136 
Mycologia 
as accounting for sufficient aeration of the algal host within the 
lichen thallus. 
Jumelle (70) worked from the physiological point of view in- 
stead of the anatomical. His results show that there is an ex- 
change of gases to and from the exterior for lichen thalli and for 
the algal hosts, both in light and darkness and at low and high 
temperatures. But he failed to differentiate between the work- 
of the lichen and its host, and his results seem more uncertain 
than those of Zukal and Schneider, who worked from the anatom- 
ical point of view. Jumelle thought that, instead of the thallus 
taking part in the process as a whole, there must be special con- 
trivances for aeration. But he did not attempt to prove this, and 
the results of others seem insufficient. 
According to more recent ideas of parasitism of the lichen upon 
the alga and the ability of the latter to receive nourishment from 
organic compounds brought up from the substratum by the 
former, the alga can get along without special provision for its 
aeration, but would doubtless thrive better were aeration abun- 
dant. In fact, it does not appear that lichen evolution has made 
any special provision for aeration of the algal host as would be 
required by the mutualism hypothesis. It seems rather that the 
lichen has developed in a manner best adapted to its own ad- 
vantage, and that the alga suffers more or less from lack of suffi- 
cient aeration, unless it chances to be a species that can secure 
carbon from the substratum through the lichen quite as well as 
from the air. 
The Relation of the Lichen to Its Algal Host 
Some algal hosts, as Nostoc, Sirosiphon, Trentepohlia, Scyto- 
nema and Phyllactidiiuii, are in contact with the substratum, from 
which they take food directly; but the conditions of parasitism 
of most lichens upon the algae are such that the algal hosts become 
completely surrounded by the parasitic lichen and raised from the 
substratum. It is this peculiar condition of parasitism, in which 
the unicellular or the filamentous host is surrounded by the para- 
site, instead of the parasite being surrounded by the host, or cov- 
ering only a portion of its surface, as we find in the more usual 
conditions of parasitism, that has led to the erroneous views re- 
