LICHENS 381 



The two plants of this association are of mutual help The 

 sponge structure formed by the Fungus holds water for the Alga 

 whUe the Alga makes carbohydrates, some of which can be used 

 by the Fungus. As a result of this mutual help, the Lichen can 

 hve on dry barren rocks where other plants cannot exist. Neither 



Fig. 335. — a much en- 

 larged section through a 

 Fig. 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 waU of the ascocarp which finally 



