Pollination Biology in the Callitrichaceae

Callitriche is the only genus in which aerial, water surface, and subsurface pollination systems are all reported. Studies of morphology, flowering phenology, patterns of seed production, and pollen tube growth suggest, however, that geitonogamy is the primary mode of pollination. Typical geitonogamy occurs between the aerial flowers of C. marginata, C. nuttallii, C. peploides, and C. stagnalis, and between the submersed flowers of C. hermaphroditica. Internal geitonogamy, in- volving pollen tube growth from within anthers of staminate flowers, through vegetative tissues, into the base of pistillate flowers, occurs in C. heterophylla, C. trochlearis, and C. verna. The submersed flowering C. hermaphroditica exhibits several characters that suggest underwater outcrossing (hy- pohydrophily), but the data are yet inconclusive. A remarkable array of pollination systems