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 :

III → I → IV → II

 The correct answer is Option (3) - III → I → IV → II

Pollen-pistil interaction is a crucial process in plant reproduction, where the pollen grains interact with the pistil of a flower. It plays a significant role in determining the success or failure of fertilization. During pollination, it is not guaranteed that only compatible pollen, which is of the same species as the stigma, will land on the stigma. Pollen from other species or even from the same plant (in the case of self-incompatible plants) can also land on the stigma.

The pistil has the ability to recognize and differentiate between compatible and incompatible pollen. If the pollen is of the right type (compatible), the pistil accepts it and promotes post-pollination events that lead to fertilization. However, if the pollen is of the wrong type (incompatible), the pistil rejects it by preventing pollen germination on the stigma or the growth of the pollen tube in the style.

Chemical components of the pollen interact with those of the pistil, enabling the recognition process.

After compatible pollination, the pollen grain germinates on the stigma, producing a pollen tube through one of the germ pores. The contents of the pollen grain move into the pollen tube. The pollen tube then grows through the tissues of the stigma and style, eventually reaching the ovary. In plants that shed pollen in a two-celled condition, the generative cell divides and forms the two male gametes during the growth of the pollen tube in the stigma. In plants that shed pollen in a three-celled condition, the pollen tube carries the two male gametes from the beginning.

Once the pollen tube reaches the ovary, it enters the ovule through the micropyle and then enters one of the synergids through the filiform apparatus. The filiform apparatus present at the micropylar part of the synergids guides the entry of the pollen tube. These events, from pollen deposition on the stigma to the entry of pollen tubes into the ovule, collectively constitute pollen-pistil interaction.