Owing to the presence of unmasked chlorophylls a and b, the species are grassy green, their pigmentation also being characteristic in green plants (mosses, ferns, and flowering plants). Starch is produced through assimilation. Some forms are symbiotic with fungi to form lichens, and symbiosis with marine invertebrate animals is also known. Sexual reproduction shows a wide range of variation, from fusion of similar motile gametes (isogamy) to that of dissimilar gametes (anisogamy or oogamy) or the fusion of the entire protoplasm content of different cells, as in the filamentous freshwater genus Spirogyra. Chlorophyceae (to which Spirogyra belongs) is the largest class within Chlorophyta, and its species vary the most in form.
Flagellate unicellular forms of the Chlorophyceae are placed in the order Volvocales, the representatives of which may be either single motile cells (such as Chlamydomonas) or colonies of cells joined together. Volvox is the best-known representative of the latter type, its colonies taking the shape of a hollow, slowly revolving sphere just visible to the naked eye. Nonflagellate unicellular or colonial algae compose the order Chlorococcales, of which Trebouxia, the algal symbiont most commonly associated with lichens, is an example.
Filamentous and membranous organization exists in the orders Ulotrichales and Ulvales, respectively. Ulothrix consists of simple unbranched filaments, each cell with a single chromatophore applied to the inner cell wall. Ulva (sea lettuce) forms leafy, platelike thalli two cell layers thick. Coenocytic structure, consisting of a multinucleate mass of protoplasm, is found in the orders Cladophorales, Siphonocladales, Dasycladales, Bryopsidales, and Caulerpales.
The desmids (order Zygnematales) are microscopic unicellular forms with minutely perforated walls. The cells are constricted in the middle and show bilateral to almost radial symmetry. They inhabit freshwater.
