Xenogamy

{{Short description|Type of pollination among plants}}

Xenogamy (Greek xenos=stranger, gamos=marriage) is the transfer of pollen grains 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.{{cite book|title=Biology textbook for XII|year=2006|publisher=Nation Council of Educational Research and Training|isbn=81-7450-639-X|url=http://ncert.nic.in/NCERTS/textbook/textbook.htm?lebo1=2-16|page=28}}

The term xenogamy (along with geitonogamy and autogamy) 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: Pollen and stigma of the flower mature at different times to avoid self-pollination.
• Self-incompatibility: 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, i.e., male and female flowers occur on separate plants, e.g., papaya, some cucurbits, etc.
• Heterostyly: The flowers of some plants have different lengths of stamens and styles so that self-pollination is not possible, e.g., Primula, Linum, etc.