Seaweeds are not classified as true plants because they lack an organised vascular system for absorbing nutrients.   Each cell is in contact with the water and they can take up nutrients, gases and fluids directly.    Every cell also contains the pigment chlorophyll, allowing the seaweed to photosynthesize the materials they need for growth from the energy of sunlight.   Some seaweeds carry additional pigment which allows them to photosynthesize in the reduced light of deeper waters and gives them other beautiful shades of red and brown.

Structure of Seaweed

Macroalgae is usually differentiated into three main sections:

Holdfast which looks similar to bundled roots but actually has the function of anchoring the seaweed in one place securely.  Unlike roots the holdfast doesn't absorb any nutrients.   The projecting fingers are called "haptera"

Stipe the supporting stem which can be up to 20 meters in macrocysts.

Blades, photosynthesizing "leaves",  some single and others with many divisions.

Thallus, is often used to refer to the whole algae.

Seaweeds may also have floats or pneumatocysts, gas filled hollows to provide buoyancy depending on the species.

Holdfast from Saccorhiza polyschides
Crissy Haydon 2009

Typical Kelp Holdfast Crissy Haydon 2009


Like plants seaweeds can be annuals or perennial and live for many years.   Annual seaweeds grow each spring and die back in the winter.   If the stipes survive the winter storms then new blades can grow from the same stipe.