A genus (family Hydrangeaceae) of deciduous or evergreen shrubs and small trees with a few rhizomatous herbs that climb by means of adventitious roots. The flowers are usually borne in terminal corymbose cymes or panicles, often with small, fertile, perfect flowers surrounded by much larger sterile flowers. In some species the ability to absorb aluminium, which depends on the soil pH, determines flower colour. There are about 50 species, native to Asia from the Himalayas to Japan and the Philippines, and the Americas. Many are widely cultivated for ornament, many cultivars being derived from H. macriphylla, also called hortensia.