HoldfastA tangle of root-like branches that anchors the kelp to a rock. Unlike a root, it takes in nothing; it only holds on.
StipeThe long, flexible stalk. It works like a stem, carrying the blades up toward the light at the surface.
BladeThe flat, leaf-like part that takes in sunlight. A giant kelp has many, spaced along each stipe.
Float (gas bladder)A bulb filled with gas at the base of each blade. The floats hold the kelp upright in the water. Botanists call one a pneumatocyst.
Rockweed 5
HoldfastOn a rockweed the holdfast is a small disc, not a tangle. It does the same job: it fixes the seaweed to the rock.
BladeThe flat body of the rockweed. It forks in two, and each branch forks again.
MidribA thickened rib down the middle of every branch of the blade.
Air bladderSmooth, gas-filled swellings, usually in pairs. One is on each side of the midrib. They float the top of the rockweed up when the tide comes in.
ReceptacleThe swollen tip of a branch. The rockweed's reproductive cells are made in small pits in it, which show as dots.
How this list was compiled: Every name checked on 2026-09-30 against sources read that day; a name is kept only where two of them use it: (1) NOAA National Marine Sanctuaries, Kelp Forest, https://sanctuaries.noaa.gov/visit/ecosystems/kelpdesc.html, with NOAA National Ocean Service, What is a kelp forest?, https://oceanservice.noaa.gov/facts/kelp.html; (2) Monterey Bay Aquarium, Giant kelp, https://www.montereybayaquarium.org/animals/animals-a-to-z/giant-kelp; (3) Diversity of Marine Plants, Man and the Gulf of Mexico Series (a school curriculum, ERIC document ED 245 925, 1984), https://archive.org/details/micro_IA41153311_0384; (4) Maine Sea Grant, Rockweed (Maine Seafood Guide), https://seagrant.umaine.edu/maine-seafood-guide/rockweed/; (5) Wikipedia, Kelp, Seaweed, Holdfast (biology), Stipe (botany), Pneumatocyst, Fucus and Fucus vesiculosus (text only). The National Park Service's Channel Islands kelp page was also read. Hints and detail lines are in our own words. Licence: own work
The drawing: Drawn for this site on 2026-09-30 from plain geometry; nothing traced or copied. Licence: own work
How it works
Seaweeds are algae, not plants, so the parts of a seaweed have names of their own. The first drawing is a giant kelp and the second a rockweed. Each part is asked on the seaweed it belongs to.
Read the full guide
Kelp first, then rockweed
On the kelp, numbers 1 to 4 are the holdfast, a stipe, a blade and a float. On the rockweed, number 5 is its holdfast and number 6 the flat, forking blade. Numbers 7 to 9 are the midrib, an air bladder and a receptacle.
Holdfast, stipe, blade: nearly every large seaweed has these three. They look like a root, a stem and a leaf, but they are not.
The whole body of a seaweed, all its parts together, is called the thallus.
Questions and answers
What are the parts of a seaweed?
A holdfast that grips the rock, a stalk called a stipe and one or more flat blades. Many seaweeds also have gas-filled floats that hold them up in the water. The whole body is the thallus.
Does seaweed have roots?
No. The holdfast looks like roots, but it takes in nothing and only anchors the seaweed to the rock.
What are the bumps on rockweed?
Two kinds. The smooth ones along the branches, usually in pairs, are air bladders that float the rockweed up at high tide. The swollen, dotted ends of the branches are receptacles, where it reproduces.
Is kelp a plant?
No. Kelp are large brown algae. Algae have no true roots, stems or leaves, which is why a kelp's parts are called holdfast, stipe and blade.