SIUM, in botany, a genus of the pentandria digynia class. The fruit is somewhat oval, and striated; the involucre consists of many leaves; and the petals are heart-shaped. There are 8 species, three of them natives of Britain, viz. the latifolium, or great water-parsley; the nodiflorum, or creeping water-parsley; and the erectum, or upright water-parsley.