Introduction to desmids

Desmids are microscopic green algae (members of the Chlorophyta) that occur in freshwaters all around the world. They make up the largest part of the class Zygnemaphyceae (formerly Conjugatophyceae), with the remainder consisting of the family Zygnemataceae (which includes filamentous taxa such as Spirogyra).

Classification
The classification of Mix (1972) for the Zygnemaphyceae is as follows:

Class Zygnemaphyceae
Order 1: Zygnematales
Family 1: Mesotaeniacaeae (saccoderm desmids; Cylindrocystis, Netrium, Ancylonema, Roya, Spirotaenia)
Family 2: Zygnemataceae (Spirogyra, Zygnema - NOT desmids!)
Order 2: Desmidiales (placoderm desmids)
Suborder 1: Archidesmidiinae
Family 3: Gonatozygaceae (Gonatozygon, Genicularia)
Family 4: Peniaceae (Penium)
Family 5: Closteriaceae (Pleurotaenium, Tetmemorus, Closterium)
Suborder 2: Desmidiinae
Family 6: Desmidiaceae (Actinotaenium, Euastrum, Micrasterias, Xanthidium, Cosmarium, Staurodesmus, Staurastrum, Spaerozosma, Spondylosium, Hyalotheca, Desmidium, Bambusina, Cosmocladium)

Morphology
The saccoderm desmids are usually simple cylindrical cells without wall ornaments and without the median constrictions that are typical of the placoderm desmids (Fig.1). Cells contain a simple chloroplast with one or two pyrenoids. The placoderm or "true" desmids are different from saccoderms in that their cells consist of two halves with a more complex wall structure and a fairly elaborate pore system. The two halves, which are referred to as semicells, are separated either by suture lines (in the Archidesmidiinae, Fig.2) or by a well-defined constriction referred to as the isthmus (in the Desmidiinae, Fig.3).



Symmetry
While saccoderm desmids usually exhibit simple cylindrical symmetries, the placoderms are diverse in their morphologies, and symmetry is often complicated. Many taxa (e.g. Euastrum or Micrasterias) have biradiate symmetry, with cells that are laterally compressed and essentially elliptical in end view (Fig. 4). Some of these taxa have protuberances which project out from the front of the cell, like in the example shown here. Other taxa, for example Staurastrum, are triradiate (Fig. 5), with processes spaced at 120 intervals, or even pluriradiate (Fig. 6), with more than three corners to the cell in end view. The degree of radiateness can sometimes vary between the two semicells of one cell; in some Staurastrum taxa, for example, cells have been observed that consist of one biradiate and one triradiate semicell (also referred to as "Janus cells").


Cell division and morphogenesis
In true (placoderm) desmids, cell division takes place at the narrow isthmus which separates the two semicells. The isthmus elongates and a septum of primary wall material forms across the narrowest section following mitosis. The newly formed daughter cells then begin to expand but remain joined together at the centre of the septum. When the new semicells are close to reaching their full size, the secondary wall is deposited, and the primary wall is eventually cast off, leading to the complete separation of the two new cells. Fig. 7 shows the two incipient daughter cells in the expansion phase, with the septum clearly visible between them.


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