They are annual or perennial, decumbent or erect stemmed plants. The leaves are opposite or whorled, sessile. Leaf blade regular, round to linear, venation palmate or pinnate. Stems hairy or smooth. The flowers are produced solitary or in pairs from leaf axil, usually radially symmetrical, sepals 5, petals 5, usually white, blue or purple in color. Dispersal and propagation is by seeds and stem fragments. Crushed leaves have a distinctive 'lemon' scent.