152 
Mycologia 
living alone under the cuticle. Fitting found that the algal host 
could bore into some leaves and not into others, so the relation of 
both alga and lichen to the leaf would vary. Since the leaf is 
injured only where the alga attacks it and is often irritated into 
an unusual thickening of epidermis and palisade and a develop- 
ment of suberin at the points affected, it would not be reasonable 
to ascribe the results to the lichen, which may, however, have a 
part in working injury where present. 
When Parmelia olivacea (L.) Ach. grows on small limbs that 
have not lost their epidermis, the rhizoids branch centripetally 
below into a hyaline layer of branches, which completely and 
closely cover the epidermis, but remain wholly superficial. A 
form of Physcia stellaris (L.) Nyl. on the perennial leaves of 
Abies pectinata also forms a complete hyphal layer over the leaf 
surface, but the rhizoids do not penetrate into the leaf. The 
lichen is easily separated from the leaf, and the hyphal layers forms 
a complete negative of the leaf surface. On young branches of 
the same tree, with epidermis still intact, Lindau (8i) found the 
the leaf and adhered closely to the upper halves of the trichomes 
that the rhizoids reached only half way down to the surface of 
the leaf and adhered closety to the upper half of each trichome 
and to organic and inorganic particles lying between them. If 
the leaves or the young branches were killed by the lichen, or 
showed signs of injury, it would seem likely that the closely 
applied rhizoids might secure food from these surfaces, or from 
the trichomes. In the absence of such evidence, it appears prob- 
able that these lichens do not secure food from the substratum. 
Winter (142, 143) seems to have been first to study the rela- 
tion of lichens to the rocks on which they grow. He found 
lichen rhizoids penetrating into the rocks to considerable depths 
and ascertained that the rock can be dissolved with fluoric acid, 
when the whole rhizoid system appears, and the relation of the 
lichen to the algal host can be seen plainly. Egeling (47) found 
that a glass surface on which a lichen was growing became 
covered with minute cracks, in which were organic and inorganic 
particles from which the lichen could secure nourishment. Glass 
is known to be soluble in carbonic acid, and the algal hosts are 
