walls, which sometimes have a row of granules on either side; plants 

 often living under semi-anaerobic conditions in stagnant water. 



In examining species of Oscillatoria care must be used not to 

 confuse them with Phormidium, a genus in which there are sticky 

 but sometimes very thin sheaths, which are best seen by staining 

 with an aqueous dye or with chlor-zinc-iodide (see Drouet, 1937). 



Key to the Species 



1. Trichomes 10m or more in diameter 2 



1. Trichomes less than 10^ in diameter 8 



2. Trichomes 25-80/^ in diameter O. princeps 



2. Trichomes smaller, 22^ or less in diameter 3 



3. Cells almost colorless, with large, conspicuous 



alveoh or vacuoles _.0. Bornetii 



3. Cells definitely pigmented, without conspicuous alveoli, 



although sometimes with pseudovacuoles.... 4 



4. Trichomes constricted at the cross walls __._ 5 



4. Trichomes not constricted at the cross walls 7 



5. Trichomes slightly capitate at the apex, with a calyptra _._ O. sancta 



5. Trichomes not capitate, without a calyptra 6 



6. Trichomes not tapering at the apex; cells very 



short, less than Vs as long as broad O. ornata 



6. Trichomes tapering at the apex; cells Ys 



or more as long as broad. O. chalybeu 



7. Trichomes straight O. limosa 



7. Trichomes hooked at the apex O. curviceps 



8. Trichomes 0.6-2^ in diameter 9 



8. Trichomes (2)-2.2-10m in diameter 13 



9. Trichomes 0.6-1. 3^ in diameter 10 



9. Trichomes 1.5-2^ in diameter _ 11 



10. Trichomes 0.6ya in diameter, 1-1.8^ long. O. angustissima 



10. Trichomes 0.8-1. 3^ in diameter, 5-7^ long O. angusta 



11. Plants golden-colored, coiled, 2^ in diameter O. minima 



11. Plants blue-green or olive-green, straight, 1.5-1. 8-(2)m in diameter 12 



12. Trichomes tapering at the apex, and bent O. acutissim^a 



12. Trichomes not tapering at the apex, straight O. limnetica 



13. Trichomes with cross walls very much thickened, sometimes 



nearly equalling the length of the cell cavity.. O. articulata 



13. Trichomes without conspicuously thickened cross walls 14 



14. Plants planktonic and purplish or red, 



usuallv coloring the water O. rubescens 



14. Plants planktonic or not, but not purple 



or red, and not coloring the water 15 



15. Trichomes 2.5-5/ti in diameter 16 



15. Trichomes 5-1 0/x in diameter 26 



16. Apical cell capitate, usually with a calyptra 17 



16. Apical cell not capitate 21 



17. Trichomes slightly constricted at the cross walls O. am,oena 



17. Trichomes not constricted at the cross walls -._ 18 



18. Trichomes tapering for a long distance in the apical region O. splendida 



18. Trichomes not at all or only briefly tapering in the apical region 19 



[ 482 ] 



