WISC0NSI2f PHYTOPLANKTON 



Family DESMIDIACEA^. 



Cells solitary or united in colonies. Naked or enclosed by a hyaline 

 gelatinous envelope that is homogeneous or radially fibrillar. Col- 

 onies filamentous or growing in three dimensions. Shape of the 

 cells exceedingly variable, generally with a pronounced constriction 

 in the median portion dividing them into two similar semicells. Semi- 

 ceRs always with a bilateral or a radial sj-mmetry. Cells frequently 

 with the walls having a characteristic ornamentation. Nucleus single 

 and lying at the median constriction, Chloroplasts of each semicell 

 generally solitary and axial, with one or more pyrenoids ; more rarely 

 with more than one axial or parietal chloroplast in each semicell or 

 extending throughout the entire length of the cell. 



Asexual reproduction by vegetative cell division, the divisions 

 taking place at the isthmus and new semicells being formed at the 

 point of division. Aplanospore formation know'n in a few instances. 



Sexual reproduction isogamous, by aplanogametes. 



KEY TO THE GENERA. 



Cells not united in colonies. 



Cell leugtli at least twice the breadtli. 

 Witiout a mediau coustriction. 

 CliJcroi.last ribl. on-like. 



Eibbon axial and straiglit GoNATOZTGOisf (p. 



Riblon parietal and spiral Spieotaenia (p.p.) (p. 



Chloroplast not ribbon-like. 

 Cells straight. 



Longitudinal ridges of chloroplast notched Netrixjm (p. 



Longitudinal ridges of chloroplast nnnotched. 



Kidgfts straight Peniuh (p.p.) (p. 



Eidges spiially twisted Spirotaenia (p.p.) (p. 



Cells curved and generally strongly attenuated at poles. .CLOSTERroM (p. 

 "With a jiiedian constriction. 



CeU apices incised TETiiEMORUS (p. 



Cell apices not incised. 



Bases of semicells not swollen Penium (p.p.) (p. 



Bases of semicells swollen. 



SemiciU bases with vertical plications DocroruM (p. 



SemiceU bases without pl-cations. 



Wall with transverse rings of verrucae Triploceras (p. 



Wall without transverse rings of verrucae. . . . Pleukotaenium (p. 

 Cell length not over twice the breadth. 

 Cells compressed. 

 Apex of semicells incised. 



Lateral margins not dee; ly incised Etjastrum (p- 



Lateral margins deeply incised Micbasterias (p.p.) (p. 



-+) 

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6) 



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20) 



7) 

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18) 

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21) 

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