(kelps) An order of the Heterogeneratae, a subclass of the *Phaeophyta
. It contains the largest known algae, Macrocystis and Nereocystis. Most species occur below the low-tide mark. The sporophyte thallus is differentiated into a holdfast, stipe, and lamina. The gametophyte generation is much smaller and simpler and shows well developed oogamy. Certain species yield alginic acid, which is commercially important as an emulsifying agent.