The ultrastructure of the freshwater flagellate Tetraselmis cordiformis Stein (Chlorophyceae) was investigated. The general morphology could be described as typical prasinophycean (Prasinophyceae sensuChristensen) and the organism shares all generic characteristics of Platymonas West. The flagellar apparatus has been examined in detail. The four flagella emerge from an apical trough in the theca and are arranged in a zig-zag row. They are covered by three types of scales. Four cruciate flagellar roots of compound type are present. One part is microtubular (4-2-4-2 system) and the other prominent part is fibrillar with distinctive cross striations. The four roots are short and terminate at the bottom of the apical through, where they attach the flagellar apparatus to the theca. The four-stranded root shows no changes in root tubule configuration. In addition to the cruciate root system there are two massive rhizoplasts. The rhizoplasts exhibit different striation patterns along their length. Taxonomic implications and flagellar root system structure as it relates to current theories of evolution in green algae are discussed. © 1979 Springer-Verlag.
