The term allogamia, or even heterogamy, refers to the fertilization of two gametes produced by cross-pollination, between two distinct individuals of the same or different species.
For example, in chard, although flowers are hermaphrodite, self-fertilization does not occur because it is prevented by protandric dichogamy (when pollen is available, the stigma is not receptive). As a result, fertilization is predominantly cross-fertilization, and the species behaves as a typical allogam.
Allogamia is distinguished, in turn, in:
– geitonogamy, when there is the union of two gametes from two flowers of the same individual;
– xenogamy, between the flowers of two different individuals of the same species;
– gnesiogamia, between the flowers of two individuals of the same species, but of different origin notogamy between individuals of different varieties of the same species.
Allogamia is the opposite mechanism to autogamy, which occurs by the union of two gametes from the same flower (self-fertilization).