Thallophyta (Algae)

Thallophyta, commonly called algae, are a highly diverse group whose defining feature is a thalloid body — the plant is not differentiated into true root, stem and leaves. Members range from microscopic single cells to giant seaweeds many metres long.

Habitat and habit. Most algae are aquatic, growing in either marine or fresh water; a smaller number grow as epiphytes on other plants, and some are symbiotic or epizoic, living non-parasitically on the surface of other organisms. The great majority are free-living, though some form symbiotic partnerships.

Range of body forms. Algal bodies span a wide size range: microscopic unicellular forms that may be non-motile (e.g. Chlorella) or motile with flagella (e.g. Chlamydomonas); multicellular unbranched filaments (e.g. Spirogyra); multicellular branched filaments (e.g. Chara); and, at the largest scale, huge macroscopic seaweeds such as Sargassum, which can grow beyond 60 metres in length.

Cell wall and stored food. The algal cell wall is built from either polysaccharides such as cellulose and glucose-based compounds, or from a variety of proteins, or from both. Reserve food is stored mainly as starch or related carbohydrate forms.

Reproduction and life cycle. Algae reproduce vegetatively, asexually and sexually. Their life cycles display alternation of generations, typically with a dominant haploid phase and a comparatively reduced diploid phase. …