138 



FRESH-WATER BIOLOGY 



29 (30) Chromatophore a single axial plate with one or more pyrenoids. 



Mesotaenium NageU. 



€i'^iki'S^^i^S\ Cells cylindrical, with rounded ends, resembling in structure 



'S'4lk'^%^S^^''^S^ ^ "^^"^ °f Mougeotia but smaller, sometimes adhering to each other 

 K:irr.'tf?'<'.--'-v-. r'f'j^^^ after division but not forming distinct filaments. 



%, Fig. 120. Mesotaenium endlicherianum Nageli. X 625. a. showiuij 



^ B the surface of the cUorophyll 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 Kiss. 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 lamellosum Brfibisson. X 200. (After Kircbner.) 



33 (23) Cells constricted at the center. . 34 



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 sUght 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 'Rails. X 465. (Original.) 



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



chromatophore of longitudinal bands; p)Tenoids many, 

 scattered Pleurotaeniopsis Lundell. 



This is regarded by Br^bisson as a Calacylindrus, by de Bary 

 as a Pleurotaenium and by West as a Cosmarium. Formerly 

 Wille recognized the genus, Pleuretaeniopsis, but now includes it 

 under Cosmarium. 



Fig. 124. Pleurotaeniofisis turgidus Lund. X 130. (After de Bary.) 



