Reproduction in Flowering Plants

Study Notes

Reproduction in Flowering Plants

Reproduction is the means by which species continue generation after generation.
Flowering plants exhibit sexual reproduction.
The flower is the reproductive organ of flowering plants (angiosperms).
Flowers consist of four whorls attached to the receptacle:
- Calyx: The outermost whorl composed of sepals, usually green.
- Corolla: The second whorl made up of petals, often brightly colored.
- Androecium: The male reproductive organ consisting of stamens.
- Gynoecium: The female reproductive organ composed of ovary, style, and stigma.

Pre-Fertilization Structures and Events

Flower formation involves structural and hormonal changes.
Inflorescence develops, bearing floral buds and eventually flowers.
Male and female reproductive parts differentiate within the flower.
- Stamens produce male gametes (pollen grains).
- Pistils produce female gametes (eggs).

Stamen, Microsporangium, and Pollen Grain

Stamen: Composed of a filament (stalk) and an anther.
Anther: Has two lobes, each containing two pollen sacs (microsporangia).
Microsporangium: Surrounded by four layers:
- Epidermis: Outermost layer.
- Endothecium: Just below the epidermis.
- Middle Layer: Middle layer.
- Tapetum: Innermost layer, provides nutrition to developing pollen grains.
Sporogenous Tissue: Compactly arranged cells within the microsporangium, forming microspore mother cells (MMCs).

Microsporogenesis

The process of microspore (pollen grain) formation from MMCs through meiotic division.
MMCs undergo meiosis to produce four haploid microspores (pollen grains).
Pollen grains develop into male gametophytes.

Pollen Grain Structure

Exine: Outer layer made of sporopollenin, a resilient organic material resistant to degradation.
Intine: Inner, thin layer made of cellulose and pectin.
Germ Pores: Apertures on the exine where sporopollenin is absent, allowing pollen tube formation.
Plasma Membrane: Surrounds the cytoplasm of the pollen grain.

Mature Pollen Grain

Contains two cells with nuclei:
- Vegetative Cell: Larger, with abundant food reserves and a large, irregular nucleus. Responsible for pollen tube development.
- Generative Cell: Smaller, involved in syngamy (fusion with an egg). Contains dense cytoplasm and a nucleus.

Pollen Grain Functions

Pollen grains of some species, like Parthenium, can cause allergies and respiratory issues.
Pollen grains are rich in nutrients and used as food supplements in pollen tablets.
Pollen Viability: The lifespan of pollen grains can vary between species. It must land on the stigma before this period to germinate.
Pollen Banks: Large numbers of pollen grains are stored in liquid nitrogen at -196°C for future use.

The Pistil, Megasporangium (Ovule), and Embryo Sac

Gynoecium: May consist of a single pistil (monocarpellary) or multiple pistils (polycarpellary), which can be fused (syncarpous) or free (apocarpous).
- Examples:
  - Multicarpellary and Syncarpous: Papaver
  - Multicarpellary and Apocarpous: Michelia
Pistil: Has three parts:
- Stigma: Receives pollen grains.
- Style: Connects the stigma to the ovary.
- Ovary: Contains the ovule(s).
Ovarian Cavity: Inside the ovary, where the placenta is located.
Megasporangia (Ovules): Arise from the placenta.

Ovule Structure

Funicle: Stalk attaching the ovule to the placenta.
Hilum: Junction between the ovule and the funicle.
Integuments: Protective envelopes surrounding the ovule.
Micropyle: Small opening at the tip of the ovule where the pollen tube enters.
Chalaza: Basal part of the ovule.
Nucellus: Mass of cells enclosed by the integuments, containing abundant food reserves.

Megasporogenesis

The process of megaspore formation from the megaspore mother cell (MMC) by meiotic division.
This occurs within the ovule.
The MMC undergoes meiosis to produce four haploid megaspores.
In most flowering plants, three megaspores degenerate.
One megaspore develops into the female gametophyte (embryo sac).

Embryo Sac Development

The functional megaspore's nucleus divides mitotically twice, forming two nuclei at opposite poles (2-nucleate embryo sac).
Further mitotic divisions result in an 8-nucleate embryo sac.
Six nuclei are surrounded by cell walls, while the remaining two (polar nuclei) are located below the egg apparatus.

Egg Apparatus

Three cells at the micropylar end of the embryo sac:
- Egg Cell: The female gamete.
- Synergids: Two cells flanking the egg cell, assisting in fertilization.

Antipodal Cells

Three cells at the chalazal end of the embryo sac, opposite the egg apparatus. Their function is poorly understood.

Description

Explore the fascinating process of reproduction in flowering plants through this quiz. Learn about the structure and functions of various flower whorls, including calyx, corolla, androecium, and gynoecium. Understanding these components is essential to grasp how angiosperms reproduce.