LICHENS 3881 
The two plants of this association are of mutual help. The 
sponge structure formed by the Fungus holds water for the Alga, 
while the Alga makes carbohydrates, some of which can be used 
by the Fungus. As a result of this mutual help, the Lichen can 
live on dry barren rocks where other plants cannot exist. Neither 
Fie. 335.— A much en- 
larged section through a 
Fic. 334. — A much branched Lichen, showing the fungal 
Lichen hanging from the branch hyphae and the globular cells 
of a tree. of the Alga. 
the Alga nor the Fungus could grow in such places alone, for the 
Alga would lack moisture and the Fungus would lack food. 
Being so little dependent upon their support for moisture and 
food, the Lichens are the pioneers on bare and exposed surfaces. 
They hasten the disintegration of rock and start soil formation. 
The materials of their dead bodies added to the disintegrate rock 
form a soil for other plants. 
Lichens multiply vegetatively by small scale-like portions, 
called soredia, which separate from the main plant body. Soredia 
are small masses of hyphae in which some algal cells are en- 
tangled and are capable of growing directly into Lichens. 
The fungal member of Lichens usually reproduces by asco- 
spores and the algal member by cell division. The asci occur in 
ascocarps which appear as small cups or disk-like bodies on the 
surface of the plant body (Fig. 336). The sex organs are quite 
suggestive of the Red Algae. The antheridia occur on branching 
hyphae and are very small cells which break off and function as 
sperms. After fertilization, sterile hyphae grow up from below 
the ascogonium and form the wall of the ascocarp which finally 
