98 EVOLUTION OF PLANTS 



years have proved conclusively that the lichen thallus 

 is a compound organism, consisting usually of an asco- 

 mycetous fungus parasitic upon an alga. W.hile the 

 alga probably derives some benefit from this association 

 with the fungus, and by the shelter afforded by the 

 fungus can probably grow where otherwise it could not, 

 still the advantage is much more on the side of the 

 fungus, which without the alga is incapable of growth 

 and soon perishes. Through this peculiar form of 

 parasitism the fungi have become decidedly altered, so 

 that they differ very considerably from any other As- 

 comycetes ; but the algse are identical, even to the 

 species, with forms which live quite free from the 

 lichen. The germinating spores of a lichen produce a 

 mass of colorless filaments, like those of other fungi, 

 and if these come in contact with the proper algal form, 

 they will attach themselves, and in time the fully 

 developed lichen thallus is produced. If, however, no 

 algal cells are within reach, the mycelium soon dies 

 unless supplied artificially with carbonaceous food. 



The lichens, no doubt, represent a very specialized 

 group of plants, but they cannot properly be separated 

 from the Fungi, as they are so obviously related to 

 them, and it is the fungus element of the lichen which 

 is the predominant one. Moreover, not all the lichens 

 are related among themselves, as it is perfectly evident 

 that this peculiar form of parasitism has arisen quite 

 independently in different groups of the Ascomycetes, 

 as well as in the Basidiomycetes. The algal elements 

 found in lichens belong also to a number of widely 

 separated groups, e.g. Protococcacese, Cyanophycese, 

 Confervacese. 



