Angiosperms: Flower Structure and Function

Flower: A Fascinating Organ of Angiosperms

• Flowers are essential for sexual reproduction in flowering plants (angiosperms).
• They exhibit a wide range of morphological adaptations aimed at producing fruits and seeds.
• Flowers possess aesthetic, ornamental, and cultural significance for humans.

Pre-fertilisation: Structures and Events

• Hormonal and structural changes precede flowering, leading to the formation of inflorescences and flowers.
• The male (androecium) and female (gynoecium) reproductive structures develop within the flower.

Stamen, Microspore, and Pollen

• The stamen consists of two parts: filament (stalk) and anther (where pollen grains develop).
• Anthers are typically bilobed with microsporangia that become pollen sacs containing pollen grains.
• The microsporangium features four wall layers, with the tapetum providing nourishment to developing pollen grains.

Pollen Grain Structure

• Pollen grains are generally spherical, ranging from 25-50 micrometers in diameter, with a tough outer wall (exine) made of sporopollenin, a resistant organic substance.
• The inner wall (intine) is thin and composed of cellulose and pectin.
• Mature pollen grains contain two cells: a vegetative cell and a generative cell.

Formation and Function of Pollen Grains

• Pollen grains must land on a stigma to facilitate fertilization; viability durations vary across species.
• Allergies and respiratory disorders in some individuals are often caused by pollen grains.
• Pollen grains are also used in palaeobotanical studies as fossils.

The Pistil: Female Reproductive Part

• The gynoecium, comprising one or multiple pistils, is the female reproductive structure.
• Each pistil includes stigma (reception platform), style (elongated part), and ovary (bulged base), which contains ovules.

Ovule Structure and Development

• Ovules are attached to the placenta by a funicle and consist of a protective integument and a nucellus that contains the embryo sac.
• Megasporogenesis involves the formation of megaspores from the megaspore mother cell (MMC), leading to female gametophyte (embryo sac) development.

Embryo Sac Formation

• The functional megaspore undergoes multiple mitotic divisions to form an 8-nucleate embryo sac.
• The egg apparatus, located at the micropylar end, consists of an egg cell and two synergids, while antipodal cells are positioned at the chalazal end.

Pollination and Types

• Pollination: transfer of pollen grains from anther to stigma essential for fertilization.
• Autogamy: self-pollination occurs when pollen is transferred within the same flower, common in plants like peas and wheat.

Summary of Pollination Mechanics

• Pollination is facilitated by external agents (wind, water, animals) that help transfer pollen to the stigma.
• Adaptations in floral structure play a significant role in attracting pollinators and ensuring successful reproduction.### Types of Pollination
• Cross-pollination occurs when pollen is transferred from the anther of one flower to the stigma of another flower.
• Examples of plants utilizing cross-pollination include sunflowers, roses, and apples.

Flower Types

• Chasmogamous Flowers: Open flowers that expose their reproductive parts.
• Cleistogamous Flowers: Closed flowers with the stigma and anther housed within, facilitating automatic self-pollination.
  • Flowers are typically autogamous, ensuring pollination without external agents.

Advantages and Disadvantages of Cleistogamy

• Advantages: Guarantees seed production even without pollinators, leading to reliable reproduction.
• Disadvantages: Results in a lack of genetic diversity due to self-pollination.

Types of Cross-Pollination

• Geitonogamy: Transfer of pollen to the stigma of a different flower on the same plant; results in genetically similar offspring.
• Xenogamy: Transfer of pollen to the stigma of a different plant, promoting genetic diversity.

Pollination Agents

• Abiotic Agents: Include wind and water, facilitating pollination without living organisms.
• Biotic Agents: Encompass animals, playing a major role in the pollination process.

Abiotic Agents

• Wind Pollination: Involves light and dry pollen grains, with flowers characterized by:
  • Exposed anthers to release pollen.
  • Large, feathery stigmas to catch pollen.
  • Non-sticky pollen that can be easily carried by the wind.
• Water Pollination:
  • Utilizes light and waterproof pollen grains.
  • Flowers are often submerged, as seen in species like Vallisneria.

Biotic Agents

• Insect Pollination: Predominantly utilized by flowering plants.
  • Flowers are large, fragrant, and colorful to attract insects.
  • Nectar production is a key attraction point.
• Bird Pollination:
  • Flowers are brightly colored and produce nectar, with little to no scent.
  • Hummingbirds are a common pollinator.
• Bat Pollination:
  • Flowers are usually white or dull in color and produce both nectar and scent.
  • Example includes the banana plant.

Efficiency of Pollination

• Wind and water pollination are generally less efficient compared to insect pollination.
• Wind-pollinated flowers typically produce more pollen than those that rely on insects.
• Water-pollinated flowers tend to be submerged and do not usually produce nectar, unlike many wind-pollinated species.