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The genus Begonia is one of the top ten most speciose angiosperm genera with about 1400 species divided into 66 sections. They are found in moist tropical and subtropical regions around the world with the most diversity in Asia (600+ species) and the Neotropics (600+ species), fewer African species (150+, about a third of which are endemic to Madagascar and nearby islands); but no presence in Australia tropical forests. Research suggests that the begonias originated in Africa relatively recently (16-43 million years ago) and colonized Asia and the New World in separate events (Tebbitt 2006 and references within).
Begonia species are terrestrial (sometimes epiphytic), shade-loving herbs, shrubs and lianas. Species range in size from a few inches to over 12 feet in height. Terrestrial species in the wild are commonly upright-stemmed, rhizomatous, or tuberous. The leaves are usually asymmetric, and often have color variegation. Although there are some exceptional annual and dioecious species, most begonias are perennial and monoecious (male and female flowers on the same plant). Male flowers have numerous stamens; female flowers have a large inferior ovary and two to four branched or twisted stigmas. Most species produce a “winged” fruit containing hundreds of tiny seeds, adapted for wind dispersal, although others produce fleshy animal-dispersed fruits and some (e.g. sections Platycentrum and Parvibegonia) show unique morphologies adapted to rain dispersal (Wikipedia 2014; Tebbitt et al. 2006).
The range of begonia flowers, foliage colors and sizes and resilience are diverse, and have been hybridized and cultivated for gardens and homes in many parts of the world. Even in temperate climates, begonias are grown outdoors as annuals, or as house or greenhouse plants. The genus is unusual in that many species throughout the genus, even those from different continents, can hybridize with each other and have been cultivated for hundreds of years, which has given rise to an enormous number of cultivars (classified into cultivar groups, which don’t reflect any phylogenetic or taxonomic structure). These hybrids cannot reproduce sexually; most are propagated vegetatively from leaf cuttings or leaf sections.
There is concern about extinction of wild begonia species; the IUCN includes 48 species on the red list of threatened species, three listed as critically endangered: B. asympeltata (Ecuador), B. pelargoniiflora (Cameroon and Equatorial Guinea), and B. salaziensis (Mauritius). The American Begonia Society (ABS) is a resource in species conservation, promoting interest, cultivar development, nomenclatural standardization, research and public education of begonias and has built a seed bank (Fuqua 2005; ABS website).
Because they accumulate free oxalic acid, a product of metabolic activity, in their leaves and petals, most begonias are sour to the taste. While some species are used as a substitute for rhubarb (e.g. Begonia rex) others are toxic, and some ornamental begonias are listed as potential household hazards (Wikipedia 2014; Kinghorn 2013).