The green, brown, and red seaweed you find washed up on the beach or floating in the water are kinds of plants called algae. These plants have green chlorophyll, just as ordinary green leaves do, and can make their own food from sunlight and nutrients in the soil or water.
But algae are as different from ordinary plants as you are from a clam. Algae are really collections of cells that feed themselves, and they don’t have the internal parts that ordinary plants do. They have no stem, no roots, and no flowers, they don’t even have real leaves!
Some of the larger algae, such as kelp, do have cells that form air sacks to keep the seaweed afloat, or suckers that attach the algae to rocks. The largest pieces of kelp can be more than 100 feet long!