Fink: Classification of Lichens 
151 
one tracheid to another through the broken-down bordered pits 
and thus deeper and deeper into the wood. 
The substratic relation of lichens that grow on the epidermis of 
twigs or leaves are peculiar and indicate a very limited depend- 
ence, if any, upon the substratum. This points to a pronounced 
dependence of the lichen upon the algal host. Lindau (8i) 
examined Biatorina bouteillii (Desm.) Lind, on the upper sur- 
face of fir leaves, where the plant forms a continuous layer. The 
lichen can be removed easily, since the thallus does not penetrate 
into the leaves. This agrees well with Ward’s (138) results with 
Strigula, which he found growing superficially on tropical leaves, 
not even attached by haustoria. It may well be doubted whether 
these lichens receive any nourishment from the leaves on which 
they grow. It is more probable that they depend wholly upon 
their algal hosts for food supply. Lindau (81) also studied the 
foliose Xanthoria parietina (L.) Th. Fr. on leaves of Abies 
pectinata. The lichen is closely attached to the leaves by its 
rhizoids and covers the whole surface. The rhizoids even fill 
the vestibules of the stomata, but do not enter the tissues of the 
leaves, which are killed, probably by suffocation. On the bark 
of the same tree, the rhizoids of this lichen pass through a few 
layers of the loose periderm. If this lichen can live on leaves 
without securing food from them, it and other foliose and fruti- 
cose lichens should be able to live in the same way on barks. 
Fitting (56) decided that certain lichens penetrate into the 
tissues of leaves and injure or even kill them. He thought that 
many more might dissolve the cuticle, while a few spread over 
the surface of the leaf without affecting it in any way. There is 
no real conflict in this, with the views of Ward and Lindau ; for 
Fitting regarded the alga part of the lichen, and it is the algal 
host that bores into the leaf before it is attacked by the lichen. 
The latter is thus brought into a position where it may extract 
food without itself boring into the leaf, which is often killed 
where attacked by the algal host, even before the latter is para- 
sitized by the lichen. It is also the alga that dissolves the cuticle, 
or works its way under this structure and spreads out between 
it and the epidermis. The TrentcpohUa-\\\<.t alga is often seen 
