The Pistil, Megasporangium (ovule) and Megasporogenesis
· The gynoecium represents the female reproductive part of the flower.
· The gynoecium may consist of
· a single pistil (monocarpellary) or
· more than one pistil (multicarpellary)
When there are more than one, the pistils may be
· fused together (syncarpous) or
· may be free (apocarpous)
· Each pistil has three parts
1. Stigma (landing platform for pollen grains)
2. Style (elongated slender part beneath the stigma)
3. Ovary (The basal bulged part of the pistil)
→ Inside the ovary is the ovarian cavity (locule).
→ The placenta is located inside the ovarian cavity.
→ Arising from the placenta are the megasporangia, commonly called ovules.
→ The number of ovules in an ovary may be
· water melon
The Megasporangium (Ovule) :
· The ovule
→ is a small structure
→ attached to the placenta (parenchymatous cushion)
→ by means of a
stalk called funicle. (or funiculus)
· The body of the ovule fuses with funicle in the region called hilum.
→ Thus, hilum represents the junction between ovule and funicle.
· Typical ovule is inverted or anatropus. Here funiculus is fused lengthwise with ovule beyond the hilum. A ridge is formed, which is called raphe. Funiculus is vascular to supply nourishment.
· Each ovule has one or two protective envelopes called integuments.
· Integuments encircle the ovule except at the tip where a small opening called the micropyle is organised.
· Opposite the micropylar end, is the chalaza, representing the basal part of the ovule.
· Enclosed within the integuments is a mass of cells called the nucellus.
· Cells of the nucellus have abundant reserve food materials.
· Located in the nucellus is the embryo sac or female gametophyte (1N).
· An ovule
→ generally has a single embryo sac
→ formed from a megaspore
through reduction division.(meiosis)
· The process of
→ formation of megaspores
→ from the megaspore mother cell
is called megasporogenesis.
· One of the cells of nucellus differentiate
→ as a single megaspore mother cell (MMC) (2N)
→ in the micropylar region of the nucellus.
· Megaspore mother cell (MMC)
→ is a large cell containing
→ dense cytoplasm and
→ a prominent nucleus.
· The MMC undergoes meiotic division.
· Meiosis results in the production of four (haploid) megaspores .