XII. LICHENES. 941 



A perfect Lichen usually consists of: 1. A THALLUS, or vegetative apparatus; 

 2. APOTHECIA, or organs of fructification ; 3. SPERMOGONIA, or organs of fertilization. 

 The thallus varies much in form, texture, and colour ; it never has stomata ; its 

 texture is usually dry and coriaceous, sometimes gelatinous. It is foliaceous when 

 it presents lobed laciniate peltate expansions, &c. ; fruticulose, when it assumes an 

 erect cylindric form, and branches (Roccella, Cladonia, &c.); filamentous, when its 

 ramifications are soft and prostrate (Ephebe, Evernia, Cornicularia, &c.); crustaceans, 

 when it forms, either on the surface of the soil, or on that of organic or inorganic 

 bodies which support it, a more or less friable crust (Opegrapha, Endocarpon}. It is 

 hypophleous when concealed under the epidermis or between the fibres of trees 

 (Verrucaria, Xylographa, &c.). It is grey, white, yellow, red or black, and usually 

 becomes greenish when moistened. 



Anatomically the substance of the thallus consists of 3-4 layers of different 

 elements : 1, cortical ; 2, gonidial ; 3, medullary ; 4, sometimes a lower layer whence 

 spring the root filaments, and called hypotkallus. 



1. The cortical layer is usually formed of colourless cells with thicker or thinner 

 walls ; its surface also presents a sort of amorphous crust, variously coloured, and 

 called epithallus. 



2. The gonidial layer is placed immediately below the cortical layer; the elements 

 composing it are continuous or disconnected, and appear under the form of bright 

 or olive-green granules (gonidia). The presence of these gonidia distinguishes the 

 tissue of Lichens from that of Fungi, in which they do not exist. 



3. The medullary layer presents three principal modifications : it is felted, i.e. 

 composed of closely interlaced filaments ; crustaceous, when the filaments, fewer in 

 number, are accompanied by white molecules mixed with numerous crystals of 

 oxalate of lime ; it is cellular when it is composed of rounded or angular utricles, 

 associated with the filaments. 



4. The lower layer, or hypothallus, is usually of a darker colour than the upper 

 it is also covered with rootlike hairs, which have been called rhizines. 



The thallus of some species (Collema, &c.), presents a more simple structure, 

 and appears to be reduced to two membranes separated by a mucilaginous mass, in 

 the middle of which filaments are suspended. 



The APOTHECIA are sometimes superficial and sessile or stipitate, and at others 

 buried in the tissue of the thallus. In the first case, they are discoid, scutellate or 

 patelliform (Usnea, &c.), or linear-elongated (Opegrapha), or globose (Roccella) ; in 

 the second, they form a sort of pouch or conceptacle, occupying the thickness of the 

 thallus. The apothecia are rarely of the same colour as the thallus ; they are usually 

 black or brown, or present all shades between bright yellow and red ; no blue ones 

 have been seen. Their size is very variable ; the smallest measure at most '--$ in., 

 the largest sometimes attain more than -/ T in. (Nepliroma). The apothecia are 

 composed of sporangia (thecce), which contain the spotfes. These thecse, more or less 

 pressed together, are usually accompanied by filaments thickened at the top 

 (paraphyses). The sporangia or thecsB are large oblong cylindric or ovoid vesicles, 

 with an attenuated base, and fixed to a layer of a special tissue denser than that of 



