3a. Inflorescence cymose, with the branches ending in 'spikes'. Flowers actinomorphous, bisexual, consisting of three stamens with large connectives and one ovary with a sessile, disc-shaped stigma. Tannin cells present... Posidoniaceae 3b. Flowers simple, unisexual, consisting of 1 or 2 ovaries or a single stamen, arranged within a specialized inflorescence or just single or in pairs. Stamens with reduced connectives. Ovary bearing a style with 1, 2, or 3 stigmata. Tannin cells present or absent 4
4a. Flowers, unisexual or bisexual, arranged in two rows on one side of a flattened spadix enclosed in a spathe. Stamens consisting of two extrorsely dehiscent thecae, connected by a ridge-like connective. Ovary horizontally placed, with a short thick style and two stigmata shed after flowering. Tannin cells absent Zosteraceae
4b. Flowers unisexual, solitary, in pairs or as a cluster on a common pedicel, terminating a short branch. Tannin cells present or absent 5
5a. Leaves with 3-many nerves and numerous tannin cells. Flowers unisexual, without a perianth, solitary or in pairs, or arranged within a cymose inflorescence. Male flowers sessile or stalked consisting of two, dorsally connate anthers. Pollen filiform. Ovaries in pairs with 1, 2, or 3 filiform styles. Tannin cells
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