Bull kelp have a long stipe topped by a single, large pneumatocyst. The bull kelp's neumatocyst has up to a dozen blades. Like giant kelp, a bull kelp's pneumatocyst is mostly filled with carbon dioxide. Pneumatocysts lift the blades closer to sunlight, necessary for photosynthesis.

Photograph by Alison Marti, MyShot
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Kelp is a giant seaweed that grows in two forms along the west coast of North America—giant kelp and bull kelp.