2/2 ALG^E 



as it may be termed, the protoplasm and chlorophyll-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 Desmidiese, 1848. 

 Nageli Gattungen einzelliger Algen. 1849. 

 Stahl Verhandl. Phys.-med. Gesell. Wiirzburg, 1880, p. 24. 

 Fischer Bot. Zeit., 1883, pp. 225 et seq. 

 Wolle Desmids of the United States, 1884. 

 Klebs Biolog. Centralblatt, 1885, p. 353. 



Cooke British Desmids, 1887 (which see for farther bibliography). 

 Hauptfleisch Zellmembran u. Hiillgallerte der Desmidiaceen, 1888. 



Class XV. Confervoideae Isogamae. 



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 Conjugate, the only known sexual mode of reproduction is an 

 isogamous 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 very 

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



