POLYMORPHISM OF MICROBES. 273 
over, these precautions often afford only a negative 
result, since some bacteria which have been reproduced 
for a long while in the same form in a given medium 
of culture, suddenly change their form and habits on 
being transferred to another medium. 
In order to give an idea of the difficulties which 
beset this branch of research, it will be enough to 
cite the history of lichens, a history well known to all 
eryptogamous botanists. The structure of these lower 
plants is at once simple and complex, since we may 
regard them as formed by the association, or symbiosis, 
as it is technically called, in each lichen of a species 
of green alga with a species of colourless fungus of the 
Ascomycetes group. 
De Bary and the botanists of his school, Schwen- 
dener, Bornet, Reess, Stahl, etc. state that in what 
is called a lichen the tissues of an alga and those of 
a fungus are intermingled in such a way as to form 
the structure which constitutes the lichen. Owing 
to this close association, a lichen can live like other 
plants, not as a parasite, like fungi: the green parts 
of the alga assimilate the carbon contained in the 
air in the form of carbonic acid, and thus supply 
nutriment to the fungus, which is consequently 
regarded as a sort of parasite to the alga. In return, 
the fungus supplies its mycelium to the lichen, by 
which the latter is enabled to fasten on the surface of 
rocks or trees. 
This attractive theory was in favour for a con- 
13 
