THE PLANT AS A HOST 41 



organisms. Every growth of Lichen must be 

 thought of as a colony composed of thousands of 

 individuals, in which, however, there is always 

 one predominant. The chief partner of this 

 strange alliance is a fungus, which encloses in its 

 embrace a countless number of green Algae. 



In a certain sense it may be said that the 

 fungus is parasitic upon these minute organisms, 

 but only to the extent of stimulating the Algae 

 to great activity and vigorous increase. Both 

 partners to the alliance appear to receive benefits 

 from thus living together. Although at first this 

 startling theory aroused an immense amount of 

 opposition, there is no longer any doubt as to its 

 correctness. The most convincing proof was that 

 advanced by Stahl, who after some patient 

 experiments at last succeeded in making Lichens 

 by artificial methods. He took a well-known 

 species of Algae, and on this scattered the spores 

 of a fungus. The result of this union was a Lichen 

 in the form of a well-recognised growth. Again, 

 it is possible to take any of the so-called species 

 of Lichen, and start the constituents of the 

 double organism on a separate existence. It is 

 then seen that the little cells of the Algae flourish 

 and multiply, whilst the fungoid element of the 

 union as surely starves. Now and again it is 



