LICH^NES. 



Perennial, aerial Algse, vegetating only under the influence of moist- 

 ure, which is imbibed by the whole surface, propagated by spores 

 (sporidia), and also by the cells (gonidia) of the green layer. 



Thallus (universal receptacle, Ach.) composed of three layers, viz. : 

 the cortical, the medullary, and the gonimoiis ; evolved from a hypo- 

 thallus (the elementary state in which the layers are confused, and dis- 

 cernible afterwards as cylindrical cells, and also as fibres on the under 

 side of foliaceous Lichenes, and forming the base, closely adnate to the 

 matrix, in crustaceous ones), typically horizontal or vertical. The 

 horizontal thallus is either crustaceous (often somewhat lobed at the 

 circumference or squamulose), or foliaceous (becoming sometimes in 

 degenerate states crustaceous). The vertical thallus is either com- 

 pressed {suifoUaceous), or terete {fruticulose) ; of both of which the 

 Jilamentous thallus and the pendulous thallus are degenerations. In 

 Cladonia and Stereocaulon a vertical thallus (podetium) arises from 

 the primary horizontal thallus, and is itself often besprinkled with a 

 kind of secondary horizontal thallus in the form of leaf-like scales. — 

 Lichenes are reproduced in two ways ; 1. by gonidia, the (normal- 

 ly green) cells of the green (gonimous) layer, which appear on the 

 surface as irregularly shaped powdery masses (soredia), and propa- 

 gate either on the original thallus, forming foliaceous or squamulose 

 expansions, or external to the original thallus, forming new individ- 

 uals of the parent thallus ; and 2. by sporidia, consisting of sub- 

 globose >or elliptical cells, which are either naked or contained in 

 other elongated more or less vertical cells (asci), and immersed in 

 the ihalaraium (or fructification proper), and propagate new indiyid- 



