1906-7.] A New Genus of Thread-Bacteria. 31 
case of Leptothrix ochracea this can be done very gradually, until the 
deposit has been removed, when it may be presumed that, owing to its dense 
membrane, the contents are untouched. When now potassium ferrocyanide 
is added, and given time to penetrate, no blue coloration can be detected. 
Molisch’s explanation certainly covers the facts of the case. He states that 
the mucilaginous layer round the cells acts like a filter, keeping back the 
iron compounds, which are later oxidised without entering into the cell. He 
further points out that the iron-bacteria presents an analogous case to 
Zygnema, the mucilaginous covering of which has been shown by Klebs to 
possess a power of attraction for aluminium, chromium, and iron compounds, 
and in consequence of the deposition of these substances, when presented to 
it, considerable swelling of Zygnema takes place. This power of attraction 
for particular inorganic or organic substances is quite common throughout 
the whole vegetable kingdom, and among other bacteria many instances may 
be mentioned, e.g. the salts of potassium are well known to have an 
attraction for bacteria, and they may be lured to their destruction by 
corrosive sublimate. We seem thus to have in the iron-bacteria a case of 
chemotaxis, with this difference, however, that the iron-bacteria, like 
Zygnema, are the attracting and not the attracted agents. We cannot call 
them iron-bacteria in any other sense than we should call Zygnema an 
aluminium or iron alga ; we cannot call them iron-bacteria in the same sense 
as we speak of the sulphur-bacteria or the nitrate-bacteria, attaching to the 
term a physiological meaning. Again, as pointed out by Beythien, Hempel, 
and Kraft (2), not only can iron be dispensed with in the cultivation of 
these bacteria, but it can be replaced by manganese, for which also the 
organism has a remarkable attraction ; and in fact, as the attraction is 
greater than that of iron, we are more justified in naming them manganese- 
bacteria. As Molisch points out, nowhere in Nature is iron necessary to a 
plant and being at the same time replaceable by manganese. Later investi- 
gations by Adler (1) show that the addition of substances which hinder the 
growth of these organisms prolongs the precipitation of ferric hydroxide, 
so that, according to this investigator, it would seem that in addition to a 
purely chemical there is also a biological factor at work in effecting the 
precipitation of the hydroxide. This is difficult to explain in face of the 
fact that these organisms can thrive very well without the presence of a 
particle of iron, and that the presence of iron has never yet been demon- 
strated inside the cell. Further, one would expect that the presence of these 
bacteria would tend to prolong the precipitation by using up the oxygen 
dissolved in the water. The only possible way out of it seems to me to be 
the supposition that in some way these organisms assimilate and thus give 
