148 
Mycologia 
only pass between the periderm cells, which they often force 
apart, but never enter. As the periderm layers are forced apart 
by the lichen hyphae and the accompanying algal filaments, they 
come to lie in irregular, isolated masses, with large areas of lichen 
and algal host tissues lying between. Both lichen and alga enter 
the bark through microscopic cracks and spread laterally in the 
periderm, the lichen preceding and the alga following and helping 
to fill the spaces produced by pushing the periderm layers apart. 
All ascomycetes found in periderm work in the same manner, and 
one can scarcely study sections of any bark of considerable age, 
without finding hyphae pushing between periderm layers, some- 
times reaching as deep as the fortieth layer. 
Cooke (39) found that cinchona bark, when it supports lux- 
uriant growths of lichens, abounds in the medicinal alkaloids 
characteristic of this tree, while these substances are largely de- 
stroyed by other fungi, which may thrive in or on the bark. 
This would indicate that lichens secure little nourishment from 
the substratum. Even if Cook’s findings are reliable for epiphloe- 
odal lichens, they can scarcely be true for hypophloeodal lichens, 
which are loosely combined with their algal hosts, some of them 
living for years in the periderm before attacking an algal host. 
It is certain that hypophloeodal lichens which live in the peri- 
derm for a year or more outside the relation with an algal host 
must take nourishment from the periderm, though they do not 
dissolve sufficient material from the periderm walls to be detected 
by microscopic examination. So we must believe that these 
plants secure nourishment from the periderm without entering 
the cells or producing appreciable diminution in walls or other 
effects that can be detected without chemical analysis. 
The rhizoids of our common Bnellia parasema (Ach.) Koerb. 
and our Rinodina sophodes (Ach.) Koerb., both epiphloeodal 
crustose lichens, penetrate through openings to a depth of five 
or six layers of periderm and spread out between these layers. 
Lindau (8r) found that such foliose lichens as Parmelia physodes 
(L.) Ach. and Pbyscia stcllaris (L.) Nyl. surround and pene- 
trate into all elevations of the periderm and also fill the depres- 
sions closely. They are often confined wholly to these surface 
elevations and depressions, but sometimes penetrate between the 
