272 ALG.-E 



as it may bu termed, the protoplasm and chloroph\ll-corpuscles are 

 already distributed symmetrically into two half-cells, which contract 

 somewhat, and the whole becomes invested by a new cell-wall. A con- 

 striction has in the meantime made its appearance between the two 

 halves, and the new individual rapidly assumes its mature form, but is 

 at first of small size. It soon divides repeatedly, and each generation 

 gradually increases in size until the full size is attained. 



The number of known species of desmids is not large compared with 

 that of diatoms ; they are found in great abundance in the midst of 

 larger algae in fresh water, especially in moor-pools, sometimes forming 

 a green scum on the surface. 



Literature. 

 Ehrenberg — Die Infusionsthierchen, 1838. 

 Ralfs — British Desmidieae, 1848. 

 Nageli — Gattungen einzelliger Algen. 1849. 

 Stahl — Verhandl. Phys. -med. Gesell. Wlirzburg, 1880, p. 24. 

 Fischer — Bot. Zeit. , 1883, pp. 22^ ei seq. 

 Wolle— Desmids of the United States, 1884. 

 Klebs— Biolog. Centralblatt, 1885, p. 353. 



Cooke — British Desmids, 1887 (which see for further l:)ibIiography). 

 Hauptfleisch — ZeHmembran u. Hiillgallerte der Desmidiaceen, 1888. 



Class XV.^Confervoideae Isogamse. 



In this class the individual still consists of a filament of cylindrical 

 cells, placed end to end, which may be branched or unbranched. As 

 in the Conjugatae, the only known sexual mode of reproduction is an 

 isogainous one between two masses of protoplasm, which are not clearly 

 differentiated beforehand into a male and a female element ; but the 

 conjugating bodies are not the contents of stationary cells, but are 

 motile ciliated swarm-spores or zoogametes^ produced by free-cell forma- 

 tion in ordinary or in slightly differentiated cells of the filament, hence 

 termed gametanges, their conjugation resulting in the production of a 

 zygosperm. The filament increases in length by the repeated transverse 

 septation of successive apical cells, or less often of intercalary cells. 

 The ordinary mode of multiplication is a non-sexual one, by means of 

 naked ciliated zoospores., closely resembling the zoogametes, but often 

 larger, and formed singly or in pairs in a cell. Vegetative propagation 

 also takes place by the formation and detachment of cysts or resting- 

 cells, which may be either akinetes or aplanospores. The cells verv 

 frequently display a plurality of nuclei, but this is not nearly so strongly 



