Process of fertilization in plants, Fertilization in Plants
Fertilization means the union of two dissimilar gametes in the process of sexual reproduction. In plants, pollination leads to fertilization. Micro sporogenesis is a process, which transforms pollen mother cells into pollen grain, containing a tube nucleus and a generative nucleus. Pollen grains are transferred to the stigma at this stage as a result of pollination. After pollen grains reach the stigma the contents of the pollen grain come out through the germ pore in the form of a pollen tube.
The growth of the pollen tube is stimulated by the secretion of sugary substance by the stigma. The pollen tube secrets some enzymes which dissolve the tissues of the stigma through which the pollen tube pushes its way through the style and the wall of the ovary while carrying the tube nucleus and the generative nucleus with it. The tube nucleus degenerates after the formation of the tube. The generative nucleus then divides to form two male gametes. The pollen tube after running through the style enters the ovule to reach the embryo-sac.
If the pollen tube enters the embryo-sac through the micropyle it is called porogamic fertilization. In some species the pollen tube enters the embryo-sac through the base of the ovule and in some through the integuments. When the pollen tube enters through the base of the ovule it is called chalazogamic and when it enters through the integuments it is called mesogamic fertilization. After the pollen tube reaches the embryo-sac its tip dissolves and the two male gametes are liberated. Of the two male gametes, one fuses with the eggcell of the embroy- sac while the other pushes further inside to meet the fusion product (2n) of the two polar nuclei called definitive nucleus. The synergids and antipodal cells of the embryo-sac do not have any function in fertilization hence they disappear sooner or later.
After fertilization the egg-cell clothes itself with a cell-wall and becomes an oospore. The definitive nucleus after fertilization transforms into an endosperm nucleus. The oospore gives rise to the embryo, the ovule to the seed, and the ovary to the fruit. The endosperm nucleus divides repeatedly to form the endosperm of the seed.
If fertilization fails the ovary withers and falls off. In many plants such as in banana, papaw, orange etc. the fruits may develop without fertilization. Development without fertilization is called parthenocarpy. In true parthenocarpic fruits seeds are not found or if found they do not germinate.
Double Fertilization and Triple Fusion
In angiosperms the fertilization occurs twice. This is evident by the fact that one of the male gametes released by the pollen tube fuses with the egg-cell and the other male gamete fuses with the definitive nucleus. Thus both the male gametes are involved in fertilization causing double fertilization.
Triple fusion: The definitive nucleus is formed as a result of fusion of the two polar nuclei at the time of the formation of the embryo-sac. During fertilization one of the male gametes fuses with the definitive nucleus. The definitive nucleus is already having diploid (2n) number of chromosomes in it. The fusion of the male gamete (n) brings the chromosome number to triploid (3n) and sometime due to presence of another polar nucleus to tetraploid (4n) condition. The fusion product is now called endosperm nucleus.
Significance of Fertilization
1. As a result of fertilization the oospore forms which gradually transforms into an embryo which on germination gives rise to the seedling.
2. The ovule is transformed into a seed which protects embryo and the endosperm.
3. Fertilization stimulates the development of ovary and leads to the formation of the fruit.
4. Fertilization helps mingling of parental characters and establishment of diploid nature of the plant. Fertilization is a link between gametophytic and sporophytic generations of the plant. The pollen grain and the embryo-sac represent the gametophytic (n) phase and the embryo represents the sporophytic (2n) phase in the life cycle of the plant.