Sexual Reproduction in Flowering Plants

This article summarises key themes and facts related to sexual reproduction in flowering plants (angiosperms), drawing from the provided excerpts of “biology-chap-1.pdf”.

1. The Essence of Biology: Life’s Continuation Through Reproduction

Biology fundamentally explores “the story of life on earth.” A core principle highlighted is that while “individual organisms die without fail, species continue to live through millions of years unless threatened by natural or anthropogenic extinction.” This continuity is critically dependent on reproduction, a “vital process without which species cannot survive for long.” Both asexual and sexual means contribute to progeny. Sexual reproduction is particularly significant as it “enables creation of new variants, so that survival advantage is enhanced.”

2. Panchanan Maheshwari: A Pioneer in Indian Botany

The document briefly introduces Panchanan Maheshwari (1904-1966), a distinguished botanist. His contributions include:

• Focus on Embryology: He “worked on embryological aspects and popularised the use of embryological characters in taxonomy.”
• Establishment of Research Centre: He “established the Department of Botany, University of Delhi as an important centre of research in embryology and tissue culture.”
• Groundbreaking Work: His “work on test tube fertilisation and intra-ovarian pollination won worldwide acclaim.”
• Educational Impact: He “made a significant contribution to school education by his leadership in bringing out the very first textbooks of Biology for Higher Secondary Schools published by NCERT in 1964.”

3. Flowers: More Than Just Aesthetic Appeal

Flowers are presented as “fascinating organ[s] of angiosperms.” While human beings have an “intimate relationship with flowers since time immemorial,” using them for “aesthetic, ornamental, social, religious and cultural value,” their primary biological purpose is for sexual reproduction. “Flowers do not exist only for us to be used for our own selfishness. All flowering plants show sexual reproduction.” To a biologist, flowers are “morphological and embryological marvels and the sites of sexual reproduction.”

4. Pre-Fertilisation: Structures and Events in Detail

Before the actual flower appears, “several hormonal and structural changes are initiated” leading to its development. The male and female reproductive structures are the androecium (stamens) and gynoecium (pistil), respectively.

4.1. Male Reproductive Structures: Stamen, Microsporangium, and Pollen Grain

• Stamen: Consists of a “long and slender stalk called the filament, and the terminal generally bilobed structure called the anther.”
• Anther: A “typical angiosperm anther is bilobed with each lobe having two theca, i.e., they are dithecous.” It is a “four-sided (tetragonal) structure consisting of four microsporangia located at the corners, two in each lobe.”
• Microsporangium Structure: Surrounded by four wall layers: “epidermis, endothecium, middle layers and the tapetum.” The outer three provide “protection and help in dehiscence,” while the “innermost wall layer is the tapetum,” which “nourishes the developing pollen grains.”
• Microsporogenesis: The process where “cells of the sporogenous tissue undergo meiotic divisions to form microspore tetrads,” which then dissociate into pollen grains.
• Pollen Grain (Male Gametophyte): Generally spherical, 25-50 micrometers in diameter, with a “prominent two-layered wall.”
  • Exine: The “hard outer layer” made of sporopollenin, “one of the most resistant organic material known.” It has “prominent apertures called germ pores where sporopollenin is absent.” The exine’s durability allows pollen grains to be “well-preserved as fossils.”
  • Intine: The “thin and continuous layer made up of cellulose and pectin.”
• Cells: At maturity, pollen grains contain “two cells, the vegetative cell and generative cell.” In over 60% of angiosperms, pollen is shed at this stage; in others, the generative cell divides to form two male gametes before shedding (3-celled stage).
• Pollen Products: “Pollen grains are rich in nutrients” and are used as food supplements, claimed to “increase the performance of athletes and race horses.” However, pollen from some species (e.g., Parthenium) can cause “severe allergies and bronchial afflictions.”
• Pollen Viability: Highly variable, from “30 minutes” in rice and wheat to “months” in some Rosaceae. Pollen can be stored for years in “liquid nitrogen (-196°C)” in “pollen banks” for crop breeding.

4.2. Female Reproductive Structures: Pistil, Megasporangium (Ovule), and Embryo Sac

• Pistil (Gynoecium): The female reproductive part, consisting of “stigma, style and ovary.”
  • Stigma: “Serves as a landing platform for pollen grains.”
  • Style: “Elongated slender part beneath the stigma.”
  • Ovary: “Basal bulged part” containing the ovarian cavity (locule) where the “placenta is located.” Megasporangia, commonly called ovules, arise from the placenta.
• Megasporangium (Ovule) Structure: A small structure attached to the placenta by a “stalk called funicle.” It is protected by “integuments” which encircle the nucellus, leaving a small opening called the micropyle. The “chalaza” is opposite the micropylar end.
• Nucellus: A “mass of cells” enclosed within the integuments, with “abundant reserve food materials.”
• Embryo Sac (Female Gametophyte): Located within the nucellus, typically formed from a single megaspore.
• Megasporogenesis: The formation of megaspores from a “megaspore mother cell (MMC)” through meiotic division, resulting in “four megaspores.”
• Female Gametophyte (Embryo Sac) Development: In most flowering plants, “one of the megaspores is functional while the other three degenerate.” The functional megaspore develops into the embryo sac (monosporic development). Its nucleus undergoes sequential mitotic divisions to form 8 nuclei. Cell walls form after the 8-nucleate stage, creating a “7-celled and 8-nucleate” mature embryo sac:
  • Egg apparatus: Three cells at the micropylar end (two synergids and one egg cell). Synergids have “filiform apparatus” to guide pollen tubes.
  • Antipodals: Three cells at the chalazal end.
  • Central cell: Large cell with “two polar nuclei.”

5. Pollination: Transfer of Pollen Grains

Pollination is the “mechanism to transfer pollen grains (shed from the anther) to the stigma of a pistil.” Since both male and female gametes are non-motile, pollination is essential for fertilisation.

5.1. Kinds of Pollination:

• Autogamy: “Pollination is achieved within the same flower.” Requires synchrony and close proximity of anthers and stigma. Cleistogamous flowers (e.g., Viola, Oxalis) “do not open at all” and are “invariably autogamous,” ensuring “assured seed-set even in the absence of pollinators.”
• Geitonogamy: “Transfer of pollen grains from the anther to the stigma of another flower of the same plant.” Functionally cross-pollination, but “genetically it is similar to autogamy since the pollen grains come from the same plant.”
• Xenogamy: “Transfer of pollen grains from anther to the stigma of a different plant.” This is the only type that brings “genetically different types of pollen grains to the stigma.”

5.2. Agents of Pollination:

Plants use “two abiotic (wind and water) and one biotic (animals) agents.”

• Abiotic Pollination:
  • Wind Pollination: More common. Pollen grains are “light and non-sticky.” Flowers have “well-exposed stamens” and “large often-feathery stigma” (e.g., corn cob, grasses).
  • Water Pollination: Rare (about 30 genera, mostly monocots). Pollen may be “long, ribbon like” and protected by a “mucilaginous covering” (e.g., Vallisneria, Hydrilla).
• Biotic Pollination (Animals): Majority of flowering plants use animals.
  • Common Agents: “Bees, butterflies, flies, beetles, wasps, ants, moths, birds (sunbirds and humming birds) and bats.” Bees are the “dominant biotic pollinating agents.”
  • Adaptations: Flowers are often “large, colourful, fragrant and rich in nectar.” Small flowers are “clustered into an inflorescence to make them conspicuous.”
  • Floral Rewards: Nectar, pollen grains, or “safe places to lay eggs” (e.g., Amorphophallus, Yucca).
  • Pollen/Nectar Robbers: Visitors that consume rewards “without bringing about pollination.”

5.3. Outbreeding Devices:

To prevent “inbreeding depression,” plants encourage cross-pollination through:

• Lack of Synchrony: Pollen release and stigma receptivity are not synchronised.
• Stigma/Anther Position: Placed at different positions to avoid contact.
• Self-Incompatibility: A “genetic mechanism” preventing self-pollen fertilisation.
• Unisexual Flowers:
  • Monoecious: Male and female flowers on the “same plant” (e.g., castor, maize) prevents autogamy.
  • Dioecy: Male and female flowers on “different plants” (e.g., papaya) prevents both autogamy and geitonogamy.

5.4. Pollen-Pistil Interaction:

A “continuous dialogue between pollen grain and the pistil,” allowing the pistil to recognise compatible pollen.

• Compatible Pollen: Germinates, grows a pollen tube through the style to the ovule.
• Incompatible Pollen: Rejected by the pistil.
• Artificial Hybridisation: Involves emasculation (anther removal) and bagging (covering the flower) to control pollination for desired hybrids.

6. Double Fertilisation: A Unique Angiosperm Event

After entering the ovule, the pollen tube releases two male gametes.

• Syngamy: “One of the male gametes moves towards the egg cell and fuses with its nucleus,” forming a “diploid cell, the zygote.”
• Triple Fusion: “The other male gamete moves towards the two polar nuclei located in the central cell and fuses with them,” producing a “triploid primary endosperm nucleus (PEN).”
• Double Fertilisation: “Since two types of fusions, syngamy and triple fusion take place in an embryo sac the phenomenon is termed double fertilisation, an event unique to flowering plants.”

The central cell becomes the primary endosperm cell (PEC), and the zygote develops into an embryo.

7. Post-Fertilisation: Development of Seed and Fruit

7.1. Endosperm Development:

“Endosperm development precedes embryo development” to provide nutrition.

• Free-nuclear endosperm: Initial stage with free nuclei (e.g., coconut water).
• Cellular endosperm: Follows cell wall formation (e.g., white kernel of coconut).
• Types of Seeds:
  • Non-albuminous/Ex-albuminous: Endosperm is consumed by the embryo (e.g., pea, groundnut).
  • Albuminous: Endosperm persists in the mature seed (e.g., castor, coconut, wheat).

7.2. Embryo Development:

The zygote develops into the embryo, passing through proembryo, globular, heart-shaped, and mature stages.

• Dicotyledonous Embryo: Has an embryonal axis and two cotyledons. The axis includes the epicotyl (terminating in the plumule) and the hypocotyl (terminating in the radicle).
• Monocotyledonous Embryo: Has one cotyledon (scutellum in grasses). The radicle is covered by the coleorrhiza and the shoot apex by the coleoptile.

7.3. Seed and Fruit Development:

• Seed: The “final product of sexual reproduction,” a “fertilised ovule.” It consists of seed coat(s), cotyledon(s), and an embryo axis. The micropyle remains for oxygen and water entry. Mature seeds undergo dehydration and enter dormancy.
• Fruit: The ovary develops into the fruit. The ovary wall becomes the fruit wall (pericarp).
  • True Fruits: Develop only from the ovary.
  • False Fruits: Other floral parts contribute (e.g., apple, strawberry).
  • Parthenocarpic Fruits: Develop without fertilisation, making them seedless (e.g., banana).

7.4. Advantages of Seeds:

• Reproductive processes are independent of water.
• Enable dispersal to new habitats.
• Provide nourishment to young seedlings.
• The hard seed coat offers protection.
• Generate new genetic combinations and variation.

8. Apomixis and Polyembryony: Alternatives to Sexual Reproduction

8.1. Apomixis:

A “special mechanism, to produce seeds without fertilisation,” found in some species like Asteraceae and grasses. It is a form of asexual reproduction that mimics sexual reproduction.

• Importance: If hybrid plants are made apomictic, there is no segregation of characters in the progeny. This allows farmers to “keep on using the hybrid seeds to raise new crop year after year” without purchasing new seeds annually.

8.2. Polyembryony:

The “occurrence of more than one embryo in a seed.” It can occur when nucellar cells develop into embryos (e.g., many Citrus and Mango varieties).