Most hydrangeaceae species are shrubs or trees, producing many small flowers along woody stems, either singly or in clusters. Leaves are usually opposite and undivided. Most species have flowers with four petals and four stamens, but some have as many as ten of each. The stamens are centrally arranged around the ovary, attached at the base. The US has 43 species in 11 genera, major western groups being fendlera, jamesia and philadelphus.