552 Ward. — Symbiosis. 
their tissues, and respire oxygen, as is well known. The pre- 
sumption is that the anaerob’lc conditions set up in the ex- 
periment described favour certain forms of soil Bacteria, such 
as Van Senus worked with, and enable them to co-operate in 
the destruction of the cell walls. 
An even more remarkableexample is given by Winogradsky, 
who found that the anaerobic Bacterium known as Clostridium 
P asteurianum is able, if supplied with abundance of dextrose 
and protected from the access of oxygen, to fix atmospheric 
nitrogen (26). In the cultures, and presumably in the soil, 
the Clostridium was found to work when protected by a 
mantle of aerobic bacteria. In fact, the nitrogen-fixing 
Clostridium was working in the meshes of the oxygen- 
consuming species, and forming gelatinous flocks like the well- 
known grains of kephir, or of ginger-beer plant. 
Yet another striking instance of symbiotic association has 
recently been given by Omeliansky (27). In experiments on 
nitrification at Bonn (28), the assertion had been made that 
the nitrifying organisms, i. e. the Bacteria known to oxidize 
ammonia to nitrous acid, and nitrous acid to nitric acid, could 
be grown and made to do their specific work in media 
containing proteids or other organic nitrogenous bodies. Now 
this was directly contradictory of the experience of Warington 
(29), Winogradsky (30), and other workers, who had found 
that one great peculiarity of these nitrifying organisms is that 
they refuse to grow on such media ; they are incapable of 
using organic nitrogen. Several workers (31) then showed 
that the Bonn observers had inadvertently employed mixtures 
of two or more species, and Omeliansky undertook a critical 
re-investigation of the whole subject, and has put forward the 
following explanation of the tangle. 
If N itrosomonas — the Bacterium which oxidizes ammonia to 
nitrous acid — and Nitrobacter— the Bacterium which further 
oxidizes nitrous to nitric acid — are sown together or separately 
on a medium containing organic nitrogen, no growth or change 
occurs. 
But if a Bacterium capable of decomposing the organic 
