FLORAL BIOLOGY AND REPRODUCTION IN POA (POEAE, GRAMINEAE)

Flowers in the cosmopolitan genus Poa L. are predominantly hermaphrodi te but many departures from this sex form occur in the New World. Dioe cism is primarily a South American breeding system with about three ti mes as many dioecious species as in the rest of the world. Gynomonoeci sm is a Central and South American trait heavily represented in Andean Peru and Bolivia. This zone of gynomonoecism separates dioecism in No rth and South America. Gynodioecism, a convenient evolutionary positio n on the pathway to dioecism, is relatively infrequent and in North Am erica is of indeterminate form in several taxa. Apomixis has long been recognised in European Pea; in western North America, apospory has in vaded dioecious species and generated populations of pistillate plants . In Peru and Bolivia, several taxa are composed exclusively of plants with pistillate flowers, but these have arisen from gynomonoecious pr ogenitors. Poa is of Eurasian origin and migrated to North America and thence to South America. Sex-form kinds and frequencies are in stark contrast in the two parts of the continent, but are explicable in evol utionary terms. The selection pressures generating the deviations from hermaphroditism and their timing are unknown.