992 COMPREHENSIVE KEY 



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 



