Ward. — Symbiosis . ' 553 
nitrogenous medium, e. g. Bacillus ramosus , is added to the 
above-mentioned N itrosomonas and Nitrobacter , the associated 
three organisms are able to carry out all the processes and 
complete the cycle of nitrification. That is to say, B. ramosus 
breaks down the gelatine and ammonia is formed, this is then 
oxidized to nitrous acid by Nitrosomonas, and the nitrous acid 
is further oxidized to nitric acid by the Nitrobacter. 
If B. ramosus and Nitrosomonas only are sown together, 
then only nitrous acid is formed, because the latter organism 
is only capable of carrying the oxidation the one stage. 
If B> ramosus and Nitrobacter only are used, then only 
ammonia is formed, because the latter organism cannot 
oxidize ammonia. 
If we try to imagine the working of this association of 
organisms in the soil, and bear in mind the frequent co- 
existence and action of the de-nitrifying Bacteria which Gayon 
and Dupetit (32), Giltay and Aberson (33), Warington (34) 
and others have familiarized us with, a glimpse is obtained 
of the very complex symbioses which may be concerned in 
the circulation of nitrogen in Nature. Moreover, some of 
these de-nitrifying Bacilli appear to be anaerobic, and can 
only work in the surface soil if protected from the access of 
oxygen ; say, by an associated aerobic Bacterium. 
Another interesting case is the following. Perdix a few 
years ago isolated from water an anaerobic Bacterium which 
converts starches into sugars, which with the aid of a yeast 
can be fermented, the whole process going on in associa- 
tion (34 a). 
Other cases of symbiotic associations of Bacteria exist among 
the forms concerned in the reductions of sulphates (35) and 
the oxidation of sulphuretted hydrogen (36), the iron bacteria 
(37), & c. ; but I propose to mention only one or two further 
examples, taken from the true Fungi. 
Symbiotic associations of Fungi are probably common, but 
only a few cases are as yet established, and these principally 
among the ferment-Fungi (38). 
Van Laer has called attention to the symbiotic co-existence 
