Pollen Tube

Pollen Tube
Definition
The tube like structure arised from the male gametophyte contain make gamete (Sperm) called pollen tube.
Role of Pollen Tube
1. Pollen tube acts as a “Vehical for Sperms”.
2. Pollen tube grow down the carpels and discharge sperm into embryo sacs resulting in the fertilization of eggs.
3. The evolution of pollen tubes parallels the evolution of seeds.
4. The obstacle has been overcome by the development of pollen tubes.
Development of Male Gametophyte

• Inside the microsporangium (pollen sac) a large number of cells called microscope mother cells are present.
• Each microspore mother cell forms four haploid microspores by meiosis.
• Each microspore divides into two cells by mitosis. One cell is called generative cell and the other is known be cell. The microspore is surrounded by a thick resistant – wall.
• In this condition the microspore is considered as immature male gametophyte.

Development of Female Gametophyte

• The female gametophyte or embryo sac developers inside the mega sporangium or nucellus of ovule.
• The megaspore mother cell (2n) form four haploid cells by meiosis, which are called linear tetrad.
• Megaspore develops into female gametophyte.
• The haploid nucleus of megaspore form eight nuclei by the mitotic divisions.
• Out of these eight nuclei, two are fused to form a diploid secondary nucleus.
• In this stage, the embryo sac contains seven nuclei three nuclei towards chalaza are called antipoda three nuclei towards the micropylar end are called egg apparatus.
• In the egg apparatus, two synergids and one egg cell are found.

Double Fertilization

• After the pollination, the pollen grain is transfered into the stigma where it germinates. \
• During the germination inner layer intine rupture the exine at any germ pore and come out in the form tube, which is called pollen tube.
• The pollen tube grown downward towards the ovule and finally it enters into ovule through microspore.
• The nuclei of tube cell and generative cells are shifted into the pollen tube.
• When the generative nucleus enters into the pollen tube, it divides to form two male gametes.
• When the pollen tube penetrates into the embryo sac its tip bursts and both male gametes enter embryo sac.
• First male gamete is fused with egg nucleus to form diploid oospore. This fusion is called syngam second male gamete is combined with secondary nucleus to form triploid (3n) endosperm nucleus.
• In this way fertilization take place twice in an embryo sac, therefore this process is called fertilization which was discovered by Nawaschin in 1989.

Changes in Ovule after Double Fertilization

• After double fertilization, the ovule is converted into a seed. In this process following changes take place.
• The oospore is change into an embryo, cotyledons, plumule and radicle.
• The endosperm nucleus forms a nutritive tissue, which provides nourishment to the developing embryo the tissue is called endosperm.
• The integuments form seed coats.
• Micropyle also persists in seeds.
• The seed is dehydrated until its water contents become 5 to 15% of its weight.