Question

how does the pollen tube occur immediately after pollen grains fall on the stigma?

Answer 1

When the pollen grains fall on the stigma, their protective wall will break down , they start to germinate in response to a sugary fluid secreted by the mature stigma.Then a tube like structure will start to grow called as the pollen tube so that it can carry the male gamete from the pollen grain to the ovary. It will grow downwards in response to the chemicals produced by the ovary.