508 LIFE-HISTORIES 
symbionts,: and their co-operative mode of life, symbiosis.1 
Plants which grow attached to some support from which 
they derive no nutriment are termed epiphytes. Lichens 
are aérial epiphytes or ‘‘air-plants.”’ 
Several aerial forms of Chlorophycesx besides Pleurococcus, and 
also a number of Cyanophycee including species of Chrodcoccus 
and Nostoc, serve as the algal symbiont in various lichens. So little 
has their structure been modified by the symbiosis, they may almost 
always be referred to forms found living independently. The fungal 
symbionts, on the other hand, have become so changed in many 
ways, that usually much uncertainty attends the effort to find their 
Fic. 336, Il.—Beard-lichen. a, b, a group of eight gonidia among which a 
hypha is branching. c, a soredium consisting of a single gonidium 
surrounded by hyphex, viewed as if cut across. d, the same in which 
the single gonidium has multiplied into several. e, a soredium which 
contains but a single gonidium, germinating by developing below in 
contact with the bark of a tree fine hypha-branches which serve as 
organs of attachment. f, the same, older, showing the multiplication 
of the gonidia, and the upward growth of the hyphe to form a thallus. 
(Schwendener.) 
near kin among non-symbiotic fungi. They may always be classi- 
fied, however, as either Ascomycetes or Basidiomycetes, and it is 
to the former class that the great majority of lichen fungi belong. 
Ascolichenes are thus symbiotic Ascomycetes. 
186. The spore-base lichens (Class Basidiolichenes) include 
only a few tropical forms of symbiotic Basidiomycetes which may be 
represented by the mushroom-lichen (Cora pavonia, Fig. 337). 
This consists of one of the tougher mushrooms associated with a 
Chroécoccus, or with another bluish alga, and assumes quite differ- 
ent shapes according to which alga is present and according as the 
nies or the fungal symbiont predominates and so determines the 
orm. 
187. The lichen subdivision, lichens in general. Li- 
chens include about 5,000 species, none of which are of 
1 Sym-bi-ont, Sym-bi-o’sis < Gr. symbiosis, living together; < syn, 
together; bios, life. 
2 Ep’i-phyte < Gr. epi, upon. 
