In the angiosperms, self-pollination is the transfer of pollen from an anther to a stigma within one single plant. It may occur within the same flower (intrafloral), or it may involve flower-to-flower (interfloral) pollen transfer within the same plant. In contrast, cross-pollination is the transfer of pollen from an anther to a stigma in another flower in a separate plant. The difference, therefore, is that the former occurs in one plant while the latter involves two plants.
However, any type of pollination may not be exclusive to a plant species. One species may be classified as naturally selfing, but outcrossing can still occur in a few flowers or in some stigmas within the same flower or inflorescence.
There are various natural mechanisms in plants which promote either type of pollination. Some are briefly described here and in the next page.
1. Perfect flowers. The occurence of this type of flower, also called bisexual and hermaphroditic flower, in a single plant favors selfing because of the presence of both male and female sexual parts in the same flower. If pollen shedding and stigma receptivity are synchronized and mechanisms promoting flower-to-flower pollen transfer are absent, intrafloral pollen transfer will be ensured. Naturally self-pollinated crops like beans, peas and members of the solanaceous family have perfect flowers only in one plant.
2. Homogamy. The male and female sexual parts mature at the same time, that is, the pollen is shed at the same time that the stigma becomes receptive. In plants having perfect flowers, this favors intrafloral self-pollination. Homogamy contrasts with dichogamy.
3. Cleistogamy. Opposite of chasmogamy, this is a plant mechanism whereby pollination and fertilization occur in an unopened flower or just before opening. Cleistogamous flowers are common in beans and peas.
Forms of cleistogamy:
There are various ways in which cleistogamy is expressed. For example, the perianth (petals and sepals) remain closed (Simpson 2010); or the stamens form a cone that encloses the stigma even after flower opening as in tomato; or the pollen grains are shed and the stigma is pollinated before the flower opens as in lettuce; or the flower remains hidden by some plant parts like the leaf sheath, ensuring intrafloral pollen transfer and effectively serving as a mechanical obstruction to cross-pollination, as in the California oatgrass (Danthonia californica); or the florets do not open as in Festuca megalura; or the anthers burst before extrusion as in wheat (Allard 1960).
ALLARD RW. 1960. Principles of Plant Breeding. New York: John Wiley & Sons, Inc. 485 p.
POEHLMAN JM. 1977. Breeding Field Crops. Connecticut: AVI Publishing Co., Inc. 427 p.
SIMPSON MG. 2010. Plant Systematics. 2nd ed. Burlington, MA: Elsevier Inc. 740 p.
(Ben G. Bareja. Jan. 4, 2014)
Read more: Mechanisms of Cross Pollination