Xenogamy (Greek ''xenos''=stranger, ''gamos''=marriage) is the transfer of

pollen grain Pollen is a powdery substance produced by most types of flowers of seed plants for the purpose of sexual reproduction. It consists of pollen grains (highly reduced microgametophytes), which produce male gametes (sperm cells). Pollen grains h ...

s from the anther to the stigma of a different plant. This is the only type of cross pollination which during pollination brings genetically different types of pollen grains to the stigma. The term xenogamy (along with

geitonogamy Geitonogamy (from Greek ''geiton'' (γείτων) = neighbor + ''gamein'' (γαμεῖν) = to marry) is a type of self-pollination. Geitonogamous pollination is sometimes distinguished from the fertilizations that can result from it, geitonogamy. ...

and

autogamy Autogamy or self-fertilization refers to the Cell fusion, fusion of two gametes that come from one individual. Autogamy is predominantly observed in the form of self-pollination, a Reproduction, reproductive mechanism employed by many flowering pl ...

) was first suggested by Kerner in 1876.{{cite book, last=Darwin, first=Charles, title=More Letters of Charles Darwin, Volume 2, date=August 2006, publisher=Echo Library, isbn=978-1-4068-0482-9 , url=https://books.google.com/books?id=X9vMhLOiTmQC&dq=Xenogamy&pg=PA668, access-date=25 February 2012, page=668 Cross-pollination involves the transfer of pollen grains from the flower of one plant to the stigma of the flower of another plant. The main characteristics which facilitate cross-pollination are: * Herkogamy: Flowers possess some mechanical barrier on their stigmatic surface to avoid self-pollination, e.g. presence of gynostegium and pollinia in '' Calotropis''. *

Dichogamy Sequential hermaphroditism (called dichogamy in botany) is one of the two types of hermaphroditism, the other type being simultaneous hermaphroditism. It occurs when the organism's sex changes at some point in its life. A sequential hermaphrodit ...

: Pollen and stigma of the flower mature at different times to avoid self-pollination. *

Self-incompatibility Self-incompatibility (SI) is a general name for several genetic mechanisms that prevent self-fertilization in sexually reproducing organisms, and thus encourage outcrossing and allogamy. It is contrasted with separation of sexes among individuals ...

: In same plants, the mature pollen fall on the receptive stigma of the same flower but fail to bring about self-pollination. *Male sterility: The pollen grains of some plants are not functional. Such plants set seeds only after cross-pollination. * Dioecism: Cross-pollination always occurs when the plants are unisexual and

dioecious Dioecy ( ; ; adj. dioecious, ) is a characteristic of certain species that have distinct unisexual individuals, each producing either male or female gametes, either directly (in animals) or indirectly (in seed plants). Dioecious reproduction is ...

, i.e., male and female flowers occur on separate plants, e.g., papaya, some cucurbits, etc. *

Heterostyly Heterostyly is a unique form of polymorphism and herkogamy in flowers. In a heterostylous species, two or three morphological types of flowers, termed "morphs", exist in the population. On each individual plant, all flowers share the same morph ...

: The flowers of some plants have different lengths of stamens and styles so that self-pollination is not possible, e.g., ''Primula'', ''Linum'', etc.

References

Symbiosis Pollination Horticulture