138 



FRESH-WATER BIOLOGY 



2 9 (3) Chromatophore a single axial plate with one or more pyrenoids. 



Mesotaenium Nageli. 



Cells cylindrical, with rounded ends, resembling in structure 

 A cells of Mougeotia but smaller, sometimes adhering to each other 

 after division but not forming distinct filaments. 



FIG. 120. Mesotaenium endlicherianum Nageli. X 625. a. showing 

 B the surface of the chlorophyll plate, b. showing the edge of the chloro- 

 phyll plate. (Original.) 



30 (29) In each cell-half several chlorophyll plates 31 



31 (32) Margins of radial plates entire; pyrenoids central in each cell-half. 



Penium de Bary. 



Cells sometimes slightly constricted at the middle, rounded or trun- 

 cated at the ends; length 3 to 9 times the breadth; membrane smooth, 

 punctate, or longitudinally striated; chromatophores radially placed 

 about a large pyrenoid in each cell-half. 



FIG. 121. Penium cucurbitinum Biss. X 295. (After West.) 



32 (31) Margins of the radial plates of the chromatophore scalloped ; pyrenoids 



several and scattered . Netrium Nageli. 



Cells shaped much as in Penium. The scallops of the outer margin of the chromatophores 

 conspicuous; pyrenoids not large and forming a center about which the chlorophyll plates 

 radiate, as in Penium, but small and scattered. 



FIG. 122. Netrium lamettosum Brebisson. X 200. (After Kirchner.) 



33 ( 2 3) Cells constricted at the center 24 



34(42) Constriction at the sides slight and usually gradual 35 



35 (38) Length of cells usually not more than six times the breadth. . . 36 



36 (37) Central constriction very gradual and shallow; a slight incision at 



the ends Tetmemorus Ralfs. 



Cells straight, fusiform, or cylindrical, slightly and broadly 

 constricted at the middle; ends rounded, each with a slight 

 linear incision; length 4 to 6 times the diameter. Chroma- 

 tophore axial with a single row of pyrenoids. 



FIG. 123. Tetmemorus granulatus Ralfs. X 465. (Original.) 



37 (36) Cells short, ends truncate, constriction rather abrupt, but not deep; 



chromatophore of longitudinal bands; pyrenoids many, 

 scattered Pleurotaeniopsis Lundell. 



This is regarded by Brebisson as a Calocylindrus, by de Bary 

 5 a Pleurotaenium and by West as a Cosmarium. Formerly 

 fille recognized the genus, Pleuretaeniopsis, but now includes it 

 ader Cosmarium. 



FIG. 124. Pleurotaeniopsis lurgidus Lund. X 130. (After de Bary.) 



