Pre-Fertilization Events in Plants
Before fertilization occurs in flowering plants (angiosperms), several important processes take place. These include the formation of male (pollen grains) and female (embryo sac) gametophytes through microsporogenesis and megasporogenesis.
A. Microsporogenesis (Formation of Pollen Grains)
1. Structure of Anther
The anther is the male reproductive organ of a flower, found in the stamens.
A transverse section (T.S.) of an anther shows that it contains four microsporangia (pollen sacs).
Each microsporangium is surrounded by layers:
Epidermis – Outer protective layer.
Endothecium – Provides support during pollen release.
Middle Layers – Thin-walled layers that degenerate as pollen matures.
Tapetum – Provides nutrients for developing pollen grains.
Diagram: T.S. of Anther Showing Microsporangia
(Labeled diagram showing four microsporangia, different layers, and pollen mother cells.)
2. Formation of Microspores (Microsporogenesis)
Inside each microsporangium, pollen mother cells (PMC, 2n) undergo meiosis to form haploid microspores (n).
Each microspore matures into a pollen grain.
This process is called microsporogenesis.
Diagram: Microsporogenesis Process
(Labeled diagram showing the stages of microspore formation and development into pollen grains.)
3. Structure of Pollen Grain
Outer Layer (Exine) – Thick, rigid, and covered with sporopollenin (protects pollen).
Inner Layer (Intine) – Thin and made of cellulose and pectin.
Two Cells Inside:
Vegetative Cell – Larger, stores nutrients, and helps in pollen tube formation.
Generative Cell – Smaller, divides into two sperm cells during fertilization.
Diagram: Structure of a Mature Pollen Grain
(Labeled diagram showing exine, intine, vegetative cell, and generative cell.)
B. Megasporogenesis (Formation of Female Gametophyte)
1. Structure of Ovule
The ovule is the female reproductive structure inside the ovary.
It is attached to the ovary wall by a stalk called the funicle.
The ovule is enclosed by two protective layers called integuments, leaving a small opening called the micropyle for pollen entry.
Inside the ovule, a diploid megaspore mother cell (MMC, 2n) is present.
Diagram: Structure of an Ovule with Its Parts
(Labeled diagram showing funicle, integuments, micropyle, nucellus, and embryo sac.)
2. Formation of Embryo Sac (Megasporogenesis Process)
The megaspore mother cell (MMC, 2n) undergoes meiosis, producing four haploid megaspores (n).
Only one megaspore survives, and it divides mitotically to form the embryo sac (female gametophyte).
The mature embryo sac contains seven cells and eight nuclei:
One Egg Cell (participates in fertilization).
Two Synergid Cells (help in guiding pollen tube).
Three Antipodal Cells (no specific function).
One Central Cell (with two polar nuclei) (fuses with a sperm to form endosperm).
Diagram: Megasporogenesis Process and Embryo Sac Formation
(Labeled diagram showing the stages of meiosis, mitotic divisions, and the final structure of the embryo sac.)
Conclusion
Microsporogenesis leads to the formation of pollen grains (male gametophytes).
Megasporogenesis results in the formation of the embryo sac (female gametophyte).
These processes ensure successful fertilization in flowering plants and the formation of seeds for reproduction.