question_answer

(i) Why are primary oocytes important in human females?

(ii) Differentiate between gametogenesis and embryogenesis.

(iii) What is double fertilisation?

Or

How outbreeding devices ensure cross-pollination? Explain.

Answer:

(i) Primary oocyte is a diploid cell formed in foetal ovary when the gamete mother cell (oogonia) is arrested at prophase-I of meiosis. It gives rise to mature ovum for fertilisation. Therefore, it is important.
(ii) Differences between gametogenesis and embryogenesis are as follows
Gametogenesis	Embryogenesis
The process by which sex cells, male (spermatocytes) and female (oocytes) are formed.	The process by which an embryo is formed from a zygote.
It occurs in the sex organs of both the male and female.	Embryogenesis usually occurs only in the female body.
Gametes formed are haploid cells.	An embryo is a diploid cell.
Both mitotic and meiotic cell division occur in gametogenesis.	Only mitotic division occurs in embryogenesis.
(iii) In angiosperms, during reproduction two male gametes are released by pollen-tube. One male gamete fuses with the egg cell forming zygote, the other fuses with two polar nuclei forming primary endosperm nucleus. Since both the fusions, syngamy and triple fusion occur in embryo sac, it is called as double fertilisation.
Or
Continued self-pollination leads to chances of inbreeding depression. Thus, flowering plants have developed many devices to discourage self-pollination and to encourage cross-pollination. These include
(a) Dichogamy In some plant species receptivity of stigma and pollen release is not synchronised, i.e. often the pollen is released before the stigma becomes receptive (protandry) or stigma becomes receptive before the release of pollen (protogyny). The condition is called dichogamy.
(b) Heterostyly In some other species, the anther and stigma are placed at different positions, so that the pollen cannot come in contact with the stigma of same flower. This condition is called heterostyly.
(c) Self incompatibility is the third device to prevent inbreeding. It is a genetic phenomenon of preventing the pollen from fertilising ovules by the same flower by inhibiting pollen germination or pollen-tube growth in the pistil.
(d) Dicliny or unisexuality effectively prevents self-pollination. It is the presence of unisexual flowers in plants that prevents autogamy, but not geitonogamy, e.g. castor, maize, etc.
(e) Herkogamy is seen in orchids where male or female sex organs themselves prove as a barrier to prevent self-pollination by some structural abnormalities.