Family Description: This order, composed of a single family, is best represented in tropical and subtropical regions of the world. Members have an alternation of isomorphic generations that are flattened and fan-shaped or repeatedly dichotomously divided. Growth is by means of a single conspicuous apical cell or a row of marginal initials. Some tropical members of the family are calcified. Reproduction is oogamous. Eggs and sperm are usually produced in sori on separate gametophytic plants. Sporophytic plants produce four or eight large, nonmotile spores per sporangium.
Species description: This delicate seaweed is composed of erect, branched blades lacking midribs. The blades, which are up to 35 cm (14 in) tall, are rather light brown above but darken near the base. They are produced by creeping horizontal branches called stolons. The branches are mostly 1 to 1.5 cm (0.4 to 0.6 in) across; they are distally rounded and have minute teeth scattered along their edges.
Sporangia are unilocular (single-compartmented) and are scattered on both sides of the blades. Each produces just four nonmotile spores. Female gametophytes have reproductive organs also on both sides of the blades. These produce a single large egg each. The analogous organs of male gametophytes are multicompart-mented and occur in oval patches (called sori) on both sides of the blades.
