DESMIDIACE^. 



[ 197 J 



DESMIDIUM. 



appear to do in Spirogyra, where the body 

 produced after conjugation usually germi- 

 nates at once into a filament, but sometimes 

 breaks up into zoospores or minute resting- 

 spores, such as are met with in unconjugated 

 cells. The reproduction of the Desmidiaceae 

 still offers a wide field for investigation. 



The Desmidiaceae may be collected in the 

 same manner as is recommended for the 

 DiATOMACE^. Their preservation is a 

 somewhat difficult matter, as almost all the 

 preservative liquids alter them more or less. 

 Those producing the smallest amount of 

 change are Thwaites's liquid, Ralfs's liquid, 

 or simple camphor-water. A few of them, 

 for example Pediastrum, are unchanged by 

 concentrated solution of chloride of calcium, 

 except that the colour becomes rather paler. 

 We believe, moreover, that the cell-mem- 

 brane, upon the forms of which the charac- 

 ters mainly depend, remains unaltered in all 

 the kinds when kept in this solution. See 

 Mounting and Preservative Liquids. 



Analysis of the Tribes and Genera. 

 (PI. 10). 



I. Closterie^. Cells single, elongated, 

 never spinous, frequently not constricted 

 in the middle (sporangia smooth). 



1. Closterium. Cell crescent-shaped or 

 arcuate, or much attenuated at the ends, not 

 constricted in the middle (figs. 40 to 45, 

 57, 58). 



2. Penium. Cell straight, not or very 

 slightly constricted in the middle, rounded 

 at the ends (fig. 36). 



3. Tetmemorus. Cell straight, constricted 

 in the middle, notched at the ends (figs. 33, 

 34). 



4. Docidium. Cell straight, constricted in 

 the middle, tnmcate at the ends (figs. 38,39). 



5. SpirotcBnia. Cell straight, not con- 

 stricted; endochrome spiral (fig. 59). 



II. CosMARiE^. Cells single, distinctly 

 constricted in the middle; segments seldom 

 longer than broad (sporangia spinous or 

 tuberculated, rarely if ever smooth). 



6. Micrasterias. Lobes of the segments 

 incised or bidentate (fig. 13). 



7. Euastrum. Segments sinuated, gene- 

 rally notched at the ends, and with inflated 

 protuberances (figs. 14 to 17). 



8. Cosmarium. Segments neither notched 

 nor sinuated, end view elliptic, cncular, or 

 cruciform (figs. 18 to 22). 



9. Xanthidium. Segments compressed, 

 entire, spinous (figs. 23 to 25). 



10. Arthrodesmus. Segments compressed, 

 each with only two spines (fig. 27). 



11. Staurastrum. End view angular, ra- 

 diate, or with elongated processes, which are 

 never in pairs (figs. 26, 28 to 31). 



12. Didymocladon. End view angular, 

 each angle with two processes, one inferior 

 and parallel ^\ith the similar one of the 

 other segment, the other superior and diver- 

 gent (figs. 32, 56). 



III. DESMiDiEiE. Cells united into an elon- 

 gated jointed filament (sporangia spherical, 

 smooth). 



13. Hyalotheca. Filament cyhndrical 

 (figs. 1,2). ^ 



14. Didymoprium. Filament cylindiical 

 or subcylindrical ; cells with two opposite 

 bidentate projections (figs. 5, 6). 



15. Desmidium. Filament triangular or 

 quadrangular; cells with two opposite bi- 

 dentate projections (figs. 7, 8). 



16. Aptogonum. Filament triangular or 

 plane, with foramina between the joints 

 (figs^. b5. 52). 



17. Sphcerozosma. Filament plane, mar- 

 gins incised or sinuated ; joints with junc- 

 tion-glands (figs. 9, 10). 



(The genus Encampia, Ehr. is placed near 

 Desmidium by Kiitzing. It is not a Diatoma- 

 cean, as it shrinks in drying (seeEucAMPiA)) . 



IV. Ankistrodesmi^. Cells elongated, en- 

 tire, small, grouped in faggot-like bundles. 



18. Ankistrodesmus (fig. 47). 



V. Pediastre^. Cells grouped in the form 

 of a disk or star, or placed side by side in 

 one or two short rows. 



19. Pediastrum. Cells forming a disk or 

 star, the outer margins bidentate (fig. 48). 



20. Scenedesmus. Cells placed side by 

 side in one or two rows (figs. 50, 51, 53, 54). 



BiBL. 'Railh, British DesmidiecB ; Ehren- 

 berg, Infusionsth. ; Pritchard, Infusoria ; 

 Hassall, Brit. Freshwater Algce; Nageli, 

 Einzell. Alg. Zurich, 1849 ; A. Braun, Ver- 

 jungung, Sfc. {Bay Soc. Vol. 1853) ; Focke, 

 Physiologische Studien, Heft i. 1848; Cas- 

 pary, Bot.Zeitung, viii. 7^^ (1850); Prings- 

 heim, Flora, 1852, p. 486. See also the Bibl. 

 of Closterium and other genera. 



DESMIDIUM, Ag.— A genus of Desmi- 

 (Uacese. 



Char. Cells united into a brittle, regularly 

 twisted, triangular or quadi-angular filament, 

 and two-toothed at the angles. 



The filaments exhibit one or two dark. 



