LICHENS. 103 
frond, and frequently flourishes upon constantly 
dripping rocks within the spray of waterfalls, 
a peculiarity contrary to the habit of lichens in 
general. The round fructification, deeply imbed- 
ded in the thallus, is similar to that of the Riccias 
or crystal-worts. This genus of lichens may 
therefore be fairly regarded as a connecting 
link with the scale-mosses. Indeed Fee placed 
it apart from the true lichens, in a section which 
he called Pseudo-Hepatice. With the fungi 
the lichens are most closely allied; the principal 
difference between many species on both sides 
being that the lichens possess a crust or thallus, 
while the fungi are destitute of it, and that the 
lichens grow on inorganic substances, and living 
structures, while fungi require dead and decay- 
ing organic substances as a matrix. There are 
other differences of course between the two orders, 
but they are of a microscopic character. Super- 
ficially the mutual resemblance is often perfect ; 
and several Pezizas and Spherias among the 
fungi have been constantly mistaken for lichens. 
To the alge or sea-weed tribe the lichens are 
related on the one side by the Lzchinas, which 
form blackish green cartilaginous fronds on rocks 
on the sea-shore, which are exposed and almost 
dry at low water, and which have received their 
name from their similarity to some of the lichen 
