Question

why cannot pollen grains be transferred from anther of one plant species to stigma of another plant species?

Answer 1

The pollen grains of one plant species may be transferred to stigma of another species but pollen grains will not germinate on the stigma due to sexual incompatibility.Sexual incompatibility is the inability of certain otherwise viable gametes to fuse with each other and form fertile offspring. Sexual incompatibility is important as it prevents the free cross pollination amongst members of different species, thus it maintains the genetic individuality of the species.