Desmid
Desmid, common name for numerous single-celled freshwater green algae. The cell is divided into two symmetrical halves, held together by an isthmus containing the nucleus. Each half of the cell contains a single chloroplast. Desmids sometimes grow in colonies, where individuals remain attached after cell division. Many species are capable of self-locomotion by gliding. They are phototropic; the direction of their movement is determined by their response to light. Desmids reproduce both sexually and asexually. Asexually, the desmids reproduce by a simple division across the isthmus; the one cell becomes two. In sexual reproduction, the desmids come together in pairs and their cell contents fuse. From this fused body come two new desmids. Although this form of sexual reproduction does not increase the population, it does maintain genetic variation within the population. Desmids of one common genus are crescent shaped and have a small vacuole at each end of the cell. These vacuoles co