SYNOPSIS OF THE GENERA
Fam. DESMIDIEAE.
Freshwater, figured, mucous and microscopic Algae, of a green colour. Transverse division mostly complete, but in some genera incomplete. Cells or joints of two symmetrical valves, the junction always marked by the division of the endochrome, often also by a constriction. Sporangia formed by the coupling of the cells and union of their contents.
* Plant an elongated jointed filament. Sporangia orbicular, smooth.
** Frond simple from complete transverse division, distinctly constricted at the junction of the segments, which are seldom longer than broad. Sporangia spinous or tuberculated, rarely if ever smooth.1. Hyalotheca. Filament cylindrical. 2. Didymoprium. Filament cylindrical or subcylindrical; joints with two opposite bidentate projections. 3. Desmidium. Filament triangular or quadrangular; joints connected by a thickened3. border. 4. Aptogonum. Filament triangular or plane, with foramina between the joints. 5, Sphaerozosma. Filament plane, margins incised or sinuated; joints with junction glands.
*** Frond simple from complete transverse division, generally much elongated, never spinous, frequently not constricted at the centre. Sporangia smooth.6. Micrasterias. Lobes of the segments incised or bidentate. 7. Euastrum. Segments sinuated, generally notched at the end, and with inflated protuberances 8. Cosmarium. Segments in front view neither notched nor sinuated, in end view elliptic, circular or cruciform. 9. Xanthidium. Segments compressed, entire and spinous. 10. Arthrodesmus. Segments compressed, and having only two spines or mucros. 11. Staurastrum. End view angular, radiate, or with elongated processes, which are never geminate. 12. Didymocladon. Segments angular, each angle having two processes, one inferior, and parallel with the similar one of the other segment, the other superior and divergent.
**** Cells elongated, entire, fasciculated.13. Tetmemorus. Frond straight, constricted at the centre and notched at the ends. 14. Penium. Frond straight, scarcely constricted at the centre. 15. Docidium. Frond straight, much-elongated, constricted at the centre, truncate at the ends. 16. Closterium. Frond crescent-shaped or arcuate, not constricted at the centre. 17. Spirotaenia. Frond straight, not constricted at the centre; endochrome spirally twisted.
***** Frond composed of few cells, definite in number, and not forming a filament. (Sporangia unknown.)18. Ankistrodesmus. Cells aggregated into faggot-like bundles.
19. Pediastrum. Cells arranged in the form of a flattened star; their outer margin bidentate. 20. Scenedesmus. Cells oblong or fusiform, entire, placed side by side in a single row, but during division in two rows.