Ulva

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Ulva (division Chlorophyta) A genus of green seaweeds in which the thallus is flat and sheet-like, and is 2 cells thick. U. lactuca (sea lettuce, green laver) is a common British species; the holdfast is perennial, producing new fronds each year. It is found attached to rocks in the intertidal zone, and in brackish water in estuaries.