General Characteristics:
1. It is a large group of algae consisting of 240 genera and over 1,500 species out of which 32 genera and 93 species are reported from India. They are commonly known as brown algae, due to the presence of golden brown xanthophyll pigment, fucoxanthin (C40 H54O6) in the chromatophores. About 99.7% members are marine and a few grow in fresh water. Some members of phaeophyceae grow both in fresh water as well as marine habitats.
2. Plant body is immobile, multicellular and highly differentiated both externally and internally.
3. They range from simple microscopic heterotrichous filament (Ectocarpus) to largest alga (Macrocystis pyrifera), which attains a length of 60 - 90 meters. The largest forms are known as Kelps or rockweeds. e.g., Lessonia davicans reaches a length of 4 meters and looks like a miniature tree. Nereocystis luelkeana, the bladder kelp which attains a length of 25 - 30 meters. Postelsia palmae appears like a palm tree and commonly known as sea palm.
4. Commonly the plant body is differentiated into hold - fast, a short or elongated stipe and an expanded blade. The blade performs photosynthesis and bears reproductive structures. Many species remain afloat by having air bladders.
5. The photosynthetic pigments include chlorophyll - a, chlorophyll - c, ß - carotene and xanthophylls like fucoxanthin, flavoXanthin and violaxanthin. The fucoxanthin is however present sufficiently which partially mask the chlorophyll and carotenoid, thereby giving the characteristic brown colouration.
6. The cell wall outer mucilagenous layer has fucinic and alginic acids, but the inner layer is mainly cellulosic. The alginic acid is used to manufacture artificial silk and adhesive, obtained commercially from sargassum, laminaria etc.
7. The cells usually have many small vesicles and white granules. The granules are called fucosan vesicles.
8. Pyrenoides are usually absent, but if present, is of single stalk type.
9. Motile structures (zoospores and gametes) have two laterally inserted unequal flagella, of which larger one is tinsel or pantonematic and the smaller one is whiplash or acronematic type.
10. The reserve foods are commonly laminarin and mannitol. Sucrose and glycerol are also present in some members.
