versely and longitudinally towards the tip to produce large numbers of coccoid ele- ments ; attached by means of a holdfast 12 Width of the sheath uniform or variable; division of cells in a transverse di- rection only 13 12. Cells within the basal portion of the sheath longer than wide; when growing in iron- bearing waters, the sheath becomes heavily impregnated with iron Crenothrix p. 212 Cells within the basal portion of the sheath much wider than long; sheaths remain colorless in iron-bearing waters Phragmidiothrix p. ^73 13. Cells within the base of the sheath 2 by 10 microns with rounded ends; divide trans- versely near the tip to produce spherical non-motile cells which are extruded either singly or in chains. The sheath becomes heavily impregnated with iron or manga- nese, becoming wide at the base and tapering towards the tip; attached by a hold- fast; false branching is common Clonothrix p. ^74 Spirally wound to straight chains up to 250 microns long; sheaths heavily encrusted with iron Leptothrix p. 26 J^ Note: The single species may be a Sphaerotilus. Not as above 14 14. Chains of cells enclosed in a sheath of uniform width; attached by means of a con- spicuous holdfast; free cells motile by means of subpolar flagella Sphaerotilus p. 263 Note: Species oi Sphaerotilus have been shown to precipitate iron in the sheath, in which state they strongly resemble species of Leptothrix. The author has shown that S. nutans will also deposit sulfur internally, though the possible relationship to Thiothrix is uncertain. Not as above ; if motile, not flagellated 15 15. Sulfur deposited internally when grown in water containing hydrogen sulfide 16 Sulfur not deposited internally 17 16. Several trichomes within a common sheath Thioploca p. 84i A single trichome within each sheath; usually attached by a holdfast Thiothrix p. 84£ 17. Colorless trichomes, attached at the base, tapering from the base to the tip; most characteristically arranged in rosettes but may occur singly. Constriction of the outer wall near the tips produces a beaded appearance. Single cells are abstricted and may exhibit a gliding motility on a solid surface. The trichomes themselves are im- mobile Leucothrix p. 850 Note: Although Harold and Stanier state that no sheath is visible, the descrip- tion of Pontothrix, which they consider identical, cites a prominent sheath. Colorless trichomes up to 500 microns in length; each cell contains one or more gas vacuoles which gleam reddish or bluish in transmitted light; enclosed in a thin trans- parent sheath; occur singly Pelonema p. 271 Note: Peloploca, which has a similar cellular morphology, although described as "no sheath evident," and which occurs in bundles, should be compared carefully with Pelonema. 18. Spiral cells 19 Not as above 23 19. Cells contain bacteriochlorophyll and carotenoid pigments; cell masses various shades of red or purple 20 Not as above 21 20. Oxidize hydrogen sulfide, depositing sulfur internally Thiospirillum p. 4-^ Do not oxidize hydrogen sulfide Rhodospirillum p. 58
