Most ocean plants live in the waters above the continental shelf. They include tiny, one-celled organisms and many kinds of seaweed and seagrasses. Some scientists estimate that the plants over the continental shelf produce more oxygen for Earth than all the forests on land.

Ocean plants are surrounded by water at all times. For this reason most have not developed the special tissues and organs needed by land plants for conserving water. Seaweeds, for example, use their "roots" only to anchor them in one spot, not to draw water from the soil.

The water also offers support to ocean plants. Even a small tree on land requires a tough, woody stem to hold it erect, but giant underwater plants do not require woody portions because the water helps hold them upright. As a result, their stems are soft and flexible, allowing them to move with the...