CHAPTER XIII. 



PURPLE BACTERIA AND THEIR BEHAVIOUR 

 TOWARDS LIGHT. 



91. Their Morphology. 



WHEN in September 1826 the " Versammlung Deutscher Natur- 

 forscher und Aerzte" (Association of German Naturalists and 

 Physicians), founded by Lorenz Okenin 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 handsbreadth across. These he found 

 to be composed of enormous swarms of a 

 unicellular cylindrical organism provided with 

 a single cilium and measuring 10-15 P l n o 

 by 5 /A broad, which he named Monas Okenii. 

 Chro JtTum'okenii. ^ ^RTY (I.) arranged this species, along 

 Magn.6oo.(J/terCoAn.) Wlth oth . er similar es, into a new genus, 

 Chromatium, 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 oppor- 

 tunity 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"). 



