Most varieties grow near coasts, where they are attached to rocks, shells, or the floor of the sea. A rootlike part, called a holdfast, anchors larger seaweeds to solid objects and prevents them from being washed away. Unlike a true root, the holdfast is not specialized to absorb water and minerals. A frond, which resembles a stem and leaf, extends from the holdfast. The soft, flexible fronds of a seaweed can sway with the water without being torn apart.
Some seaweeds have gas-filled swellings on their fronds.
These swellings keep the frond afloat. Seaweeds belong to the brown, red, and green algae groups. Most brown algae prefer cold water, where they may grow extremely large. Brown algae called gulfweeds prefer warm water. Gulfweeds grow in large, floating masses over an area of the North Atlantic Ocean known as the Sargasso Sea.