Giant Kelp (or Macrocystis) has the distinction of being the largest marine plant (and seaweed) in the world; with the largest attached plant recorded being 65m long. The kelp plant has a root-like holdfast that fixes to rocky surfaces; a long slender stalk or stipe; and long, leaf-like blades or fronds, that are the major site of photosynthetic activity.
The kelp plant is supported in the water by gas-filled bladders on each frond called pneumatocysts. The holdfast is cone-shaped and can grow up to 60 cm in diameter in large plants.