Fink: Classification of Lichens 
117 
the species which form the connections between lichens and other 
fungi will be at all satisfactory. 
The lichen may be defined thus : A lichen is a fungus which 
lives during all or part of its life in parasitic relation with an algal 
host and also sustains a relation with an organic or an inorganic 
substratum. It need scarcely be pointed out that this definition 
is a biological rather than a morphological one, and that we are 
treating lichens from a biological rather than a morphological 
or a taxonomic point of view. Our definition denies the mutual- 
ism hypotheses, to one of which some of us have adhered for 
many years ; nor does it meet very well the requirements of 
those who hold that the group Lichenes should be retained. 
Of course, the lichen is usually parasitic on many individuals 
of some species of alga, instead of living on a single in- 
dividual. One may speak of the alga or the algal host, meaning 
the species on which the lichen is parasitic, or one may, with 
equal propriety, speak of the algae or the algal hosts on which 
the lichen is parasitic, meaning thereby the individuals. The defi- 
nition above follows the first method. Likewise, as pointed out 
by Bonnier (31), Friederich (59), Stahlecker (125) and Zukal 
(149), what is called a lichen individual is often compound, being 
composed of a number of simple individuals united. Our defini- 
tion defines the simple individual. Again, the lichen is probably 
partly saprophytic on the alga. It does not seem necessary to 
recognize this point in a definition of the lichen, for the present 
at least. One or two of Zukal’s half-lichens are not known to 
sustain the relationship with the external substratum and if they 
do not, may be excluded from lichens on this account. 
The Algal Hosts of Lichens 
For some of us, it would not be necessary to consider the nature 
of lichens further ; but for those who still cling to the dual hypo- 
thesis, the last word will not be said until the relation of the 
lichen to its algal host has been thoroughly reviewed. A con- 
siderable number of algae have been enumerated as lichen hosts, 
but most of these rarely or never function as hosts for lichens. 
Fiinfstuck (64) gives ten algae known as lichen hosts. These are 
Chlorococcum (Cystococcus) humicola, Palmella botryoides, 
