The following are the events in pollen-pistil interaction:
I. Germination of the pollen grain to form a pollen tube.
II. Entry of the pollen tube into the ovule through the micropyle.
III. Arrival of the pollen grain at the stigma.
IV. Growth of the pollen tube through the tissues of the stigma and style, ultimately reaching the ovary.
The sequential order of their occurrence is :
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → III → I → IV → II
Pollen-pistil interaction covers all the events from the deposition of pollen on the stigma until the pollen tube enters the ovule. The events follow one another along the path of the pollen tube:
III. Arrival of the pollen grain at the stigma – the pollen grain lands on the stigma during pollination.
I. Germination of the pollen grain to form a pollen tube – after compatible pollination, the pollen tube comes out through one of the germ pores.
IV. Growth of the pollen tube through the tissues of the stigma and style – the tube grows down and reaches the ovary.
II. Entry of the pollen tube into the ovule through the micropyle – this is the last of the four events.
So the order is III → I → IV → II.
NCERT Extract:
Pollination – “… As mentioned earlier, following compatible pollination, the pollen grain germinates on the stigma to produce a pollen tube through one of the germ pores (Figure 1.12a). The contents of the pollen grain move into the pollen tube. Pollen tube grows through the tissues of the stigma and style and reaches the ovary (Figure 1.12b, c).”
Pollination – “… Pollen tube, after reaching the ovary, enters the ovule through the micropyle and then enters one of the synergids through the filiform apparatus (Figure 1.12d, e). … All these events–from pollen deposition on the stigma until pollen tubes enter the ovule–are together referred to as pollen-pistil interaction.”