Most plants cannot live in saltwater, since the water drowns their roots and the salt poisons their systems. Seaweed, however, is not a true plant and does not use systems that can be waterlogged. It has thick, rubbery stems that protect it from the corrosive ocean water, and uses simplified versions of roots and leaves to hold it in place and absorb sunlight. More complicated types of seaweed even have specialized bladders that allow it to float.
Although it resembles a plant, seaweed is actually a type of complex algae. Simple types of algae form plant plankton and the small colonies that live in pools and other still-water habitats. Seaweed algae, on the other hand, builds itself into more complicated, multi-cellular versions that can withstand the turbulent and deep waters of the ocean. Like plants, seaweed depends on sunlight to create energy through photosynthesis and has simplified leaf and root structures that help anchor it in place.
Seaweed can survive in many different water environments but almost always requires some type of solid surface from which to grow. The organism grows a specialized structure called a holdfast that cements it to rock or into soil, and absorbs necessary nutrients from the water around it. Some types of seaweed can grow to hundreds of feet to reach near the water's surface, where they can receive needed sunshine. Many types of seaweed eventually develop into entire colonies or forests, which can grow for miles. Seaweed that washes up on the shore has broken or died and lost contact with its holding surface.
Seaweed can produce either asexually or sexually. Simpler, smaller types of seaweed use asexual reproduction, creating small spores that swim away from the parent, establish themselves in new locations, and grow into individual organisms. Other types create male and female cells that join and produce a new organism, similar to the way that moss reproduces.
Seaweed comes in three different varieties: green, red and brown (although blue-green algae is sometimes considered a type of seaweed as well). Green algae grows only three feet long at the most, and is most useful for sea creatures, who eat and hide in it. Red algae has the most applications to human industry, but brown algae includes the kelp family and grows by far the largest out of the three varieties. The forests that brown algae create are home to unique environments of marine life that can survive nowhere else.
Certain kelps are edible and used in different dishes, but red algae is harvested in large quantities to produce a vegetable gelatin used in a large number of foods and cosmetic products. Some of the larger kelps are also used to create fertilizers, medicines and dietary chemicals.