50 ORDER I. PSEUDOMONADALES 



in a medium rich in calcium ions, so that identity of this species with Rhabdomonas 



calcium carbonate is precipitated as the rosea would become evident, 



alkalinity increases, has not yet been estab- Source: From a pond near Graz, Austria, 



lished but seems possible. In that case the Habitat: Fresh water. 



Genus XII. Rhodothece Molisch, 1907. 



(Die Purpurbakterien, Jena, 1907, 19.) 



Rho.do.the'ce. Gr. noun rhodum the rose; Or. noun thece box (capsule); Rhodothece the 

 rose capsule. 



Sulfur purple bacteria, occurring singly, not aggregated in families. Cells spherical, each 

 surrounded by a rather wide capsule which is, however, rarely visible without special stain- 

 ing. Motility not observed. Contain bacteriochlorophyll and carotenoid pigments, coloring 

 the cells reddish. Capable of photosynthesis in the presence of hydrogen sulfide; the cells 

 then store sulfur globules, arising as an intermediate oxidation product of the sulfide. 



In view of the experiences of Bavendamm and others that a number of representatives of 

 the sulfur purple bacteria, characterized by typical colonial aggregates when found in 

 nature, may develop as single cells in pure culture, it is quite conceivable that the genus 

 Rhodothece is identical with some other genus, e.g., Thiopedia or Lamprocystis, and that 

 these genera represent different growth forms induced by environmental conditions. 



The type species is Rhodothece pendens Molisch. 



1. Rhodothece pendens Molisch, 1907. nomena due to the pseudovacuoles and to 



(Die Purpurbakterien, Jena, 1907, 19.) the sulfur globules distort the cell shape 



pen'dens. L. part. adj. pendens hanging. under ordinary illumination so that bacteria 



Cells spherical, frequentlj^ occurring as appear as polygons rather than round cells, 



diplococci, occasionally as very short chains Usually 2 aerosomes and 2 sulfur globules 



or clumps of 3 to 5 individuals. 1.8 to 2.5 per cell. Color not observable in individual 



microns in diameter. Produce rather abun- bacteria. Cell groups are rose-red. Motility 



dant slime. Cells embedded in individual not observed. 



capsules which are rarely visible without Habitat: Mud and stagnant water con- 

 staining (India ink). Characteristic is the taining hydrogen sulfide and exposed to 

 regular occurrence of pseudovacuoles (aero- light. Not reported from sulfur springs, 

 somes) which are supposed to keep the cells Illustrations: Molisch, Die Purpurbak- 

 suspended in liquid media. Refractive phe- terien, Jena, 1907, PI. II, fig. 13-14. 



Genus XIII. Chromatium Perty, 1852. 



(Zur Kenntniss kleinster Lebensformen, Bern, 1852, 174.) 



Chro.ma'ti.um. Gr. noun chromatium color, paint. 



Cells occur singly, more or less ovoid, bean- or vibrio-shaped or short rods. The last- 

 mentioned are often thick-cj'lindrical with rounded ends. Motile by means of polar flagella. 

 Contain bacteriochlorophyll and carotenoid pigments, coloring the cells various shades of 

 red. Capable of photosynthesis in the presence of hydrogen sulfide and storing elemental 

 sulfur as an incomplete oxidation product in the form of globules inside the cells. 



At present the genus contains twelve described species. Differentiation of these species 

 has, in the past, been based almost entirely upon size and shape of individual cells, often 

 with complete disregard for the variability of these criteria. The unsatisfactory and arbi- 

 trary nature of such a classification has occasionallj^ been pointed out, and with much 

 justification. Winogradsky (Schwefelbacterien, Leipzig, 1888, 98) mentions the many transi- 

 tional stagfes that can be observed between Chromatium okenii and Chromatium weissei; 

 Strzeszewski (Bullet. Acad. Sci., Cracovie, Ser. B, 1913, 321) holds that it is impossible to 

 distinguish, on the basis of sizes or otherwise, between Chromatium weissei and Chromatium 

 minus. Such contentions, derived from observations on material from natural collections or 



