POLYMORPHISM OF MICROBES. 273 



over, these precautions often afford only a negative 

 i-esult, 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 branqji of research, it will be enough to 

 cite the history of lichens, a history well known to all 

 cryptogamous 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 



