The cell wall of the brown algae consists of a cellulose differing chemically from that of plants. The outside is covered with a series of gelatinous pectic compounds, generically called algin; this substance, for which the large brown algae, or kelps, of the Pacific coast are harvested commercially, is used industrially as a stabilizer in emulsions and for other purposes. The normal food reserve of the brown algal cell is a soluble polysaccharide called laminarin; mannitol and oil also occur as storage products. The body, or thallus, of the larger brown algae may contain tissues differentiated for different functions, with stemlike, rootlike, and leaflike organs, the most complex structures of all algae.

Some groups of brown algae have evolved an interesting type of alternation of generations, in which physiologically independent haploid gametophyte plants produce gametes, the fusion of which initiates the diploid sporophyte generation. The mature sporophyte plant produces, through meiosis, haploid spores, which develop into new gametophytes. The two generations, or phases, may be indistinguishable in size and form, or they may differ greatly. The genus Ectocarpus, for example, is found growing attached to larger algae. It has similar-looking gametophyte and sporophyte plants. In the kelps, however, the gametophyte is only a microscopic filament, in contrast to the occasionally tree-sized sporophyte.