CHAPTER XIII. 



PURPLE BACTERIA AND THEIR BEHAVIOUR TOWARDS LIGHT. 



91. Their Morphology. 



WHEX in September 1826 the " Versammlung Deutscher Naturforscher und 

 Aerzte" (Association of German Naturalists and Physicians), founded by Lorenz 

 Oken in 1822, was held in Jena, Ch. Ehrenberg was among those present. In 

 the course of a stroll to the neighbouring village of Ziegenhain he observed in a 

 pool in the brook below the church a number of large red patches, about a 

 hand's-breadth across. These he found to be composed of enormous swarms of 

 a unicellular cylindrical organism provided with a single cilium and measuring 

 10-15 /* l n ky 5 M broad, which he named Monas Okenii. Later on, PERTY (I.) 

 arranged this species, along with other similar ones, into a new genus, Chroma- 



FIG. 34. FIG. 35. 



Chromatium Okenii. Monas Wariningii. 



Magn. 600. (After Cohn.) Magn. 600. (After Cohn.) 



tium, and the Monas in question then received the name Chromatium Okenii, 

 which it still retains. This organism is shown in Fig. 34. 



The main reason for considering this organism here, separately from the 

 other red bacteria previously noticed, is on account of its physiological action 

 rather than its morphological character, which action places this bacterium (and 

 many other similar species) in quite a distinct category from all other pigment 

 bacteria, and, in fact, all other bacteria whatsoever. Even in 1875 COHN (II.) 

 expressed a doubt as to whether the ciliated Chromatium Okenii and allied forms 

 could be classed as bacteria at all, since it was at that time assumed that the 

 latter organisms were not endowed with special organs of locomotion. However, 

 as explained in chapter iii., the improvements made in the methods of optical 

 and micro-chemical examination have led to this opinion being modified. 



The chief characteristic of the Chromatia and their congeners is their 

 behaviour towards light ; but before considering this more closely we will throw 

 a glance over the multitude of organisms now in question, with some of which 

 we have already had an opportunity of becoming acquainted. Thus we know 

 from 68 that Lankester studied the ciliated red monads and classified them all 

 as special forms of growth of a single species (" Bacterium rubescens "). This un- 

 proved pleomorphism we declined to recognise at the time, but must now revert 



