160 OUTLINES OF BACTEEIOLOGY 



stage in the development the thickened edge is developed, and move- 

 ment stops. It is probable that multiplication by cell-division also 

 occurs, but so far the process has not been observed. 



Not only do the individuals twist round themselves, but also round 

 other individuals of the same kind (Fig. 104). The twisting is probably 

 due to contact irritability.^ 



§ 5, THE PHYSIOLOGY OF THE IRON-BACTERIA. 



We are not yet in possession of a completely satisfactory explanation 

 of this remarkable deposition of iron on the membranes of these 

 bacteria. Cohn compared the deposition in Crenothrix polyspora 

 to the deposition of silicon in diatoms. Zopf, who examined the 

 same organism, declared that the deposition was purely mechanical, 

 the iron being retained in the mucilaginous layer surrounding the 

 cell in the same way that the coloured matter is retained by the 

 gelatine in certain coloured jellies. Later, Winogradsky came to a 

 quite different conclusion, maintaining that the bacteria derived some 

 benefit, in the shape of an acquisition of energy, by the oxidation 

 of the bicarbonate FeH2(C03)2 to the hydroxide Fe2(0H)g. These 

 statements, however, were not supported by his researches on this 

 matter, and Molisch's still later experiments show that a different 

 explanation is more in accordance with the facts. This experimenter 

 showed that these bacteria could be cultivated in solutions which did 

 not contain a particle of iron. It was also demonstrated that the 

 iron could be replaced by manganese with the same result, all the 

 manganese being attracted to the membranes of the bacteria, just 

 as the iron was. Further it is known that certain algae, e.g. Zygnema, 

 have a power of attraction for certain metallic salts, e.g. compounds 

 of chromium, aluminium, and iron, with the result that the alga 

 becomes considerably swollen in appearance. 



These experiments point to conclusions different from those come 

 to by Winogradsky. They indicate that the accumulation of ferric 

 hydroxide on the surface of these organisms is due rather to a 

 purely chemiotactic power of attraction, than to a process vitally 

 connected with the activities of the organisms, for on the one hand 



' The author has lately published a preliminary notice of five new species of 

 iron-bacteria, all of which were found in various samples of iron- water collected 

 from different parts of Great Britain. See Proc. Roy. Soc. Edinburgh, 

 Vol. XXVIII. Part V. No. 19. 



