Salvinieae
A small natural order of annual, floating herbs, not attached to the soil, resembling large Lemnae (Salvinia) or a Jungermannia (Azolla), with no true stem. Salvinia is met with throughout the Northern hemisphere, as well as in tropical and South America; and Azolla, the only other genus, inhabits Asia, Africa, Australia, and America from Canada to the Straits of Magellan. Fronds with margins reflexed before expansion, usually claret-coloured on the under surface, sometimes composed of cellular tissue, without nerves and stomata (Salvinia); sometimes with a stomatiferous epidermis (Azolla), rounded or lobed, sessile or sub-sessile, alternate or distichous, imbricated. Reproductive organs of two kinds, similar to those of Marsileaceae, inserted at the base of the fronds. The order embraces about eighteen species.