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(a) Explain the post-pollination events leading to seed production in angiosperms.                      (3)

(b) List the different types of pollination depending upon the source of pollen grain.                    (2)

Explanation:

(a)  (i) Pollen grain germinates on the stigma to produce a pollen tube through one of the germ pores. 

      (ii) Pollen tube grows through the tissues of the stigma and style and reaches the ovary.

      (iii) If  pollen grain sheds at two celled condition, then generative cell inside the pollen tube divides to form two male gametes. In plants which shed pollen at the three celled condition, pollen tube already has two male gametes from the beginning.

    (iv) Pollen tube enters in the ovule. After entering in one of the synergids, the pollen tube releases the two male gametes into the cytoplasm of the synergid.

  (v) One of the male gamete moves towards the egg cell and fuses with its nucleus to form zygote (syngamy).

  (vi) The other male gamete moves towards the two polar nuclei located in the central cell and fuses with them to produce primary endosperm nucleus. As this involves fusion of three haploid nucleus so it is termed as triple fusion.

  (vii) There are two fusions-(1) Syngamy , (2) triple fusion, which take place in the embryo sac, so it is known as double fertilization.

 (viii) The central cell after triple fusion becomes the primary endosperm cell (PEC) and develops into endosperm while the zygote develops into an embryo which leads to formation of seed in angiosperms.

(b) Different types of pollination:

(i) Self pollination

Autogamy. When pollen of same flower reaches the stigma of same flower.

Geitonogamy. Pollen grain of one flower reaches the stigma of another flower of same plant.

(ii) Cross pollination:

Xenogamy. When pollen grain of a flower from one plant pollinates the stigma of a flower on another plant.                                                                                                                                (5)

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