il8 
Mycologia 
Trentepohlia (Chroolepus) umbrina, Pleurococcus vulgaris, Dac- 
tylococcus infusionum, Nostoc lichenoides (?) , Rivularia nitida, 
Polycoccus punctiformis, Gloeocapsa polyderrnatica ^nd Sirosiphon 
pulvinatus. Of these the larger number are blue-green algae, yet 
the two species of green algae, Chlorococcum humic ola and Tren- 
tepohlia umbrina, form the hosts of many more lichens than all 
the others combined. So far as the writer knows, Palmella 
botryoides is the host of only one little-known lichen, viz., Epi- 
gloea bactrospora Zuk. ; and there is better reason for including 
among lichen hosts one or more Scytonemas which are algal hosts 
of the better known Dictyonemas, also of one of Zukal’s half- 
lichens, and may sometimes be the host of Stereocaulons as well. 
Only a few lichens are known to live facultatively on different 
algae, most lichens being obligative parasites. Lecanora grana- 
tina Sommerf., Solorina crocea (L.) Ach., the Cypheliums, Pan- 
naria tryptophylla (Ach.) Mass, and probably a few others are 
facultative. The facultative species are either those which have 
never settled ujx)n a definite host, or those which are changing from 
one host to another. But such change is probably rare and so 
slow a process as scarcely to come within the range of observation. 
Lichens usually parasitize terrestrial algae which grow in habi- 
tats comrrionly invaded by lichens. This accounts for the small 
number of algal hosts of lichens. It is well known, as will be 
brought out below, that lichens frequently attack free algae and 
that parasitizing algal individuals or colonies, of species which 
have long formed lichen hosts, is going on constantly. On ac- 
count of the modification of the algal hosts, due to parasitism, 
there is still some uncertainty regarding certain species of algae 
that serve as lichen hosts ; and it would not be strange if the num- 
ber of closely related species that function as such hosts is larger 
than we now suppose. Lichens are known to parasitize approx- 
imately one per cent, of known species of blue-green algae and a 
very much smaller proportion of the numerous green algae. 
Finding the Algal Hosts Growing Near Lichens 
Schneider (m) thinks that the spores of lichens are degen- 
erate and do not often function for reproduction. We often find 
algae which, under the microscope, show plainly the presence of 
