Green, but not a plant
Is Seaweed a Plant?
It is green and it lives on sunlight, but seaweed has no roots, no leaves and no stems. It is not a plant at all.
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Step 1 of 5
The thing gripping the rock is not a root.
It is called a holdfast, and it only grips. It takes in nothing at all, which is why seaweed grows just as well on a rock as on a shipwreck.
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Step 2 of 5
It has no tubes inside, and does not need any.
A land plant must haul water and minerals up from the soil. Seaweed is surrounded by water carrying everything it needs, so it absorbs through its whole surface.
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Step 3 of 5
The flat blades are not leaves, and they float on purpose.
Many seaweeds carry little gas-filled bladders that lift the fronds towards the light instead of leaving them slumped on the seabed.
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Step 4 of 5
So it belongs with algae, not with plants.
Land plants all descend from one green ancestor. Seaweeds come from several separate lines — the greens, the browns and the reds — that never made that journey.
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Step 5 of 5
Being floppy is the point.
A stiff plant standing in the surf would snap. Seaweed bends flat with every wave and springs back, which is why it survives a coastline that breaks rock.
Kelp forests off some coasts grow taller than most trees, and a kelp can add half a metre in a day.
The short version
Seaweed is an alga, not a plant — its holdfast only grips, it absorbs everything through its surface, and being floppy is what lets it live in the surf.
Try it yourself
Look at a piece of washed-up seaweed. Find the holdfast at the base and see that it is a claw for gripping, not a root system for drinking.