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

pollen grain Pollen is a powdery substance produced by seed plants. It consists of pollen grains (highly reduced microgametophytes), which produce male gametes (sperm cells). Pollen grains have a hard coat made of sporopollenin that protects the gametophyt ...

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 fusion of two gametes that come from one individual. Autogamy is predominantly observed in the form of self-pollination, a reproductive mechanism employed by many flowering plants. However, species o ...

) 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 Herkogamy (or hercogamy) is the spatial separation of the anthers and stigma in hermaphroditic angiosperms. It is a common strategy for reducing self-fertilization. Common forms *Approach herkogamy - (called "pin flowers") is displayed when the ...

: Flowers possess some mechanical barrier on their stigmatic surface to avoid self-pollination, e.g. presence of gynostegium and pollinia in ''

Calotropis ''Calotropis'' is a genus of flowering plants in the family Apocynaceae, first described as a genus in 1810. It is native to southern Asia and North Africa. They are commonly known as milkweeds because of the latex they produce. ''Calotropis'' ...

''. *

Dichogamy Sequential hermaphroditism (called dichogamy in botany) is a type of hermaphroditism that occurs in many fish, gastropods, and plants. Sequential hermaphroditism occurs when the individual changes its sex at some point in its life. In particular, ...

: 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 a species, meaning that it has distinct individual organisms (unisexual) that produce male or female gametes, either directly (in animals) or indirectly (in seed plants). Dioecious reproductio ...

, 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