Geitonogamy (the pollination between flowers on the same plant) has important implications for sex-allocation
theory, the evolution of dioecy and other issues in evolutionary biology. Internal geitonogamy is a unique self-pollination
system that occurs in the genus Callitriche L. In this system self-fertilization is effected by pollen tube growth through
vegetative tissues from the staminate to pistillate flowers. Internal geitonogamy occurs in seven of ca 50 species of
Callitriche (C. heterophylla Pursh, C. heteropoda Engelm., C. lechleri (Hegelm.) Fassett, C. nubigena Fassett, C. rimosa
Fassett, C. trochlearis Fassett, C. verna L.). Internal geitonogamy is correlated with the amphibious growth habit and
polyploidy. However, not all polyploid amphibious species of Callitriche have internal geitonogamy. Correlation between
internal geitonogamy and geographic distribution is not apparent. There is a tendency for species that possess internal
geitonogamy to occur at higher elevations than those that lack this system. Callitriche is the genus in which aerial, water
surface, and submerged pollination systems are all reported. Geitonogamy, the primary mode of pollination in this genus,
can be realized in two ways: 1). with contact between anther and stigma ("contacters"); 2). without contact between anther
and stigma ("non-contacters")
