150 Chlorophycew 



A. Point of division of cells variable or sometimes fixed (at the isthmus). 



Tribe 3. Peniese. Cells of moderate length, straight, cylindrical, some- 

 times with a slight median constriction. Cell-wall with or without pores. 

 Point of division of cells often variable. The cell often arrives at maturity 

 by periodical growth. 8. Penium. 



Tribe 4. Closterieae. Cells elongate, generally curved ; symmetrical in 

 one longitudinal plane only. . Cell-wall commonly with pores. Points of 

 division regularly placed in the middle region of the cell. 



* Cells almost cylindrical, scarcely attenuated. 



Chloroplast single, without apical moving 



grannies 9. Roya. 



** Cells strongly attenuated towards each ex- 

 tremity. Two chloroplasts in each cell, 

 with apical moving granules 10. Closterium. 



B. Point of division of cells always fixed (at the isthmus). 



Tribe 5. Cosmarieae. Cells exhibit great variety of form, and the cell- 

 wall consists of two thin, firm layers with pores. There is no periodical 

 growth, the cell becoming adult immediately after division by the mature 

 growth of the young semicell. 



a. The point of division of the cell, where the new and old parts of the 

 cell-wall are obliquely fitted together, remains plane during division. Solitary 

 or colonial. 



* After division the cells remain free and solitary. 



f Cells elongated and cylindrical ; constriction slight. 

 Apices of cells truncate or rounded. 



\ Base of semicells plicate 11. Docidium. 



1 I Base of semicells plane 12. Pleurotcenium. 



Apices of cells cleft, incision open or narrow. 



I Cell-wall adorned with rings of 



furcate processes 13. Triploceras. 



II Cell-wall plane. 



Apical incision widely open, 

 each apical angle furnished 



with a spine 14. Ichthyocercus. 



Apical incision narrow ... 15. Tetmemorus. 

 tt Cells relatively short, commonly compressed or radiating, con- 

 striction usually deep. 

 Cells compressed (at right angles to the plane of the front 

 view) ; from the vertical view fusiform or elliptical. 

 I Cells generally with an apical in- 

 cision and a central protuber- 

 ance, moderately lobed 16. Euastrum. 



I I Cells very compressed and deeply 



lobed or incised 17. Micrastenas. 



HI Cells with a more or less entire margin, often fur- 

 nished with warts or spines. 



