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The Chlorophyceae (“greens”) have cellulose cell walls, contain chlorophylls a and b, and produce a-linked storage reserve (starch). For these reasons they are considered ancestral to Charophyceae and all plants (Viridae).

The desmids, along with other greens with similar advanced characteristics, are elevated to the class Charophyceae.

Some authors group the Chlorophyceae and Charophyceae along with two or three smaller classes (Trebouxiophyceae, Ulvophyceae, Prasinophyceae) into Division Chlorophyta to indicate their evolutionary proximity (see Graham and Wilcox 2000).

Most “greens” appear as green as their successors, while some (Haematococcus, Trentepohlia) produce a “sun screen” from carotenoids and can be quite red. They are separated from other classes that contain chlorophyll alone, or chlorophylls a and c (Bacillariophyceae, Chrysophyceae, Cryptophyceae, Eustigmatophyceae, Haptophyceae, Phaeophyceae, and Xanthophyceae, and/or beta-linked storage reserves.

Cell division is also ancestral ("phragmoplast”), and embryos are neither retained by the parent nor provided with a protective cover of somatic cells.

Similar to many other photosynthetic classes of protists, the Chlorophyceae can be grouped morphologically as unicells, colonies, or filaments. A recent taxonomic treatment of the class(es) is by Moestrop ('primitive greens' called Prasinophyta) and John (greens ca;;ed Clorophyta) both elevating the class Chlorophyceae to Phylum level, in John, Whitton and Brook (2011).

Chlorophyll a and b absorbance spectra

The primary photopigments in the photosynthetic system include chlorophyll a -- common to all photosynthetic protists, and chlorophyll b -- present only in green (Chlorophyceae, Charophyceae) and euglenoid (Euglenophyceae) protists, along with yellow carotenoids.

Chlorophyll a, b, and carotenoid absorbance spectra:

Accessory pigments, the carotenoids, present in many photosynthetic protists, increase absorbance of green light.

John, D.M., B.A. Whitton and A.J. Brook  2011.  The Freshwater Algal Flora of the British       Isles.  Cambridge Press (878 pp).