SCENEDESMUS Meyen 1829, p. 774 

 Colony of 2-4-8-32 ovoid, fusiform, crescent-shaped, or oblong 

 cells lying side by side in a single series, or in a double row with the 

 cells alternating; cell walls smooth or with spines, teeth, and ridges; 

 chloroplast a parietal plate covering most of the cell wall and often 

 showing a median lateral notch; 1 pyrenoid. 



This genus contains some species which are perhaps more widely 

 distributed than any other fresh-water algae. Whereas a few are 

 commonly found as euplankters, especially those which bear spines, 

 most forms occur in tychoplankton. In favorable habitats one or 

 two species may completely dominate the flora, and often a small 

 artificial pool or an aquarium will be densely colored by their 

 tremendous numbers. See Chodat (1926) and Smith (1916a) on 

 the taxonomy and pure-culture studies of the genus. 



Key to the Species 

 1. Wall smooth, without decorations such as spines, teeth, or ridges..— ._.. 2 



1. Wall decorated with spines, teeth, or ridges 7 



2. Poles of cells rounded 3 



2. Poles of cells narrowed, sometimes pointed 4 



3. Cells definitely arranged in a double series S. arcuatus 



3. Cells arranged in a single series ( sometimes in 



an indefinitely alternating series) S. bijuga 



4. Cells all in one plane __ 5 



4. Cells not all in one plane ___ 6 



5. Cells fusiform, all the same shape in the colony S. obliqutis 



5. Cells both fusiform and crescent-shaped in the same colony S. dimorphus 



6. Cells adjoined alternately at their poles 



only, forming a twisted chain __ _____ S. Bernardii 



6. Cells adjoined along their longitudinal walls, 



forming a cirque_ S. acuminattis 



7. Cells with longitudinal ridges only, no teeth S. acutifonnis 



7. Cells with teeth, spines, and sometimes ridges also 8 



8. Cells with teeth or spines only 9 



8. Cells with both teeth (or spines) and ridges 18 



9. Wall uniformly beset with very short teeth S, hystrix 



9. Wall not beset with uniformly distributed teeth 10 



10. Cell wall with a longitudinal row of small 



teeth, as well as teeth at the poles _____ S. serratus 



10. Cell wall without a longitudinal row of teeth 11 



11. Cells with only a single, blunt, papilla-like 



spine at the apex of each cell S. incrassatulus 



11. Cells with long, sharp teeth or spines 12 



12. Cells with 2 or 3 sharp teeth at the poles S. denticulatm 



12. Cells with long spines, not teeth alone at the poles 13 



13. Spines on only the outer cells of the series 14 



13. Spines on both the outer and inner cells of the series 16 



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