Plants that live in the sea are surrounded by water at all times. For this reason, they have no need to develop the special tissues and organs for conserving water that are needed by plants on land. Seaweeds, for example, use their rootlike structures called holdfasts only to anchor them in one spot. They do not use them 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. Giant underwater plants do not require woody portions; the water holds them upright. As a result, their stems are soft and flexible, allowing them to move with the current without breaking.