Trees, evergreen, often dioecious. Leaves alternate (rarely subopposite), very large, with an often fine glandular serration; petiole long, not glandular. Flowers unisexual; male flowers in erect terminal panicles, female flowers singly or in short terminal or axillary racemes. Sepals 5, nearly free, fleshy, valvate, ovate. Petals absent. Stamens many. Female flowers with many staminodes and 6–8 very short styles; stigmas lobed, branched, tortuous. Fruit a woody egg-shaped capsule, splitting from apex and base into (5–)6–8 segments which can remain attached alternately at the top and bottom end; seeds small, flattened and surrounded by a broad triangular or rectangular wing, stacked inside the fruit (Sleumer 1954; Flora of China 2022; Hodel & Henrich 2020).
Itoa comprises two species, one distributed from Bhutan to Indochina (Itoa orientalis), which is in cultivation in the west, and another in tropical eastern Indonesia (I. stapfii (Koord.) Sleumer) which has much smaller leaves (to 15 cm long) and larger seeds (wing 4 cm long) (Hodel & Henrich 2020). William Hemsley’s generic name commemorates the Japanese botanist and physicist Keisuke Ito, and his grandson Tokutaro Ito (Quattrocchi 2000).
The genus was originally placed within the family Flacourtiaceae, but phylogenetic analyses have now shown this to be an artificial aggregate; Itoa itself belongs in the Willow Family, Salicaceae, and is quite close in evolutionary terms to the poplars and willows themselves (Zhang, Zeng & Liu 2021). Its nearest allies which are in cultivation in the west are the deciduous east Asian genera Carrierea, Idesia and Poliothyrsis.