138 
^Iycologia 
Nylander once thought that the algae were always free from 
the lichen hyphae, but later he abandoned this erroneous view. 
Schwendener (119) made the lichen a parasite and the alga a 
slave and preceded Warming in applying the term “ helotism.” 
He decided that the algae are seized, held firmly by the lichens 
and become so modified as to be identified with great difficulty if 
at all. Even such a filamentous alga as Sirosiphon parasitized by 
Polychidium muscicoliim (Sw.) Gray, and Rivularia parasitized 
by species of Lichina, he proved to have the filaments broken up 
into groups of algae scarcely recognizable as belonging to these 
genera. He found Scytonema so modified, when parasitized by 
lichens, that he could not decide certainly whether it belonged to 
the Scytonemaceae or the Rivulariaceae. Much less did he claim 
any knowledge of the genus or the species. One can distinguish 
between Rivularia and Scytonema parasitized by lichens if the 
apices are normal, but this is very rarely, if ever, the case. 
Schwendener, in some instances, reached a conclusion regarding 
the species of the algal host by finding the free algae growing 
in proximity with the parasitized forms, or was able to dis- 
tinguish the algae in very young lichens growing in groups of 
older lichens, which in turn were growing among the free algae. 
He records finding the algal host of Pannaria brunnea (Schw.) 
Mass., occurring in unrecognizable conditions in convoluted 
masses. These unroll, when boiled a moment in water or heated 
in dilute acid, into filaments of 100 or more cells, which belong to 
Polycoccus punctiformis or some closely related Nostoc. 
The constancy of the dependence of the lichen upon the algal 
host has been much magnified by those who hold to the theories 
of mutualism or individualism. Frank was first (57) to note the 
long duration of growth of certain lichens independently of the 
host. His first statement was regarding the common Arthonia 
radiata (Pers.) Ach., which he found growing on bark a year or 
more without the alga, often remaining outside the parasitic rela- 
tion until the apothecia were fully developed and the spores were 
forming in the asci. Then the algal filaments appear within the 
thalli, at first in small numbers, but soon become abundant. Again, 
Frank (58) records finding Arthonia radiata (Pers.) Ach. in its 
natural habitat, grown to large size without attacking Trente- 
