Pistil, Megasporangium, Embryo sac

Define the gynoecium and describe its types.

The gynoecium is the female repoductive part of a flower. It may have 1 pistil (Monocarpellary) or more than 1 pistil (Multicarpellary). When there are more than one pistil, they could be fused (Syncarpous) or free (Apocarpous)

What is pistil? Name its parts and describe them.

Pistils collectively constitute the female reproductive part of a flower. A pistil consists of i) Stigma ii) Style iii) Ovary.

The stigma is the landing platform for the pollen grains

The style is the elongated slender part beneath the stigma

The basal blushed part is the ovary

Describe the ovary and ovule in detail.

The ovary

1) The ovary is the basal bulged part of the pistil.

2) Inside the ovary is the ovarian cavity (Locule)

3) Inside the locule is the placenta

4) Arising from the placenta are the ovules (Megasporangia)

5) The number of ovules arising from the placenta may be 1 as in mango, wheat, paddy or many as in watermelon papaya orchids.

The ovule

1) The ovule is a small structure attached to the placenta of the ovule by the means of a stalk called the funicle.

2) The body of the ovule is fused with the funicle in the region called Hilum.

3) Each ovule has one or two protective envelops called integuments

4) These integuments encircle the nucellus except the tip where a small opening called the micropyle is present.

5) Opposite to the micropyle, is the chalaza which is the basal part of the ovule.

6) The nucellus which is enclosed by the integuments is a mass of cells. These cells have abundant reserve food material

7) In the nucellus, an embryo sac is located which is the female gametophyte.

Give example of species with one ovule arising from the placenta in the Ovary.

Example: Wheat, Paddy, Rice

Give example for species with more than one ovules arising from placenta of ovary.

Example: Watermelon, Papaya, Orchids

Junction between ovule and funicle ?

Hilum

Define megasporogenesis in detail.

The process of formation of megaspores from the megaspore mother cell through meiosis is called Megasporogenesis.

A single cell from the nucellus differentiates into the megaspore mother cell at the micropylar region of the nucellus.

The megaspore mother cell is a large cell containing dense cytoplasm and a prominent nucleus. This megaspore mother cell undergoes meiosis and gives rise to 4 megaspores.

What is monosporic development ?

After megasporogenesis , only one megaspore is functional while the other three degenerate.

This functional megaspore develops into the embryo sac (female gametophyte)

This method of embryo sac formation is called monosporic development.

Describe the formation of embryo sac in detail.

• The nucleus of the functional megaspore divides mitotically to form two nuclei which move to the opposite poles, forming 2 nucleate embryo sac

• Two more mitotic nuclear divisions occur leading to the formation of the 4 nucleate embryo sac and 8 nucleate embryo sac.

• These mitotic divisions are strictly free nuclear, the nuclear divisions are not immediately followed by cell wall formation.

• After the 8 nucleate stage, the cell walls are formed leading to the organization of a typical embryo sac.

• 6 of the 8 nuclei are surrounded by cell walls and organized into cells while the remaining 2 nuclei called polar nuclei are situated below the egg apparatus in the large central cell.

• There is a characteristic distribution of cells in the embryo sac. 3 cells are grouped together at the micropylar end and constitute the egg apparatus

• The egg apparatus consists of 2 synergids and one egg cell

• These synergids have cellular thickenings at the micropylar tip called the filiform apparatus which guide the pollen tubes into the synergid.

• Three cells are at the chalazal end and are called antipodals.

• The large central cell has 2 polar nuclei and hence the embryo sac is 8 nucleate and 7 celled.