Question

Growth of pollen tube towards embryo sac is
(A) Geotropism
(B) Thigmotaxis
(C) Chemotropic
(D) Phototaxis

Answer 1

Hint:In angiosperms and few gymnosperms, the male gamete has to reach the female gamete, after compatible pollination in order to undergo fertilization. The male gamete of the pollen grain moves through the pollen tube towards the embryo sac. This happens with the help of chemical interaction and is known as pollen-pistil interaction.

Complete answer:When the right type of pollen grain gets deposited on the stigma, the pistil accepts the pollen and promotes post-pollination events that lead to fertilization. After compatible pollination, the pollen grain germinates on the stigma to produce a pollen tube through one of the germ pores. The contents of the pollen grain move into the pollen tube. The pollen tube grows through the tissue of the stigma and style and reaches the ovary. This is due to certain chemicals released by the ovule which attracts the pollen tube towards it. This is called chemotropism. It is defined as the growth or motion of an organism or part of it in response to chemical stimuli.

Understanding the options one by one,
(A) Geotropism
It is defined as the growth or motion of parts of the plant in response to gravity. This is not the correct answer.
(B) Thigmotaxis
It is defined as the motion of an organism in response to touch. It doesn’t contribute to the growth of pollen tubes towards the embryo sac. Hence, this is not the correct answer.
(C) Chemotropic
It is defined as the growth or motion of an organism or part in response to chemical stimuli. The growth of the pollen tube towards the embryo sac is because of chemicals released by the ovule. Therefore, this is the correct answer.
(D) Phototaxis
It is defined as a locomotive movement when an organism moves towards or away from the stimulus of light. This is not the correct answer.
Therefore, Option (C) Chemotropic, is correct.

Note:It is important to understand the mechanism behind the growth of the pollen tube towards the embryo sac. Usually, the movement of male gametes towards the female gamete is due to the chemical released by the female gamete, irrespective of whether it’s a plant or an animal body. Moreover, the filiform apparatus of synergids guides the entry of pollen tubes into the egg apparatus, in the later stage.