26 
Proceedings of the Royal Society of Edinburgh* [Sess. 
seen in the youngest stages, when no trace of iron deposition is observable 
on the organism and before twisting has begun (figs. 11, 21). They are 
found in individuals on which a very slight deposit is observable and in 
which twisting is already well advanced, and, judging from the huge 
numbers that are often found attached to the mother organism (figs. 26, 29), 
they must be produced in large numbers even when a thick deposit has 
been laid down. In fact I have traced the various stages of conidia 
formation in Leptothrix (as will be explained in a later publication), from 
which it was clear that the conidia arise even when a thick deposit of iron 
is present, and they seem to be able to make their way through it without 
difficulty. It thus is evident that conidia formation takes place in many 
cases throughout the whole life of the individual ; and often individuals are 
seen so loaded with conidia — for most still cling to the parent organism — 
that the original outline of the latter is completely lost, and an appearance 
similar to fig. 26, which represents a part of an individual, is seen. Thus 
hundreds, even thousands, of conidia may be attached to an individual. The 
deposit on the bed of the stream into which the iron-water runs consists 
chiefly of organisms in this condition, and the cause of the sinking of the 
organisms to the bottom of the water is evidently due to the increase of 
weight caused by a further deposit of iron. It is obvious that if the conidia 
remain attached, the surface upon which iron can be deposited is very much 
increased thereby, and the density very soon becomes greater than that of 
water. The further development of the conidium consists in an increase 
in size of the protrusions from the cell, until it becomes a well-defined 
protuberance (figs. 21, 26, 27, 29). By examining an individual in which 
the deposit of iron is very small, and on which the number of conidia is 
comparatively few, all stages in conidia formation can be readily observed. 
When the protuberance has attained the normal size, a constriction is 
formed at its base, and the newly formed conidium, as it can now be called, 
is physiologically independent of the parent plant. The process is repeated 
until the conidia are present in large numbers. Every portion of the 
individual can take part in this process, and in fact in many cases it may 
be safely stated that every square ju of the organism has upon it a conidium 
either mature or in process of development. The increase of thickness 
caused by conidia formation is diagrammatically represented in fig. 29. 
The reason why the presence of the iron deposit presents no difficulty is 
most probably due to the fact that the cell is surrounded as in other bacteria 
by a coating of mucilage, and that the iron is held by this coating so that 
the iron deposit cannot be very dense and closely compacted. Whatever 
the explanation, there is no doubt of the capacity of the conidia to be 
