i i 2 PURPLE BACTERIA 



to it in order to append the remark that, though we are not indebted to the last- 

 named worker for any revelations regarding the morphology of these organisms, 

 we have to thank him for an exhaustive study of their colouring matter, which 

 he termed bacterio-purpurin. A form similar to Chromatium Okenii was dis- 

 covered by E. WARMING (I.) on the coast of Seeland and afterwards examined 

 by Cohn, and by him entitled Monas Warmingii (Fig. 35), so that Perty's pro- 



FIG. 36. 



Spirillum volutans. 

 Mag;n. 600. (After F. Cohn.) 



FIG. 37. 



Ophidomonas sanguinea. 

 [Magn. 600. (After F. Cohn.) 



FIG. 38. 



Rhabdomonas rosea. 

 Magu. 600. (After F. Cohn.) 



position with respect to the generic name Chromatium seems not to have 

 been accepted by the Breslau bacteriologist. Whilst the species hitherto 

 mentioned differ among themselves in point of size, but not appreciably in 

 form, and all more or less correspond with the plump, short rods shown in 

 the drawing, a second group of similar species exhibits the spirillum form of 

 growth. One example of this is afforded by Spirillum rubrum (Fig. VI. 

 Plate I.), a second being the Spirillum volutans, shown in Fig. 36, and a third 

 the Ophidomonas (Fig. 37), described by Ehrenberg. A third sub-group finally 

 comprises organisms of elongated spindle form, and therefore resembling a 

 whetstone in outline ; e.g. Rhabdomonas rosea (Fig. 38) 4-5 /i wide and 

 20-30 p, long first observed and described by Cohn. These organisms are not 

 infrequently to be found in ponds and lakes ; sometimes being so abundant 

 as to colour the water red. A series of observations respecting such occurrences 

 was made by CHARLES MORREN (I.). 



92. Influence of the Individual Colours of the Spectrum. 



The necessity for a separate consideration of these red species, grouped 

 together by ENGELMANN (V.) as purple bacteria, is soon apparent when an 

 attempt is made to study their physiology. The bacteria described in the 

 previous chapter, of which Micrococcus prodigiosus may serve as an example, 

 also produce red colouring matter ; but, in their case, the latter is a mere inert 

 waste product, appearing under certain conditions, or absent under others, 

 without the growth of the cell being thereby seriously affected. Contrary to 

 this, the purple bacteria do not excrete any colouring matter into the environ- 

 ment, but the pigment occurs exclusively within the cells (Fig. 39) in the part 

 of the contents which immediately adjoins the cell-wall, and which is described 

 in chapter ii. as the parietal layer. The bacterio-purpurin is not always distri- 

 buted throughout the whole of this layer, but is frequently restricted to isolated 

 spots therein, and, in exceptional instances, is altogether absent. It is not 

 present in any definite shape, such as granules, bands, or plates, like the chloro- 

 phyll of the higher plants, but occurs diffused in the plasma. 



