any of numerous large coastal seaweeds growing in colder seas and belonging to the order Laminariales (about 30 genera) of brown algae. Until early in the 19th century the ash of such seaweeds was an important source of potash and iodine. Giant kelps, of the genus Macrocystis, are rich in minerals and produce algin, a complex carbohydrate (polysaccharide) useful in various industrial processes, including tire manufacture. Algin is added to ice cream before freezing to prevent ice crystallization and is also used as a suspending and emulsifying agent in other food products.

Laminaria, a large brown seaweed (1 to 3 metres 【3.3 to 9.8 feet】 long) abundant along the Pacific and Atlantic coasts, has a stipe that superficially resembles the stem of land plants. Growth extension occurs at the meristematic region between the stipe (which is perennial) and the blade (which is shed annually).

Macrocystis, the largest known kelp, up to 65 metres (215 feet) long, is limited in distribution because it reproduces only at temperatures below 18–20 °C. The complicated plant body, in some ways similar in appearance to that of higher plants, has a large rootlike holdfast for attachment to the ocean floor, a stemlike stipe for the internal transport of organic material, and long branching stalks with blades that stay afloat by means of gas bladders.

Nereocystis, an annual kelp that grows primarily in deep waters and rapid tideways, can attain lengths up to 40 metres (130 feet). Internally the plant structure is similar to Macrocystis; externally the stalk is tough and whiplike, terminating in a single large bladder containing up to 10 percent of carbon monoxide. The long leafy outgrowths from the stalk carry out photosynthesis and reproduction.