Seaweeds are the large visible algae that commonly grow in the intertidal and subtidal zones along the coast. Scientifically called macrophytic marine algae. They are not plants, but have some basic plantlike
features. They photosynthesis, are sedentary and are not overtly responsive to external stimuli; they have cell walls that give them rigidity; they are prolific reproducers. Seaweeds do not share other more advanced features of plants. They do not have flowers, cones or other elaborate enclosed reproductive structures. They do not have specialized systems for transporting nutrients internally (although some have primitive systems). They do not protect themselves with bark and waxy coverings. Their root-like holdfasts function only to anchor them and are not important in extracting nutrients and water from the soil. They are mostly multicellular and conspicuous. Their holdfasts may be disc-shaped, branched or claw-like. Seaweeds either
have a single leaflike blade or numerous blades and a stipe, which closely resembles a stem. They are actually in the Protista family
Other than needing some sunlight, seaweeds are self-sufficient in their environment; they can absorb everything they need to sustain life directly from the seawater that surrounds them. The various varieties of these marine plants have several characteristics in common: all live in saltwater; all reproduce by
means of spores (sporophylls) rather than seeds; and all are autophylic, meaning they are capable of providing their own food and absorbing nutrients directly from the seawater through their cellular walls, rather than via a root system.
-Compiled from Pacific Seaweeds by Louis Druehl
and Seaweed by Valerie Gennari Cooksley.