Phaeophyceae

The members of Phaeophyceae are the brown algae, and they belong chiefly to the sea — marine habitats are their main home.

Size and form

Brown algae differ greatly in size and shape. At the simple end they are branched, filamentous forms such as Ectocarpus. At the other extreme are the kelps, which are profusely branched and can grow to a towering height of about one hundred metres.

Colour and pigments

These algae carry chlorophyll a and chlorophyll c, together with carotenoids and xanthophylls. Their colour ranges from olive green to various shades of brown, and how brown they look depends on how much of the xanthophyll pigment fucoxanthin they contain.

Food storage

Brown algae store their food as complex carbohydrates, held in the form of laminarin or mannitol.

Cell wall and protoplast

The vegetative cells have a wall of cellulose, which is usually coated on the outside by a gelatinous layer of algin. Inside, the protoplast contains — besides its plastids — a centrally placed vacuole and a nucleus.

Body structure

The plant body is generally anchored to the surface it grows on by a holdfast. Above the holdfast comes a stalk called the stipe, and attached to the stipe is a leaf-like, photosynthetic organ known as the frond.

Reproduction

• Vegetative reproduction takes place by fragmentation.
• Asexual reproduction in most brown algae is carried out by biflagellate zoospores. These are pear-shaped and bear two unequal flagella attached at the side. …