156 
]\Iycologia 
4. The fundamental problem concerns the nature of lichens, 
and this must be settled before we can hope to agree regarding 
the classification of these plants. 
5. Due to peculiar ideas about the relation of the lichen to its 
algal host, this problem of relationship has become the main part 
of the consideration of the nature of the lichen. It is therefore 
treated at length in this paper. 
6. Recent researches prove that all hypotheses of mutualism be- 
tween the lichen and the symbiotic alga are erroneous, and that 
the lichen is a fungus pure and simple. 
7. The following are the main arguments against mutualism. 
Lichens commonly grow where there are free algae of the same 
species as those parasitized by these lichens. The spores of the 
lichens germinate and attack the free algae as other fungi attack 
their hosts. Lichens perform like other fungi on culture media 
and may be made to produce their reproductive organs on these 
media. Their development on such media does not differ from 
that reached when growing with their algal hosts more than other 
fungi vary from their usual appearance when grown on culture 
media. Lichen spores also attack the algal hosts, when the 
spores and the algae are introduced into cultures together ; and 
the resulting lichen is normal and sometimes fructifies in the cul- 
tures. Algal hosts extracted from lichen thalli grow in cultures 
like free algae of the same species grow on similar culture media. 
Some lichens live for years in their substrata outside the rela- 
tion with their algal hosts. The researches of Elenkin and Dan- 
ilov prove that lichen hyphae absorb food from the algal host 
cells, which are killed by severe parasitism or more probably by 
parasitism and saprophytism combined. The relation of the 
lichen to its substratum proves that higher lichens can take com- 
paratively little food from it and must depend more than lower 
lichens upon the algal hosts ; and this shows that the parasitism 
of the lichen upon the algal host has become more severe in the 
evolution of higher lichens. Finally, the algae parasitized by 
lichens are in a disadvantageous position with reference to carbon 
assimilation. 
8. The following are the main arguments for the fungal nature 
of lichens. Lichens are like other fungi with respect to vegetative 
