CHAPTER 2

Sexual Reproduction in Flowering Plants

Flower

• A typical flower consists of four floral parts, or whorls: calyx (sepals), corolla (petals), androecium (stamens) and gynoecium (carpels). Calyx and corolla are accessory parts.

• Androecium(Male reproductive part) and Gynoecium(Female reproductie part) are essential parts.

Microsporangium

• The terminal part of the stamen bears the anther. It consists of two lobes. Each lobe has two chambers or loculi, called the pollen sacs or microsporangia. Thus a typical anther is tetrasporangiate. The four pollen sacs appear to lie in the four corners of an anther.

Wall of the anther• The anther wall consists of four layers. These are

epidermis, endothecium, middle layers and tapetum.• Epidermis: This is the outermost layer. It is only one

cell in thickness.• Endothecium: This layer is situated just below the

epidermis. It is a single layer of radially elongated cell with fibrous thickening.

• Middle layers: The middle layers consist of three to four layers of thinned wall cells. These layers are situated below the endothecium.

• Tapetum: Tapetum is the innermost layer of the wall and is multinucleate. The tapetal cells provide nutrition to the developing microspores.

Stamen, Microsporangium and Pollen Grain

Microsporogeneis OR Development of male gametophyte

Male gametophyte=microspore or pollen grain

The process of formation of microspores from a microspore mother cell by meiosis is called microsporogenesis.

Exine is composed of sporopollenin. Intine is composed of cellulose and pectin.

Parthenium or Carrot Grass causes pollen allergy.

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

2) Nucellus : The primordium grows into a mass of cells forming nucellus, the body of ovule. Cells of nucellus are rich in reserve food.

3) Intgegument: The two protective covering of nucellus are integuments.

4) Micropyle : Integuments encircle the ovule except at the tip where a small opening called the micropyle.

5) Chalaza: Basal part of ovule is called chalaza that lie opposite to micropyle.

6) Hilum: The body of the ovule fuses with funicle in the region called hilum.

7) Embryo sac: A single embryo sac or female gametophyte located in the nucellus, which is developed from megaspore. It contains egg apparatus, antipodal cells and secondary nucleus.

Structure of the ovule (Megasporangium)

(Anatropous ovule)

Megasporogenesis

• It is the process of formation of megaspores from megaspore mother cell.

• The ovules are present inside the ovary in multiple lobes. A cell in the ovule differentiates into a megaspore mother cell. The megaspore mother cell is diploid. This megaspore mother cell undergoes meiosis to form 4 haploid megaspores. 3 of the 4 megaspores degenerate and only one megaspore is left in each ovule.

Development of Female Gametophyte (Megasporogenesis)