The cell containing generative nucleus (generative cell) is small and attached to the wall. It, later, loses contact with the pollen wall and float freely in the cytoplasm of the larger vegetative or tube cell. During the germination of the pollen grain, the pollen tube develops through a germ pore.
The vegetative nucleus moves first into the pollen tube which is followed by the generative cell. The generative cell divides mitotically forming two male nuclei or sperms.
(a) Male gametophyte:
Pollen grain (microspore) transferred to the stigma of the flower develops into male gametophyte. Its nucleus divides to form a vegetative nucleus (tube nucleus) and a generative nucleus. This division takes place even before the pollen reaching the stigma. The division results in two unequal cells.
(b) Female gametophyte:
The megaspore is the first cell of the female gametophyte. The lowermost megaspore enlarges and produces an embryo sac. In all angiosperms, development of female gametophyte is endosporous, i.e. within the megaspore. In typical case, the functional lowermost megaspore gives rise to 8-nuclcate embryo sac. Out of these eight nuclei, the uppermost 3-nuclei towards the micropyle form egg apparatus containing middle egg cell and on its either side, two synergies.
(c) The lower 3-nuclei towards the chalazal end of the embryo sac are called antipodal. Two nuclei present at the centre of the embryo sac fuse forming secondary nucleus or definite nucleus. However, there are instances where two megaspore or even four megaspore nuclei may take part in the embryo sac formation. This embryo sac is respectively called bisporic or tetrasporic embyo sacs.
D. Pollination :
Transference of pollen grains from the anther to the stigma of the same or a different flower is known as pollination When anther matures, it liberates the pollens through dehiscence of the wall layers of anther. However, a pollen does not possess the capacity of locomotion and it requires some agents for its transference In some flowers, the anther is placed just above the stigma so that pollen may fall directly on the stigma of the same flower.
The pollination is, thus, of two types : (1) self pollination or autogamy - transference of pollen grain to the stigma of the same flower or the flower borne by the same plant (2) cross pollination or allogamy- transference occurs to the stigma of another flower borne by another plant.
a. Self pollination:
It is the transference of pollen grains from the anther of a flower to the 'stigma of the same flower. Hence, the flowers, in this case, need to be bisexual.
This process also takes place by transference of pollen grains from one flower (male or bisexual) to another flower (female or bisexual), borne by the same individual plant. This type of pollination is called geitonogamy. Here, only one plant is involved. The flowers may be bisexual or unisexual borne by the same plant.
Contrivances of self pollination:
There are certain adaptations in the flowers to effect the self pollination. The following are the contrivances or adaptations commonly met within flowers to achieve self pollination.
(i) Fertilization and production of the progeny are always certain by this method.
(ii) It maintains purity of race and superiority of the variety.
(iii) There is less wastage of pollens here.
(i) It leads to loss of viability and vigour of the plant (ii) If lethal genes become homozygous, the effect may be disastrous, b. Cross pollination
It is the transference of pollen grains from one flower to another borne by two separate plants of the same or allied species. Cross pollination within a species (different variety) is called xenogamy.
Since it occurs between two different strains of the plant, it yields hybrids. It is affected by external agents which carry the pollens of one flower to the stigma of another flower. These flowers are borne by two seperate plants of the same or allied species.