Question

Why does the pollen grains of Plant of one species does not react with another species of Plants ?

Answer 1

When the pollen grains fall on the stigma, certain chemical signals are exchanged between both. This is called pollen -stigma interaction. If the interaction occurs, the pollen grain germinates to form a pollen tube which enters the style and moves towards the ovary. When pollen grains of some other species falls on the stigma, this interaction does not occur as a result the pollen fails to germinate and form a pollen tube. These pollen grains simply wither or fall off.