Question

With the help of suitable diagrams, trace the growth of the pollen tube right from the deposition of the pollen grain on the stigma up to fertilization.

Answer 1

Step 1: Deposition of a pollen grain on the stigma:

• During pollination, a pollen grain (male gametophyte) from the anther lands on the stigma of a flower (female reproductive part).
• The stigma is sticky, so it helps the pollen grain to adhere firmly.
• If the pollen grain is compatible, it absorbs water and nutrients from the stigma and germinates.

Step 2: Entry of pollen tube into the ovule:

• The pollen tube goes into the ovary and then into an ovule.
• It enters the ovule through a small opening called the micropyle.
• Inside the ovule, it penetrates the embryo sac, where the eggcell and two polar nuclei are located.

Step 3: Germination of pollen grain:

• The pollen tube comes out of one of the pollen grain’s germ pores.
• The tube nucleus and the two male gametes (sperms) move inside the pollen tube.
• The pollen tube grows through the style by digesting its tissues with enzymes secreted by the tube nucleus.

Step 4: Fertilization (Double fertilization):

• The tip of the pollen tube bursts inside the embryo sac, releasing the two male gametes.
• These two male gametes take part in two types of fusion processes:
  1. Syngamy:
     • One male gamete fuses with the egg cell to form a zygote.
     • The zygote later develops into an embryo.
  2. Triple fusion:
     • The other male gamete fuses with the two polar nuclei present in the central cell.
     • This forms a triploid (3n) endosperm nucleus, which develops into the endosperm that nourishes the developing embryo.

• This entire process is known as double fertilization, which is a unique feature of flowering plants (angiosperms).