What is 'double fertilisation', and why is it special to flowering plants?
A pollen grain delivers two male gametes into the embryo sac.
A pollen grain delivers two male gametes into the embryo sac. One fuses with the egg cell to form the diploid zygote (the future embryo); the other fuses with the two polar nuclei to form the triploid primary endosperm, which becomes the food store for the growing embryo. Because two fusion events happen together — one making the embryo, one making its food — it's called double fertilisation, and it's found only in angiosperms (flowering plants).
Key point
Two fusions: gamete + egg → zygote, and gamete + polar nuclei → nutritive endosperm — unique to angiosperms.
Common mistake
thinking plants fertilise with a single fusion like animals.
Memory tip
one fusion builds the embryo, the other builds its pantry (endosperm).
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