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NCERT Exemplar · Q26

Q.A vertical section of a pistil is described as follows: its top bears a stigma on which two pollen grains have landed; below the stigma a slender style connects downward to a swollen ovary at the base; inside the ovary lies a single ovule that contains an embryo sac, and at the micropylar (upper) end of that embryo sac sits a cluster of three cells forming the egg apparatus. Trace, in correct order, the path a pollen tube takes after a pollen grain germinates on the stigma until it reaches the embryo sac, and name the cells that make up the egg apparatus.

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When a compatible pollen grain lands on the stigma it germinates and puts out a pollen tube. The tube grows down through the style and ovary, enters the ovule through the micropyle, passes the nucellus, and delivers its two male gametes into the embryo sac by entering one of the synergids. The egg apparatus at the micropylar end of the embryo sac is made of one egg cell and two synergids.

Concept

Double fertilisation in angiosperms requires the male gametes, carried in the pollen tube, to reach the female gametophyte (embryo sac) deep inside the ovule. The pollen tube therefore has to travel the whole length of the carpel and find its way into the embryo sac.

Path of the pollen tube (in order)

  1. Stigma – the pollen grain absorbs water, germinates and produces a pollen tube.
  2. Style – the tube grows down through the tissue of the style (in solid styles) or along the stylar canal.
  3. Ovary – the tube reaches the ovary and is guided toward the ovule.
  4. Micropyle – in most plants the tube enters the ovule through the micropyle (this route is called porogamy).
  5. Nucellus – it grows a short way through the nucellus.
  6. Embryo sac – the tube enters the embryo sac by penetrating one of the two synergids, guided by the filiform apparatus of that synergid, and bursts to release the two male gametes.

Egg apparatus …

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