Wikipedia
Desmids (Gr. desmos, bond or chain) are an order (Desmidiales) in the Charophyta, a division of green algae that forms a sister group to the land plants ( Embryophyta).
The desmids are often treated as members of the Zygnematales, as family Desmidiaceae. The Desmidiales comprise around 40 genera and 5,000 to 6,000 species, found mostly but not exclusively in fresh water. Many species may be found in the fissures between patches of sphagnum moss in marshes. With a pH level of approximately 4.0, sphagnum peat provides the ideal environment for this flora.
The structure of these algae is unicellular, while the cell is sometimes divided into two symmetrical compartments separated by a narrow bridge or isthmus, wherein the spherical nucleus is located. Each semi-cell houses a large, often folded chloroplast for photosynthesizing. One or more pyrenoids can be found. These form carbohydrates for energy storage. 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. The largest among them may be visible to the unaided eye.
Desmidiales lack flagella. Desmids commonly reproduce by asexual fission, however, in adverse conditions, Desmidiales may reproduce sexually, through a process of conjugation, which are also found among the Zygnematales. These two groups are closely related, and comprise the class Zygnematophyceae. They resemble the algae genus Spirogyra as some of them are filamentous, such as the species Desmidium swartzii. However, these filaments are arranged in a helix pattern.
Desmids possess characteristic crystals of Barium sulphate at either end of the cell which exhibit a continuous Brownian type motion.
Many desmids also secrete translucent, gelatinous mucilage from pores in the cell wall that acts a protecting agent. 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.
In the inner layer of the wall the pore is a simple canal, but in the outer, except in Closterium, the canal is surrounded by a specially differentiated cylindrical zone, not composed of cellulose, through which the canal passes. This is termed the pore-organ. The canals are no doubt in all cases occupied by threads of mucilage in process of excretion. 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. In most cases, however, these are absent or only represented by small perforated buttons.