Some species are minute filamentous plants, requiring the microscope for their detection; others, like Lessonia, are of considerable bulk, or, like Macrocystis, of enormous length.
In Nereocystis and Macrocystis a zone of tubes occurs, which present the appearance of sieve-tubes even to the eventual obliteration of the perforations by a callus.
The blade is centrally-ribbed in Alaria and laterally-ribbed in Macrocystis.
In Fucus vesiculosus they arise in lateral pairs; in Ascophyllum they are single and median; in Macrocystis one vesicle arises at the base of each thallus segment; in Sargassum and Halidrys the vesicles arise on special branches.
Macrocystis sp., tubular elements from the medulla, with sieve-like transverse walls.