Pollen-pistil interaction and double fertilisation

Pollen lands on stigmas all the time, much of it from the wrong plant. The pistil can recognise its own kind: it accepts compatible pollen and lets it grow, and rejects incompatible pollen. This 'conversation' between the pollen and the pistil is the pollen-pistil interaction, carried out by chemicals from both.

Compatible pollen germinates: it grows a pollen tube out through one of its germ pores. The tube grows down through the stigma and the style, carrying the two male gametes, enters the ovule through the micropyle, and then enters one of the synergids, guided by the filiform apparatus.

Plant breeders use this knowledge to cross plants on purpose, in artificial hybridisation. They must make sure that only the chosen pollen reaches the stigma:

• Emasculation: if the female parent's flowers are bisexual, its anthers are removed with forceps before they open, so they cannot pollinate their own flower.
• Bagging: the flower is then covered with a bag, often of butter paper, to keep out stray pollen. When the stigma is ready, the chosen pollen is dusted on, and the flower is bagged again so nothing else can reach it.

Then comes fertilisation. The pollen tube releases its two male gametes into a synergid, and each has its own partner:

• One male gamete fuses with the egg cell: this is syngamy, and it gives a diploid zygote, which will grow into the embryo.
• The other fuses with the two polar nuclei in the central cell: this is triple fusion, since three nuclei are involved. It gives the triploid primary endosperm nucleus (PEN), and the central cell becomes the primary endosperm cell.

Two fusions happen in the same embryo sac, so the process is called double fertilisation. It is found only in flowering plants.