Question

1. How is fertilization in conifers different than fertilization in angiosperms?
2. Draw and label the steps in Angiosperm double fertilization.

Answer 1

Answer. Conifers means gymnosperms. In gymnosperms, the female gametophyte is represented by various archegonia. The egg cells originate in the archegonia.

Pollen has wings in conifers so generally they are wind pollinated. As pollen reaches ovule, it forms pollen tube and penetrates the archegonia to reach an egg for fertilisation. As fertilisation occurs zygote is formed which by further divisions forms embryo. Double fertilisation is absent in the gymnosperms. Seed encloses the embryo and seed grows it forms the sporophyte plant.

In angiosperms, pollen are the male gametes. 4 functional pollens are produced by one micropore mother cell inside the anther. Each pollen grain has haploid nucleus. Just after completing meiosis divide by mitosis, to form 2-cells inside a pollen, one is called generative nucleus and the other tube nucleus.

Pollination in 60% of angiosperms family takes place at 2-celled stage. It could be through wind, water, insects, birds, snails etc.

Pollen reaches the stigma surface, compatibility reaction occurs and it forms pollen tube. The generative nucleus again divided mitotically to form 2 nuclei. As pollen tube penetrates the ovule mostly through the micropyle, one male gamete fuses with the egg to form zygote. Zygote by embrogeny forms embryo.

Another male gamete fuses with the polar nuclei of the ovule to form the endosperm. The latter is triploid in nature as it is formed by the fusion of 3 haploid cells.

This fusion of 1 male gamete with an egg and another male gamete with the polar nuclei is known as double fertilisation.