Sexual Reproduction in Flowering Plants – Class 12 Biology Chapter 1 Notes & NCERT PDF

Introduction

To a biologist, flowers are "morphological and embryological marvels and the sites of sexual reproduction". This chapter follows the full story inside an angiosperm: the stamen and pistil and the gametophytes they form, pollination, the pollen–pistil dialogue, double fertilisation, and how the endosperm, embryo, seed and fruit develop. It ends with apomixis and polyembryony, which matter a great deal for the hybrid seed industry. Board papers draw heavily on this chapter for diagrams (the anther T.S., the ovule, the embryo sac) and for "differentiate between" questions.

Key Concepts

1.1 Flower – A Fascinating Organ of Angiosperms

The androecium (a whorl of stamens) is the male reproductive organ. The gynoecium is the female reproductive organ. The decision to flower is taken well before a flower appears: hormonal and structural changes lead to the floral primordium, then to inflorescences, floral buds and finally flowers.

1.2 Pre-fertilisation: Structures and Events

1.2.1 Stamen, microsporangium and pollen grain

- It is generally spherical and 25–50 micrometers across.

- The exine is made of sporopollenin, "one of the most resistant organic material known". It withstands high temperatures and strong acids and alkali, and no enzyme is known to degrade it. This is why pollen fossilises well. Where sporopollenin is absent, the exine has germ pores.

- The intine is thin and made of cellulose and pectin.

- A mature pollen grain has a large vegetative cell with food reserve and an irregular nucleus, plus a small, spindle-shaped generative cell.

- In over 60 per cent of angiosperms, pollen is shed at this 2-celled stage. In the rest, the generative cell first divides to form two male gametes (the 3-celled stage).

- Rice and wheat pollen lose viability within 30 minutes of release. In some members of Rosaceae, Leguminoseae and Solanaceae, pollen stays viable for months.

- Pollen can be stored for years in liquid nitrogen at −196°C, in pollen banks.

- Parthenium (carrot grass) came to India as a contaminant of imported wheat and causes pollen allergy.

1.2.2 Pistil, megasporangium (ovule) and embryo sac

- monocarpellary: a single pistil;

- multicarpellary: more than one pistil, which may be syncarpous (fused, as in Papaver) or apocarpous (free, as in Michelia).

- the funicle (stalk);

- the hilum (the junction of the ovule and funicle);

- one or two integuments, which leave an opening called the micropyle;

- the chalaza (the basal part);

- the nucellus, which holds reserve food and the embryo sac.

- At the micropylar end is the egg apparatus: 2 synergids and 1 egg cell. The synergids carry the filiform apparatus, which guides the pollen tube.

- At the chalazal end are 3 antipodals.

- The large central cell holds 2 polar nuclei.

- The mature embryo sac is therefore 8-nucleate but 7-celled.

1.2.3 Pollination

Pollination is the transfer of pollen grains from the anther to the stigma of a pistil.

- Autogamy takes place within the same flower. Viola, Oxalis and Commelina produce chasmogamous flowers (open) and cleistogamous flowers (never open). Cleistogamous flowers are invariably autogamous and give assured seed-set without pollinators.

- Geitonogamy moves pollen to another flower of the same plant. It is functionally cross-pollination but genetically like autogamy.

- Xenogamy moves pollen to a different plant. It is the only type that brings genetically different pollen to the stigma.

- Wind-pollinated flowers have light, non-sticky pollen, well-exposed stamens and large, often feathery stigmas. They often have a single ovule per ovary. Examples are grasses and the corn cob, whose waving threads are the stigma and style that trap pollen.

- Water pollination is rare, limited to about 30 genera, mostly monocots. Examples are Vallisneria, Hydrilla and Zostera. In Vallisneria, the female flower reaches the surface on a long stalk and the pollen floats to it. In sea-grasses, ribbon-like pollen is carried under water, and a mucilaginous covering protects it from wetting.

- Wind- and water-pollinated flowers are not colourful and produce no nectar.

- Insect-pollinated flowers are large, colourful, fragrant and rich in nectar. Their rewards are nectar and pollen. Amorphophallus (a flower about 6 feet tall) and the Yucca–moth pair offer safe places to lay eggs. Visitors that take the reward without pollinating are pollen/nectar robbers.

- pollen release and stigma receptivity are not synchronised;

- the anther and stigma are placed at different positions;

- self-incompatibility, a genetic mechanism that stops self-pollen germinating or its tube growing;

- unisexual flowers. Monoecious plants such as castor and maize prevent autogamy but not geitonogamy. Dioecious plants such as papaya prevent both.

- Emasculation: anthers are removed from the bud with forceps before they dehisce. This is only needed when the female parent has bisexual flowers.

- Bagging: the flower is covered with a butter-paper bag. When the stigma becomes receptive, the chosen pollen is dusted on and the flower is rebagged.

1.3 Double Fertilisation

The pollen tube releases two male gametes into a synergid.

1.4 Post-fertilisation: Structures and Events

- The PEN divides into free nuclei (free-nuclear endosperm), and cell walls form later (cellular endosperm). Tender coconut water is free-nuclear endosperm with thousands of nuclei, and the white kernel is cellular endosperm.

- The endosperm is fully consumed in pea, groundnut and beans, but persists in castor and coconut.

- A dicot embryo has an embryonal axis and two cotyledons. The epicotyl ends in the plumule and the hypocotyl ends in the radicle, which is covered by a root cap.

- A monocot (grass) embryo has one lateral cotyledon, the scutellum. The radicle and root cap are sheathed by the coleorrhiza, and the shoot apex is enclosed by the coleoptile.

- Non-albuminous seeds (pea, groundnut) have no residual endosperm. Albuminous seeds (wheat, maize, barley, castor) retain some.

- A persistent nucellus is called perisperm (black pepper, beet).

- The integuments become the seed coat, and the micropyle stays as a pore that admits O₂ and water.

- Mature seeds hold 10–15 per cent moisture and may become dormant.

- The ovary wall becomes the pericarp.

- False fruits, where the thalamus also forms the fruit, include apple, strawberry and cashew. Parthenocarpic fruits develop without fertilisation and are seedless, as in banana; parthenocarpy can be induced with growth hormones.

- Seed viability records: Lupinus arcticus germinated after about 10,000 years, and a date palm (Phoenix dactylifera) seed about 2000 years old was found at King Herod's palace near the Dead Sea.

1.5 Apomixis and Polyembryony

- a diploid egg forms without reduction division and develops into an embryo;

- more often, as in many Citrus and Mango varieties, nucellar cells grow into embryos.

Diagrams to Know (in words)

Key-Facts Sheet

Board Exam Focus

Frequent questions (from the exercises) cover microsporogenesis versus megasporogenesis, outbreeding devices, bagging, triple fusion, false fruits, the tapetum, and apomixis.

Key Terms

Common Mistakes