19. Cells granular at the cross walls .— O. Agardhii 



19. Cells not granular at the cross walls (rarely granular) 20 



20. Apical cell witli a flattened calyptra O. prolifica 



20. Apical cell with a cone-shaped calyptra O. amoena 



21. Trichomes tapering in tlie apical region.. 22 



21. Trichomes not tapering in the apical region 24 



22. Trichomes straight throughout O. Agardhii 



22. Trichomes twisted and bent, or straight in the basal portion and 



hooked or curved at the apex 23 



23. Trichomes crooked or spirally twisted at the 



apex, cross walls not granular O. terebriformis 



'23. Trichomes curved but not hooked or twisted 



at the apex; cross walls granular O. granulata 



24. Cells not granular at the cross walls — O. subbrevis 



24. Cells with distinctly granular cross walls 25 



25. Cells with rows of granules at the cross walls... ...O. tenuis 



25. Cells with 1 or 2 conspicuous granules at the cross walls O. amphibia 



26. Apical cell capitate, usually with a calyptra 27 



26. Apical cell not capitate -29 



27. Trichomes bent or hooked at the apex, forming a plant mass O. anguina 



27. Trichomes scattered in tychoplankton or euplankton, straight — 28 



28. Cells 6— 8/i in diameter, trichomes usually scattered, 



appearing red or purple when in bundles O. rubescens 



28. Cells 4-6|U in diameter, trichomes soHtary, or, 



when in bundles, appearing blue-green O. Agardhii 



29. Trichomes straight 30 



29. Trichomes spirally twisted, vmdulate, or at 



least hooked or curved at the apex 36 



30. Trichomes constricted (sometimes slightly) at the cross walls 34 



30. Trichomes not constricted at the cross walls 31 



31. Trichomes not appearing red or purple 32 



31. Trichomes appearing red or purple, coloring the water when 

 abundantly present; solitary or in bundles O. rubescens 



32. Cells 8-9.2-( 10 )m in diameter (sometimes curved at the apex) O. nigra 



32. Cells (3.5)-5-6m in diameter - 33 



33. Cells granular at the cross walls, with pseudovacuoles O. Agardhii 



33. Cells not granular at the cross walls, without pseudovacuoles ..O. subbrevis 



34. Trichomes in parallel, free-floating bundles O. lacustris 



34. Trichomes not in parallel, free-floating bundles 35 



35. Cells long, cylindrical O. Hamelii 



35. Cells shorter than wide, or about quadrate — .. O. tenuis 



36. Cells long, cylindrical O. Hamelii 



36. Cells shorter than wide, or slightly longer 



than broad, but nearly quadrate 37 



37. Trichomes 8-9-(10)m in diameter 38 



37. Trichomes less than S/i in diameter 40 



38. Trichomes crooked and much twisted in the apical region O. ornata 



38. Trichomes regularly curved at the apex ( often 



straight or only slightly bent) — 39 



39. Cells granular at the cross walls, very little or 



sometimes not at all constricted O. nigra 



39. Cells not granular at the cross walls, definitely 



constricted at the cross walls O. chalybea 



[483] 



