​ SEAWRACK, the detached seaweeds thrown up, often in great quantities, by the sea and used for manure, also formerly for making kelp. It consists largely of species of Fucus — brown seaweeds with flat branched ribbon-like fronds, characterized in F. serratus by a saw-toothed margin and in F. vesiculosus, another common species, by bearing air-bladders. Also of Zostera marina, so-called sea-grass, a marine flowering plant with bright green long narrow grass-like leaves.