ASCOLTCHENES. 



137 



tradistinction to other Fungi, need light for the development of 

 their nutritive organs, and are therefore, in any case internally, of 

 a more or less greenish colour. The form and condition of the 

 thallus is unusual among the Fungi, and they can grow upon rocks 

 and in other places where no dead organic matter, such as would 

 be required by other Fungi, is obtainable. 



Two cellular forms are therefore to be found in each Lichen: 

 1. The cells which belong to the Fungus. These are generally 

 septate, branched J/ypJice without any trace of chlorophyll. In 



Fie. 131. Transverse section through the thallus of Stictafuliginosa (x500) : r-r rhizoid- 

 strands which arise from the under side ; g-g gonidial layer ; m medullary layer; o upper, 

 it lower cortex. 



the thallus of the majority of Lichens there may be found a 

 medullary layer (Fig. 131 m) of loosely-woven hyphae, between 

 which there are large air chambers ; and an external layer (cortex) 

 (Fig. 131 o, u) formed of closely-woven hyphse without any 

 intercellular spaces. In some Lichens (Collemacea3) the hypht 

 wind about in the thallus, being equally distributed throughout, 

 without forming any decided strata. These Lichens moreover be- 



