(5) Post-pollination development: (a) The liberated pollen g | CBSE OFFIClAL

(5) Post-pollination development:

(a) The liberated pollen grains are transferred to the receptive surface of the carpel (i.e., stigma) by the process called pollination.

(b) On the stigma, the pollen grain absorbs water and swells within a few minutes.

(c) The vegetative (or tube) cell enlarges and comes out through one of the apertures in the form of a pollen tube.

(d) The wall of pollen tube is the extension of intine. The tube secretes exogenous pectinases and other hydrolytic enzymes to create a passage for its entry.

(e) The vegetative and generative nuclei are carried by the pollen tube, the farmer lying at its tip.

(f) The generative cell divides to form two non-motile male gametes.

(g) The tube nucleus has no important function and may disintegrate.

Megasporogenesis
The process of formation of megaspore from megaspore mother cell by meiotic division is known as megasporogenesis. This process takes place in ovule.

Structure and Functions of Parts of An Ovule(1) Structure of ovule:

Ovule is considered to be an integumented megasporangium. The ovule consists of the stalk and the body. The stalk is called funicle. One end of the funicle is attached to placenta and the other end to the body of the ovule. The point of attachment of funicle with the body is called hilum. Sometimes funicle gets fused with the body of the ovule one side and forms a ridge known as raphe. The body of the ovule shows two ends: the basal end, often called the chalazal end and the upper end is called micropylar end. The main body of the ovule is covered with one or two envelopes called integuments. These leave an opening at the top of the ovule called micropyle. The integuments enclose a large parenchymatous tissue known as nucellus.

(2) Development of female gametophyte (Megagametogenesis):

The process of development of female gametophyte or embryo sac from megaspore is called megagametogenesis.

(i) Monosporic type (Polygonum): In this type, only one megaspore situated towards chalazal end takes part in the development of embryo sac.

(ii) Bisporic type: In this type two megaspore nuclei take part in embryo sac formation.

(iii) Tetrasporic type: This type of embryo sac develops from four megaspore nuclei.

Pollination

(1) The process of transfer of pollen grains from an anther to the stigma of the same flower or of different flower.

(2) It is of two types:

(i) Self pollination: This process involves the transfer of pollen grains from the anthers to the stigma of the same flower or of another flower borne by the same plant.

(ii) Cross pollination: Cross pollination involves the transfer of pollen grains from the flower of one plant to the stigma of the flower of another plant. It is also called xenogamy.

Fertilization
The fusion of two dissimilar sexual reproductive units (gametes) is called fertilization. This process was discovered by Strasburger (1884).

(1) Germination of pollen grain on stigma and growth of pollen tube:

Pollen grains reach the receptive stigma of the carpel by the act of pollination. Pollen grains, after getting attached to the stigma, absorb water and swell. Subsequent to mutual recognition and acceptance of pollen grains, the pollen grain germinates (in vivo) to produce a pollen tube which grows into stigma towards the ovarian cavity.

(2) Entry of pollen tube into ovule:

After reaching ovary, the pollen tube enters the ovule. Pollen tube may enter the ovule by any one of the following routes:

(i) Porogamy: When the pollen tube enters the ovule through micropyle, it is called porogamy. It is the most common type. e.g. Lily.

(ii) Chalazogamy: The entry of pollen tube into the ovule from chalazal region is known as chalazogamy. Chalazogamy is less common. e.g. Casuarina, Juglans, Betula, etc. It was first observed by Treub (1981) in Casuarina.

(iii) Mesogamy: The pollen tube enters the ovule through its middle part i.e. through integument (e.g. Cucurbita, Populus) or through funicle (e.g. Pistacia).