In oogamous conditions, which is found in Bryophyta onwards in every plant group, the male gametes are motile and the female egg cells are non-motile. The motility of the sperm may depend on two conditions, namely, (i) if the water is available, the flagellated sperms may swim to the egg and fertilize the same, or, (ii) if there is pollen tube formation and the male gametes are eventually transferred via the pollen tube. The second practice is observed in Gymnosperms and angiosperms, which is called as Siphonogamous.