Pre-Fertilisation : Structures and Events

Pre-fertilisation covers every structure and event in a flowering plant before the male and female gametes meet. This page explains the flower as the site of sexual reproduction, the stamen and anther, the wall layers of the microsporangium, microsporogenesis and the pollen grain, the pistil and ovule, megasporogenesis and the 7-celled, 8-nucleate embryo sac. It follows the NCERT Class 12 chapter Sexual Reproduction in Flowering Plants. NEET often asks pre-fertilisation facts as match-the-list and statement questions on anther wall layers, the pollen wall and the cells of the embryo sac.

1. Sexual Reproduction and the Flower

1.1 Why sexual reproduction matters

• Biology, in essence, is the story of life on earth.
• Individual organisms always die, but a species survives for millions of years through reproduction, unless natural or human-made (anthropogenic) extinction threatens it.
• Reproduction is a vital process; without it a species cannot survive for long.
• Each individual leaves progeny by asexual or sexual means.
• The sexual mode creates new variants, which enhances the survival advantage of the species.
• ★ Exam imp All flowering plants show sexual reproduction.
• The colours, scents and variety of flowers all exist as an aid to sexual reproduction.
• Inflorescences, flowers and floral parts show an amazing range of adaptations. They all ensure the end products of sexual reproduction: fruits and seeds.
• Reproduction in flowering plants and in humans, and human reproductive health, form one connected unit of study.

1.2 The flower: a fascinating organ

• Flowers have aesthetic, ornamental, social, religious and cultural value.
• They are used as symbols of human feelings such as love, affection, happiness, grief and mourning.
• To a biologist, flowers are morphological and embryological marvels and the sites of sexual reproduction.

• ★ Exam imp The two most important units of sexual reproduction develop in two parts of the flower: the male gametophyte (pollen grain) in the anther, and the female gametophyte (embryo sac) in the ovule, inside the ovary.

1.3 Panchanan Maheshwari (1904-1966)

• Born in November 1904 in Jaipur (Rajasthan); one of the most distinguished botanists of India and the world.
• Obtained his D.Sc. in Allahabad. Dr W. Dudgeon, an American missionary teacher, inspired his interest in botany, especially morphology.
• Dudgeon said it would give him great satisfaction if his student progressed beyond him. This made Panchanan ask what he could do for his teacher in return.
• Worked on embryological aspects and popularised the use of embryological characters in taxonomy.
• Established the Department of Botany, University of Delhi as an important centre of research in embryology and tissue culture.
• Stressed the need to start work on the artificial culture of immature embryos; tissue culture has since become a landmark in science.
• ★ Exam imp His work on test tube fertilisation and intra-ovarian pollination won worldwide acclaim.
• Honoured with fellowship of the Royal Society of London (FRS), the Indian National Science Academy and other institutions.
• Encouraged general education and contributed greatly to school education, leading the preparation of the very first Biology books for Higher Secondary Schools, published in 1964.

2. Pre-fertilisation: An Overview

1. The plant decides to flower much before a flower is seen on it.
2. Several hormonal and structural changes begin. They lead to the differentiation and development of the floral primordium.
3. Inflorescences form; they bear floral buds, which open into flowers.
4. In the flower, the male and female reproductive structures, the androecium and the gynoecium, differentiate and develop.

• Androecium: a whorl of stamens, the male reproductive organ.
• Gynoecium: the female reproductive organ, made of one or more pistils.

3. The Stamen and the Anther

• A typical stamen has two parts: the filament, a long and slender stalk, and the anther, the terminal and generally bilobed structure.
• The proximal end of the filament is attached to the thalamus or to the petal of the flower.
• The number and length of stamens vary in flowers of different species.
• Anthers of different species also vary widely in size, shape and attachment.
• ★ Exam imp A typical angiosperm anther is bilobed, and each lobe has two theca. So the anther is dithecous.
• A longitudinal groove often runs lengthwise and separates the theca.

• In transverse section (T.S.) the bilobed nature of the anther is very distinct.
• ★ Exam imp The anther is four-sided (tetragonal), with four microsporangia at its corners, two in each lobe.
• The microsporangia develop further and become pollen sacs.
• Pollen sacs extend longitudinally through the whole length of the anther and are packed with pollen grains.

4. Structure of the Microsporangium

• In transverse section, a typical microsporangium is near circular in outline.
• It is generally surrounded by four wall layers: epidermis, endothecium, middle layers and tapetum.

• The outer three layers (epidermis, endothecium, middle layers) protect the microsporangium and help in dehiscence of the anther.
• ★ Exam imp Tapetal cells have dense cytoplasm and generally more than one nucleus.
• When the anther is young, a group of compactly arranged, homogeneous cells, the sporogenous tissue, occupies the centre of each microsporangium.

5. Microsporogenesis

Microsporogenesis: the formation of microspores from a pollen mother cell (PMC) through meiosis.

1. As the anther develops, each cell of the sporogenous tissue can give rise to a microspore tetrad. So each is a potential pollen mother cell (microspore mother cell).
2. Each pollen mother cell (2n) undergoes meiosis and forms four haploid (n) microspores.
3. The microspores are arranged in a cluster of four cells, the microspore tetrad.
4. As the anther matures and dehydrates, the microspores dissociate from each other and develop into pollen grains.
5. Each microsporangium forms several thousand pollen grains. They are released when the anther dehisces (splits open).

6. The Pollen Grain

6.1 Pollen as the male gametophyte

• ★ Exam imp Pollen grains represent the male gametophytes.
• Touching the opened anthers of Hibiscus leaves a yellowish, powdery deposit of pollen on the fingers.
• Under a microscope, pollen of different species shows a great variety of sizes, shapes, colours and designs.

• Pollen grains are generally spherical, about 25-50 micrometres in diameter.
• They have a prominent two-layered wall: the outer exine and the inner intine.

6.2 The pollen wall

• ★ Exam imp Pollen grains are well preserved as fossils because of the sporopollenin in their exine.
• A hard exine protects the pollen; the pollen tube later emerges through one of the germ pores.

6.3 Cells of the mature pollen grain

• ★ Exam imp In over 60 per cent of angiosperms, pollen grains are shed at the 2-celled stage (vegetative cell + generative cell).
• In the remaining species, the generative cell divides mitotically to give two male gametes before shedding: the 3-celled stage.

6.4 Pollen, allergy and food supplements

• Pollen of many species causes severe allergies and bronchial afflictions in some people.
• These often lead to chronic respiratory disorders such as asthma and bronchitis.
• Parthenium (carrot grass) came into India as a contaminant with imported wheat. It is now everywhere and causes pollen allergy.
• Pollen grains are rich in nutrients. Pollen tablets are used as food supplements.
• In western countries, many pollen products are sold as tablets and syrups.
• Pollen consumption has been claimed to increase the performance of athletes and race horses.

6.5 Pollen viability and storage

• After shedding, a pollen grain must land on the stigma before it loses viability to bring about fertilisation.
• The period of viability is highly variable. It depends to some extent on the prevailing temperature and humidity.
• ★ Exam imp In cereals such as rice and wheat, pollen loses viability within 30 minutes of release.
• In some members of Rosaceae, Leguminosae and Solanaceae, pollen stays viable for months.
• Pollen of a large number of species can be stored for years in liquid nitrogen ($-196{}^{\circ }\mathrm{C}$), much as semen or sperms are stored for artificial insemination.
• Such stored pollen forms pollen banks, similar to seed banks, for crop breeding programmes.

7. The Pistil and the Ovule

7.1 The gynoecium and the pistil

• The gynoecium is the female reproductive part of the flower.
• It may have a single pistil (monocarpellary) or more than one pistil (multicarpellary).
• In a multicarpellary gynoecium the pistils may be fused (syncarpous), as in Papaver, or free (apocarpous), as in Michelia.

• Each pistil has three parts: stigma, style and ovary.
• ★ Exam imp The stigma serves as a landing platform for pollen grains.
• The style is the elongated, slender part beneath the stigma.
• The ovary is the basal, bulged part of the pistil.

• Inside the ovary is the ovarian cavity (locule). The placenta is located inside this cavity.
• The megasporangia, commonly called ovules, arise from the placenta.
• The number of ovules in an ovary may be one (wheat, paddy, mango) or many (papaya, water melon, orchids).

7.2 Structure of the ovule (megasporangium)

• The ovule is a small structure attached to the placenta by a stalk, the funicle.
• ★ Exam imp The body of the ovule fuses with the funicle in the region called the hilum. So the hilum is the junction between ovule and funicle.
• Each ovule has one or two protective envelopes, the integuments.
• The integuments encircle the nucellus except at the tip, where a small opening, the micropyle, is organised.
• Opposite the micropylar end lies the chalaza, the basal part of the ovule.
• Enclosed within the integuments is a mass of cells, the nucellus. Its cells have abundant reserve food.
• The embryo sac or female gametophyte lies in the nucellus. An ovule generally has a single embryo sac, formed from a megaspore.

Examples to Remember

8. Megasporogenesis

Megasporogenesis: the formation of megaspores from the megaspore mother cell (MMC).

1. The archesporium differentiates in the nucellus. One archesporial cell becomes the megaspore mother cell.
2. Ovules generally differentiate a single MMC in the micropylar region of the nucellus. It is a large cell with dense cytoplasm and a prominent nucleus.
3. The MMC undergoes meiosis, passing through a dyad, and forms four megaspores (a tetrad).
4. In a majority of flowering plants, one megaspore is functional and the other three degenerate.
5. Only the functional megaspore develops into the female gametophyte (embryo sac).

9. The Female Gametophyte (Embryo Sac)

9.1 Formation of the embryo sac

1. The nucleus of the functional megaspore divides mitotically. The two nuclei move to opposite poles: the 2-nucleate embryo sac.
2. A second mitotic nuclear division gives the 4-nucleate stage.
3. A third mitotic nuclear division gives the 8-nucleate stage.
4. These divisions are strictly free nuclear: nuclear division is not followed immediately by cell wall formation.
5. After the 8-nucleate stage, cell walls are laid down and the typical female gametophyte (embryo sac) is organised.

9.2 Distribution of cells in the mature embryo sac

• Six of the eight nuclei are surrounded by cell walls and organised into cells.
• The remaining two nuclei, the polar nuclei, lie below the egg apparatus in the large central cell.
• ★ Exam imp Three cells grouped at the micropylar end form the egg apparatus: two synergids and one egg cell.
• The synergids have special cellular thickenings at the micropylar tip, the filiform apparatus. It plays an important role in guiding the pollen tubes into the synergid.
• Three cells at the chalazal end are the antipodals.
• The large central cell has the two polar nuclei.

10. Exam Essentials

Pairs to Match

Numbers to Remember

• 4 wall layers around a microsporangium; 4 microsporangia per anther, 2 per lobe.
• Pollen diameter: about 25-50 micrometres.
• Over 60 per cent of angiosperms shed 2-celled pollen.
• Rice and wheat pollen: viable for about 30 minutes; some Rosaceae, Leguminosae, Solanaceae: months.
• Pollen storage in liquid nitrogen: $-196{}^{\circ }\mathrm{C}$, for years.
• Integuments: one or two; MMC per ovule: generally one; megaspores formed: 4, functional: 1.
• Mitotic divisions in the embryo sac: 3 (2-, 4-, 8-nucleate).
• Mature embryo sac: 8 nuclei, 7 cells; egg apparatus 3 cells; antipodals 3; polar nuclei 2.
• Panchanan Maheshwari: 1904-1966; first Biology books for Higher Secondary Schools: 1964.

11. Quick Revision

• All flowering plants reproduce sexually; flowers are the sites of sexual reproduction.
• The androecium (stamens) is male; the gynoecium (pistils) is female.
• Stamen = filament + anther; the anther is bilobed, dithecous and tetrasporangiate.
• Microsporangia become pollen sacs running the length of the anther.
• Microsporangium wall: epidermis, endothecium, middle layers (protect, help dehiscence) and tapetum (nourishes).
• Sporogenous tissue → pollen mother cells → meiosis → microspore tetrads → pollen grains.
• Pollen (male gametophyte): 25-50 micrometres; exine of sporopollenin with germ pores; intine of cellulose and pectin.
• Mature pollen: large vegetative cell and small spindle-shaped generative cell; over 60 per cent shed 2-celled.
• Pollen causes allergies (Parthenium), is used as a food supplement, and can be stored at $-196{}^{\circ }\mathrm{C}$ in pollen banks.
• Pistil = stigma, style, ovary; syncarpous in Papaver, apocarpous in Michelia.
• Ovule: funicle, hilum, integuments, micropyle, chalaza, nucellus and one embryo sac.
• MMC in the micropylar nucellus undergoes meiosis; in most flowering plants only 1 of the 4 megaspores is functional (monosporic).
• Three free-nuclear mitoses give 8 nuclei; walls form after the 8-nucleate stage.
• Mature embryo sac: 8-nucleate, 7-celled; egg apparatus at the micropylar end, antipodals at the chalazal end.
• The filiform apparatus of the synergids guides the pollen tube.

12. Solved Examples

Common Mistakes to Avoid