In general, desmids prefer acidic waters (pH between 4.8 and 7.0), so many species may be found in the fissures between patches of sphagnum moss in marshes.

The cell-wall, of two halves (termed semicells), which, in a few species of Closterium and Penium, are of more than one piece, has two distinct layers, the inner composed mainly of cellulose, the outer is stronger and thicker, often furnished with spines, granules, warts et cetera.

It is made up of a base of cellulose impregnated with other substances including iron compounds, which are especially prominent in some species of Closterium and Penium and is not soluble in an ammoniacal solution of copper oxide.

Desmids assume a variety of highly symmetrical and generally attractive shapes, among those elongated, star-shaped and rotund configurations, which provide the basis for their classification.

[7] Desmids possess characteristic crystals of barium sulphate at either end of the cell[12] which exhibit continuous Brownian motion.

These pores are either, as in Micrasterias, uniformly distributed across the cell-wall but always appear to be absent in the region of the isthmus, or, in highly ornamented forms, as many genera of Cosmarium, grouped symmetrically around the bases of the spines, warts and so on with which the cell is provided.

At the inner surface of the wall they terminate in lens- or button-shaped swellings, while from the outer end of the pore-organ there sometimes arise delicate radiating or club-shaped masses of mucilage through which the canal passes and which appear to be more or less permanent in character.

[9] In adverse conditions, desmids may reproduce sexually through a process of conjugation, which are also found among other closely related taxa in the Zygnematophyceae.

[8] Desmid species are generally found attached to aquatic vegetation, such as Utricularia, or tychoplanktonic; that is, free-floating in the water column after being disturbed.

[10] Although the Desmidiales are cosmopolitan, a number of species appear to be restricted to continents or biogeographical realms; this is likely because desmids have strict ecological requirements and do not produce resting spores, making successful dispersal less likely.