The floral development of Syngonium angustatum is analysed in the context of a recently published molecular phylogeny of the Araceae. The initiation of discoid floral primordia occurs acropetally on the surface of the inflorescence. Female flowers, atypical bisexual flowers, sterile male flowers and male flowers are inserted on the same phyllotactic spirals on the spadix. Stamen primordia are initiated simultaneously on the periphery of more or less circular floral primordia. There are four stamens per flower (rarely three). In a synandrium, the fusion of stamens occurs very early during their developmental cycle. In some flowers one or two stamens remain free. The staminodes are also initiated on the periphery of the discoid floral primordium and their number varies from four to six. The growth of the fused staminodes will eventually form a longitudinal cavity in the centre of the mature synandrode. On the synandrodes located near the female zone, one or two staminodes remain free during development. No atypical bisexual flowers were observed on the inflorescence of Syngonium. The presence of a few calcium oxalate crystals was observed on the surface of all types of flowers. All the atypical flowers located at the base of the sterile zone corresponded to sterile male flowers and resulted from a more or less random disorganisation of the typical structure of a synandrode. In the Aroideae, free stamens or staminodes represent a plesiomorphic condition. The association of synandria and synandrodes is present in all early diverging genera of the tribe Caladieae and could represent the ancestral state. It is not clear if free stamens have evolved once or twice in the tribe Caladieae, both scenarios are possible.
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Vol. 42 • No. 2