This is the collective process that involves the events from the deposition of pollen on the stigma until the pollen tube enters the ovule or merely the dynamic process that requires pollen recognition followed by inhibition.
The pistil can recognize the right compatible pollen from the same species and reject pollen grains that are incompatible originating from the same species or of another species. The recognition of the compatible pollen leads to fertilization, the process of the fusion of the male gamete with the female gamete to form a diploid zygote within the embryo sac. The male gametes are attracted to the female gametes, which produce chemicals for this attraction.
The pollen tube grows from the germ pores on the pollen grains where all the contents of the pollen grain move into the pollen tube. The growth is maximum in the tip region, where the cytoplasm is mostly confined. Callose plugs are formed at regular intervals in the pollen tube to restrict cytoplasm to the tip of the pollen tube.
The pollen grains upon deposition on the stigma, absorb liquid from its wet surfaces, thus expand in size protruding through germ pores. The stigma is very significant in the germination of pollen grains by secreting fluids that contain gums, sugar, and resins protecting the pollen as well as the stigma itself.
In the case the stigma has been removed, such as in some cleistogamous flowers, the pollen grains can also germinate on the style, surface of petals or on another sac.
The periodin which pollen grains remain viable is dependent on the existing temperature and humidity. For example, pollen grains in cereals such as rice and wheat lose the viability within thirty minutes and is some members of Families Leguminosae, and Solanaceae maintain their viability for months. Liquid nitrogen(-196C) can be used to store pollen grains for years.
The viable pollen germinates stigma, regardless of the germ tube position. The pollen absorbs water from the stigma, swells up hence germinating to produce a pollen tube which grows in the direction of the ovary. The pollen tube path in the pistil is guided by the secretion of an ovule. Stigmatic fluids containing chemicals such as sugars, resins, and lipids provide a suitable medium for pollen germination.
Usually, one pollen tube forms from a pollen grain(monosiphonous), but there are cases where more than one pollen tube may emerge from a grain(polysiphonous), but only one of them grows further, for example in Asclepiadaceae and Orchidaceae.
Due to pollen_pistil interaction, there develops intense competition in compatible gametes. Only the ones with vigorous pollen tube can reach the female gamete for fertilization.