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
To label the kelp structures, name the four numbers on the giant kelp, drawn from its rock up to the surface. Five more numbers are on the rockweed under it, a smaller brown seaweed to compare with the kelp.
Read the full guide
Kelp structures word bank
In alphabetical order, to copy onto the board: air bladder, blade, float, holdfast, midrib, receptacle, stipe. Blade and holdfast are each written twice, once on the kelp and once on the rockweed.
The numbers that get swapped
The float (4) is the kelp's: one small bulb where each blade (3) joins the stipe (2). The air bladders (8) are the rockweed's smooth ovals, in pairs beside the midrib (7). The receptacles (9) are swollen too, but dotted, and only at the branch tips. The two holdfasts look nothing alike: a tangle on the kelp (1), a small disc on the rockweed (5).
For the kelp alone, set Start from to Giant kelp: four numbers, four lines, and the rockweed printed faint.
When marking, pneumatocyst is right on line 4, and lamina on lines 3 and 6.
Questions and answers
Is rockweed a kelp?
No. Kelps are the large brown algae of one order, the Laminariales. The rockweed on this sheet is a Fucus, a brown alga of another order, the Fucales, that lives between the tides on rocky shores.
What is a pneumatocyst?
The scientific name for a seaweed's float: a structure that holds gas, found on brown seaweeds. It lifts the blades toward the surface, where they get more sunlight.
What is a kelp frond?
A stipe together with its blades. The sheet asks for the stipe and the blade on lines of their own, so frond has no number.
What is the stem of a kelp called?
A stipe. It is tough but flexible, so the kelp can sway with the ocean currents. Kelp are algae, not plants, which is why the stalk has a name of its own.