneously inside one embryo sac: (1) Syngamy: one male gamete fuses with the egg to form a diploid zygote (2n). This is the "true" fertilisation. (2) Triple fusion: the second male gamete fuses with the two polar nuclei (or with the secondary nucleus formed from their fusion) to form a triploid primary endosperm nucleus (3n). The zygote develops into the embryo, while the primary endosperm nucleus divides to form the endosperm, which nourishes the developing embryo. Double fertilisation is unique to angiosperms; it does not occur in gymnosperms or other plant groups.

Sporopollenin is the most resistant biological material known. It makes up the exine (outer wall) of pollen grains and is responsible for their extraordinary durability. Sporopollenin can resist high temperatures, strong acids, strong alkalis, enzymes, and microbial decay. Because of sporopollenin, pollen grains can be preserved for thousands or even millions of years (palynology, the study of fossil pollen, depends on this). The pollen grain wall has germ pores (apertures) where sporopollenin is absent; this is where the pollen tube emerges during germination. Sporopollenin also makes pollen grains useful as forensic evidence and for tracking ancient climate.

Three types of pollination based on the source of pollen: (1) Autogamy: pollen transferred to the stigma of the SAME flower (self-pollination in the strictest sense). Examples: Pisum sativum (pea) typically self-pollinates. Cleistogamous flowers (e.g., Viola, Commelina) are always autogamous because they never open. (2) Geitonogamy: pollen transferred from one flower to a DIFFERENT flower OF THE SAME PLANT. Genetically still self-pollination (same plant = same genotype) but functionally similar to cross-pollination because it requires a pollinator. (3) Xenogamy: pollen transferred from a flower of one plant to a flower of a DIFFERENT plant of the same species. This is the only TRUE cross-pollination that brings genetic variation. NEET trap: geitonogamy and xenogamy both look like cross-pollination but only xenogamy is genetically cross-pollination.

Apomixis is the formation of seeds WITHOUT fertilisation (asexual reproduction through seeds). The diploid embryo develops directly from the diploid nucellar cells (without meiosis or syngamy). Found in many grasses (Poaceae), some Asteraceae, and citrus. Apomictic seeds give rise to plants that are genetically IDENTICAL to the parent (no genetic recombination). Importance in agriculture: (1) Hybrid varieties of crops (e.g., hybrid maize, tomato) usually lose their hybrid vigour after a few generations (because of segregation in F2). If apomixis could be introduced into hybrids, the hybrid character would be preserved generation after generation, saving farmers from buying new seeds every year. (2) Allows propagation of desirable types without sexual recombination. Many citrus varieties propagated through apomictic seeds because some embryos are nucellar (asexual).

Both develop from the zygote following double fertilisation, but their final structures differ. Dicot embryo (e.g., capsella): has TWO cotyledons (which often store food), an epicotyl (above cotyledons; gives shoot apex), a hypocotyl (between cotyledons and radicle; gives stem base), a radicle (root tip), and a plumule (developing shoot tip with leaves). Monocot embryo (e.g., grasses, maize, wheat): has ONE cotyledon, called the SCUTELLUM, which absorbs nutrients from the endosperm. The shoot is enclosed in a protective sheath called the COLEOPTILE. The root is enclosed in a protective sheath called the COLEORHIZA. The endosperm is large and persistent in monocots (provides food during germination). NEET trap: scutellum, coleoptile, coleorhiza are MONOCOT-specific terms.