30 GUIDE TO THE STUDY OF LICHENS. 



The many generations of a life of dependent luxury 

 had greatly crippled its energies; functionally it be- 

 came more and more weakened, until finally it could 

 not even digest a sufficient quantity of the food already 

 prepared, or, in other words, it no longer possessed suf- 

 ficient vitality to feed itself. In order to regain its 

 lost vitality it must again appropriate a larger amount 

 and better quality of food ; inorganic food was not 

 available, because it had lost its chlorophyll. *If the 

 fungus could, however, prevail upon some chlorophyll- 

 bearing organism to prepare inorganic food-substances 

 for its assimilation the problem would be solved ; that 

 is, it must press into its service some plant having 

 chlorophyll. As already indicated, ordinary parasitism 

 with higher plants did not suffice: these plants were 

 not sufficiently adapted to its needs. The fungus was 

 therefore obliged to cast about for different plants, 

 such as were specially suited to its physiological re- 

 quirements. Splendid opportunities were offered in 

 the numerous single-celled algae (Protococcus vulgaris), 

 known as " green mould," which everywhere covered 

 trees, rocks, etc. The fungus made the attempt and 

 entered into a biological relationship with this alga. 

 It is highly probable that this first relationship was of 

 a parasitic nature but the fungus soon found that these 

 minute organisms were scarcely able to supply the 

 extra food materials demanded by the attaching para- 

 site ; the climatic conditions were not sufficiently favor- 

 able, and, furthermore, the alga itself showed a slight 

 preference toward leading a parasitic life, so that the 

 fungus was considered as a very undesirable intruder. 



