Xenogamy

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 pollination which during pollination brings genetically different types of pollen grains to the stigma.^[1]

The term xenogamy (along with geitonogamy and autogamy) was first suggested by Kerner in 1876.^[2] 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.

References

1. ↑ Biology textbook for class XII. Nation Council of Educational Research and Training. 2006. p. 28. ISBN 81-7450-639-X. 
2. ↑ Darwin, Charles. More Letters of Charles Darwin, Volume 2. Echo Library. p. 668. ISBN 978-1-4068-0482-9. Retrieved 25 February 2012.