seaweed, name commonly used for the multicellular marine algae. Simpler forms, consisting of one cell (e.g., the diatom) or of a few cells, are not generally called seaweeds; these tiny plants help to make up plankton. The more highly developed types of seaweed usually have a basal disk, called a holdfast, and a frond of varying length and shape, which often resembles a plant in having stemlike and leaflike parts.
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