Fertilization in Orchids

Pollinia with each pollen grain containing a vegetative and a generative nucleus ,are carried to the stigma of the same of a different flower ,where they start to germinate . The pollen grow down through the narrow cavity in the centre of the column and towards the ovules. The generative nucleus divides once to give rise to two sperms, which enter the embryo-sac through the microphyle and fertilization takes place. The Vegetative nuclesus degenerates in the pollen tube. Stort (1972) reports pollen tubes with upto 8-nuclei in Cyrtopodium species . The fate of these supernumerary nuclei are , however , not known .

The time limit between pollination and fertilization varies with species from 4 days in Gastrodia elata to 6-10 months in Vanda Suavis.

One of the two sperm cells unites the egg nucleus . The behaviour of the second sperm nucleus varies. In the more advanced forms , it simply degenerates . In others triple fusion may occur with the two polar nuclei, which may then degenerate. In still others the endosperm nucleus may undergo a few divisions without any wall formation , before degenerating .