Question

Please describe – in detail - the following processes that occur during

Pollination and double fertilization in an angiosperm

Answer 1

Ans.

(A) Pollination is simply transfer of pollen grains (male gametophyte) from pollen sac of anther of male reproductive part / stamen (androecium) to the stigma of the female reproductive part / pistil (gynoecium / carpel). The pollination can be carried out by some pollinating agents : Biotic pollinators- animals like insects (entomophily), birds (ornithophily), bats (chiropterophily) or by human and abiotic pollinating agents like wind (anemophily), water (hydrophily) etc.

There are two braod type of pollination in angiosperms (plants with seed inside fruit):

1. Self pollination: When pollen grain from anther are transfered on stigma of same flower (autogamy self pollination) or stigma of other flower of same plant (geitonogamy self pollination) then it is called as self pollination. In cleistogamous flowers (flowers which never open), 100% self pollination (cleistogamy) occurs.

2. Cross-Pollination / allogamy: When pollens are transfered from stigma of flower of a plant to the stigma of flower of other plant of same species.

After pollination, further development of male gametophyte happens which includes initioation and enlarment of pollen tube on the stigma. This process of formation of pollen tube is called as germination of pollen grains.

After germination, generative cell nucleus (haploid by ploidy) further divided and produce two male nuclei / male gametes.

When pollen tube reaches to the ovule of carpel, it enters into it (mainly through micropyle pore) and both male gametes are transfered in to the embryo sac (female gametophyte) in the ovule.

(B) Fertilization / syngamy / First fertilization: Its the fusion of one male gamete with the egg cell / female gamete in the embryo sac. After fertilization, fusion of egg cell and male gamete, diplod zygote is developed from which seed embryo will be formed.

Triple fertilization: Its the fusion of one male gamete with the secodary nucleus, 2N, (formed by fusion of two haploid polar nuclei in the central cell) in the central cell of the embryo sac. Here since three haploid nuclei fused: one male gamete, two haploid polar nuclei, hence it is called as triple fusion. After triple fusion triplod (3N) nucleus (PEN- primary endosperm nucleus) is formed and the central cell is converted into triploid PEC- primary endosperm cell. PEC further divides and form the endosperm which provides nutrition to the growing embryo.

Double Fertilization: In angiosperms, since two fertilization occures: fertilization/ syngamy and triple fertilisation, it is called as double fertilization.

After fertilization, zygote develops into seed embryo, PEC develops into endosperm, ovule develops into seed and ovary is developed into fruit.