What is Pollination & Role of Pollen Tube

The transfer of pollen grains from anther to stigma of a flower is called pollination. Pollination is two types.

1. Self Pollination
2. Cross Pollination

Self Pollination: Transfer of pollen grain from anther to stigma of same flower is known as self-pollination. In this type only single flower is involved. Deocious plants cannot self-pollinate because they are uni sexual, being either staminate or carpel late.

Cross Pollination: Transfer of pollen grain from the anther to stigma of another flower of same species. Cross-pollination takes place by different agents like wind, water, insects and animals. In cross-pollination two flowers involved.

    When pollen grains reached to stigma after some rest they begin to germinate. The outer layer of pollen grain (exine) bursts and a pore from called germ pore, the inner layer (intine) develop into a tube called pollen tube.

Role of Pollen Tube: Pollen tube is a source or vehicle to take the male gametes to ovule because there is a little between male gamete and ovule, so pollen tube helps there to take the male gametes to the ovule. Therefore, the development of pollen tube is absolutely necessary, without it the process of fertilisation is not possible.

Development of Male Gametophyte (Pollen): 

    In microsporangia (Pollen Sac) of an anther a large number of diploid microspores mother cells are present. These microspore mother cells divided by meiosis and each from four haploid microspores. Each microspores ultimately divides once by mitosis and produces two cells, one is called generative cell and the other is called tube cell. These two cells surrounded by a thick resistant wall. Two cells and their wall collectively form a pollen grain or immature male gametophyte.

Development of Female Gametophyte (embryo Sac): 

     Ovules contain megasporangia. In megasporangium of each pvule has a megaspore mother cell. This megaspore mother cell grows and divided by meiosis to produce four haploid megespores. In many angiosperms only one of them survive. This megaspore continues to grow and is nucleus divides by mitosis three time. In this way eight haploid nuclei formed, then these nuclei surrounded by partitions. This multicellular structure is called embryo sac, which is the female gametophyte.

        In mature embryo sac there are two groups of three cells at each end. The three cells lying towards micropyle form egg apparatus. One larger cell of this group is called Egg-cell or ovum and the other two are called synergids. The other group of three cells lying towards chalaza is called Antipodal. The other two nuclei called pollar nuclei anther not partitioned into separate cells but these two fused together and form a large central cell called Definite or secondary nucleus. The ovule now consists of the embryo sac. (female gametophyte) and the integuments protective layers of sporophytes tissue around the embryo sac.

Double Fertilization: 
       When pollen grains reach at the stigma of the carpal then development of pollen tube begins. The pollen grains absorb nectar, which initiate the development of tube. The nucleus of pollen grain divided to form two cells one is tube cell and other is generative cell.Tube cell control the growth of pollen tube. The generative cell divides to form two sperms or male gametes. The pollen tube grows towards ovary through style, it carry two sperm cell into the ovule. Now sperm reach the embryo sac. One sperm cell cell fertilizes the egg to form zygote, it is diploid and develops to form embryo. The other sperm fuses with the diploid secondary nucleus (definitive nucleus) to form tiploid endosperm nucleus. It develops to form endosperm, which stores food for the development of embryo, this union of two sperm cell with different cell of the embryo sac is called double fertilization.
      After double fertilization ovule develops into seed and ovary develops into fruit. The seeds are present inside the fruit.