Why Is Seaweed Not a Plant?

Seaweed resembles plants but instead is algae.

Seaweed has parts that resemble plant leaves, roots and stems. The ocean-dwelling organism can even perform photosynthesis like plants. Another commonality is that, like plants, many types of seaweed are edible. However, seaweed is not a plant.


Seaweed belongs to the marine algae class of aquatic organisms that are neither plants or animals. Seaweed provides food for aquatic life and protective cover for sea animals.


Seaweed attaches to the ocean floor or other solid structures with a holdfast, which is similar to a plant root. However, unlike roots, holdfasts do not gather nutrients. Seaweed does not have stems or leaves like plants. The part of the seaweed that resembles a leaf is called a blade. The part that looks like a stem is called the stipe, and the body of seaweed is called the thallus.

Photosynthesis and Water Absorption

While most plants only complete photosynthesis in their leaves, seaweed can carry out the process in all tissues. Since seaweed does not have roots, it absorbs water though all of its tissues.