SEXUAL REPRODUCTION IN FLOWERING PLANTS
The Pistil, Megasporangium (ovule) and Embryo sac
The pistil is the female reproductive organ of a flower in flowering plants (angiosperms). It plays a central role in the process of sexual reproduction by containing the female reproductive structures and facilitating fertilization. Here’s an overview of the structure of the pistil:
Stigma:
- The stigma is the uppermost part of the pistil and is typically located at the top of a stalk called the style.
- Its main function is to receive pollen grains during the process of pollination. The stigma is often sticky or has specialized structures that help capture and hold onto pollen.
Style:
- The style is a slender, elongated tube-like structure that connects the stigma to the ovary.
- Its purpose is to provide a pathway for pollen tubes to grow from the stigma to the ovules within the ovary during fertilization.
Ovary:
- The ovary is the swollen, basal part of the pistil located at the base of the style.
- Within the ovary are one or more ovules, each containing a female gametophyte (embryo sac) with an egg cell.
- The ovary’s primary function is to house and protect the developing ovules, which will eventually become seeds after fertilization.
Ovules:
- Ovules are the female reproductive structures found within the ovary.
- Each ovule contains an embryo sac, which includes the female gametes: the egg cell, synergids, and antipodal cells.
- The egg cell is the female gamete that fuses with a sperm cell during fertilization to form a zygote.
Septum (Placenta):
- In some flowers, the ovary may be divided into sections by a septum or placenta.
- The septum provides support for the ovules and can facilitate the development of multiple seeds within the ovary.
Carpels:
- The pistil is often composed of one or more carpels, which are modified leaf structures.
- Each carpel consists of a stigma, style, and ovary.
- A single carpel may function as an individual pistil or multiple carpels can be fused together to form a compound pistil.
Comparison of gynoecium terminology using carpel and pistil
Gynoecium composition
Carpel terminology
Carpel terminology
Examples
Single carpel
Monocarpous (unicarpellate) gynoecium
A pistil (simple)
Avocado (Persea sp.), most legumes
Multiple distinct ("unfused") carpels
Apocarpous (choricarpous) gynoecium
Pistils (simple)
Strawberry (Fragaria sp.), Buttercup
Multiple connate ("fused") carpels
Syncarpous gynoecium
A pistil (compound)
Tulip (Tulipa sp.), most flowers
The Megasporangium (Ovule)
The megasporangium, also known as the ovule, is a female reproductive structure found within the ovary of a flower. It plays a central role in the process of sexual reproduction in flowering plants (angiosperms). Here is an explanation of the structure of the megasporangium in point form:
Location:
- The megasporangium is located within the ovary of the pistil, which is the female reproductive organ of a flower.
Covering Layers:
- The megasporangium is covered by several protective layers, which include: a. Integuments: These are one or more layers of protective tissue that surround the megasporangium, leaving a small opening called the micropyle at one end. b. Outer Epidermis: The outermost layer of the integuments provides an additional protective barrier. c. Inner Epidermis: The inner layer of the integuments is in direct contact with the megasporangium’s contents.
Nucellus:
- Inside the integuments, the central part of the megasporangium is called the nucellus.
- The nucellus contains the female gametophyte, also known as the embryo sac, which develops from a megaspore.
Megaspore Mother Cell:
- Within the nucellus, a single megaspore mother cell undergoes meiosis to produce four haploid megaspores.
Surviving Megaspore:
- Typically, only one of the four megaspores survives and continues to develop, while the others degenerate.
Embryo Sac (Female Gametophyte):
- The surviving megaspore undergoes several rounds of mitotic divisions to form the embryo sac or female gametophyte.
- The embryo sac consists of several cells, including: a. Egg Cell (Ovum): The female gamete that fuses with a male gamete (sperm) during fertilization. b. Synergids: Cells that assist in guiding the pollen tube to the egg cell. c. Antipodal Cells: Cells located at the opposite end of the embryo sac from the egg cell. Their precise function is not fully understood.
Micropyle:
- The micropyle is a small opening at one end of the integuments.
- It allows for the entry of the pollen tube during pollination, facilitating fertilization.
Megasporogenesis
Megasporogenesis is the process in the life cycle of flowering plants (angiosperms) that results in the formation of megaspores. These megaspores are essential in the development of the female gametophyte (embryo sac) within the ovule (megasporangium) of a flower. Here’s an explanation of megasporogenesis:
Location:
- Megasporogenesis occurs within the megasporangium, which is typically located within the ovule, a structure found within the ovary of a flower.
Diploid Mother Cell:
- The process begins with a diploid (2n) cell known as the megaspore mother cell. This cell is located within the nucellus of the ovule.
Meiosis I:
- The megaspore mother cell undergoes the first division, meiosis I.
- Meiosis I results in the formation of two haploid (n) cells called megaspore daughter cells.
- These two megaspore daughter cells are genetically different due to the random assortment of chromosomes during meiosis.
Degeneration of Three Megaspores:
- Out of the two megaspore daughter cells produced in meiosis I, typically only one of them survives and continues to develop.
- The other three megaspores typically degenerate and do not participate in the formation of the female gametophyte.
Surviving Megaspore:
- The surviving megaspore begins to grow and undergoes further development.
- It enlarges and becomes the functional megaspore, which will eventually give rise to the female gametophyte or embryo sac.
Mitotic Divisions:
- The functional megaspore undergoes several rounds of mitotic divisions without cytokinesis.
- These divisions result in the formation of multiple cells within the megaspore.
Formation of the Female Gametophyte (Embryo Sac):
- Through mitotic divisions, the functional megaspore develops into the female gametophyte, also known as the embryo sac.
- The embryo sac typically contains eight nuclei, which are distributed among various cells.
- The nuclei include the egg cell (ovum), synergids (cells that assist the pollen tube during fertilization), and antipodal cells (cells located at the opposite end of the embryo sac from the egg cell, with a less understood role).
Micropyle Formation:
- As the female gametophyte matures, a small opening called the micropyle forms in the integuments (protective layers) of the ovule.
- The micropyle allows for the entry of a pollen tube during pollination.
