A seaweed may belong to one of several groups of multicellular algae: the red algae, green algae, and brown algae. As these three groups are not thought to have a common multicellular ancestor, the seaweeds are a polyphyletic group. In addition, some tuft-forming bluegreen algae (Cyanobacteria) are sometimes considered as seaweeds — "seaweed" is a colloquial term and lacks a formal definition.
Structure of Seaweed
- Seaweeds' appearance somewhat resembles non-arboreal terrestrial plants.
- thallus: the algal body
- lamina: a flattened structure that is somewhat leaf-like
- sorus: spore cluster
- on Fucus, air bladders: float-assist organ (on blade)
- on kelp, floats: float-assist organ (between lamina and stipe)
- stipe: a stem-like structure, may be absent
- holdfast: specialized basal structure providing attachment to a surface, often a rock or another alga.
- haptera: finger-like extensions of holdfast anchoring to benthic substrate
- The stipe and blade are collectively known as the frond.
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