512 THE LICHEN FAMILY. 



The main body or substance of a lichen is termed the " thallus." 

 It corresponds at once to the root, the stem, and the foliage of a 

 flowering-plant, all three parts being here fused into one. In colour 

 it is usually gi'ay, brown, whitish, or yellow. Green scarcely ever 

 occurs, a circumstance furnishing a ready means of distinguishing 

 lichens ffom mosses, with which they are frequently confounded, the 

 latter being almost invariably green, as well as provided with distinct 

 leaves. The thallus appeal's to be universally perennial, and nothing 

 is more remarkable in the family than the slow growth and the 

 longevity of individuals. In their most simple form. Lichens often 

 appear as a mere powdery or scurfy coating, various in colour, and 

 resembling white, green, or yellow washes. These abnormal or 

 undeveloped states, which are exceedingly common, were until lately 

 believed to be genuine species, and collected into genera called Isidium, 

 Leprdria, and Varioldria. In its perfect or mature condition, the 

 thallus is of two principal kinds, horizontal and simple, and vertical 

 (or pendulous) and branched, each of these again presenting several 

 modifications, viz. : — 



A. — Thallus horizontal. 



1. Crustaceous. 



Tartareous, or thick, dense, and hard, growing chiefly on weather-beaten 



rocks. 

 Leprose, forming scaly expansions, and found on rocks and trees. 

 Pulverulent, consisting of a mere powdery scurf, and growing chiefly on 



wood and bark. 



2. FoLiACEOtis. These comprehend the flattened and leaf-hke species, often 



spreading in wide patches, and brightly coloured, and growing chiefly 

 upon the bark of trees or on the ground. They are either — 

 Membranous, 

 Leathery, or 

 Cartilaginous. 



B. — Thallus vertical. 

 In this division come the tliread and hair-like licliens, and those which 

 resemble little shrubs. They are either — 



3. Shrubby, or 



4. Filamentous. 



The fructification commonly resembles minute saucers, for the 

 most part sessile on the surface of the thallus, and tcchnicall)'- called 

 " apothecia." They are generally circular and open, the spores lying 

 in the interior like the fruit in an open tartlet. Some species produce 

 their apothecia regularly every season, and in great abundance ; others 



