Question

What happens when a pollen grain falls on a stigma?

Answer 1

Hint: Angiosperms (flowering plants) are considered to be the most successful and dominating group of land plants on Earth due to their unique flower structures. A complete flower consists of vegetative parts, including petals and sepals and reproductive parts, including stamen and pistil. Stamen or androecium, the male reproductive organ, consists of anther and filament. While pistil or gynecium, the female reproductive organ consists of stigma, style and ovary.

Complete answer:
The landing and subsequent germination of the pollen grains formed in another on the stigma of the female reproductive organ are known as pollination. The pollen grains carry the tube cells and generative cells. The generative cell gives rise to male gametes that help in fertilization. Pollination can take place within the same flower (autogamy), between the flowers in the same plant (geitonogamy) or between flowers of different plants (xenogamy).
The landing of any viable pollen grains or microspores on receptive stigma does not always result in fertilization. Several different types of pollen-stigma incompatibility systems have evolved in plants, such as interspecific incompatibility that prevent fertilization by another species; intraspecific incompatibility that prevents fertilization by any individual within the species; and self-incompatibility that prevent fertilization by the same plant. Hence, only the pollination of compatible pollen grains results in fertilization.
If the pollen and the stigma are compatible, the pollen gets hydrated by taking up water, and the pollen tube emerges. The pollen tube descends through the style toward the micropyle region of the ovule present in the ovary. Through pollen tubes, male gametes reach the embryo sac and one male gamete fuse with the egg and another fuse with polar nuclei to form the zygote, primary endosperm nucleus, respectively.

Note:
The life cycle of plants consists of an alternation of generation between gametophytic generation and sporophytic generation. The haploid gamete producing structure is called the gametophyte, and the diploid multicellular generation is the sporophyte. Hence, pollination is considered to be the interaction between male gametophyte and female gametophyte residing inside the female sporophyte.