Any aquatic or marine plant with air-filled sacs or bladders that help it float near the water surface.
From Old English 'blædre' (bladder) + 'wēod' (weed/plant). This refers to seaweeds and water plants with characteristic buoyant, gas-filled chambers called pneumatocysts.
Kelp and many seaweeds have these built-in flotation devices that work like scuba tanks—they're filled with gases that keep the plant standing upright in water so it can get sunlight, which is genius for underwater survival!
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