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. While 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 alge 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. Protococcacee, Cyanophycee, 
Confervacee. 
