Multi-cellular Algae: Seaweeds Seaweeds are more formally referred to by biologists as macrophytes or macroalgae Seaweeds are not plants and so lack true leaves, stems, and roots The complete body is known as the thallus whether it is a filament, a thin leafy sheet, or a giant kelp

General Structure All portions of the thallus are capable of photosynthesis The leaf-like, flattened portions of the thallus are known as blades – large surface area; main photosynthetic region Gas-filled bladders known as pneumatocysts help keep the blade close to the sea surface Some seaweeds have a distinctive, stem-like structure, the stipe, which provides support

Plant Algae

General Structure A structure resembling roots, the holdfast, attaches the thallus to the bottom – NOT involved in any significant absorption of water and nutrients – Does not penetrate through sand and mud as true plant roots do Restricts seaweeds to hard bottoms; not found in sandy, soft bottom habitats Water and nutrients are transferred directly across the surface of the thallus