The dictionary defines 'kelp' as: '1. any large brown seaweed; 2. the ash of such seaweed, used as a source of iodine and potash.' To the first definition, Chambers Biology Dictionary adds: 'especially Laminaria and its allies.' In Britain, the family Laminariaceae includes six species of large brown seaweeds with broad, flattened blades, which can grow to impressive lengths of 6m or more.