OF ARTS AND SCIENCES. 195 



Mr. Tuckerman commimicated the following arrangement 

 and description of the Lichenes of the northern portion of 

 North America, viz. : — 



A Synopsis of the Lichenes of the Northern United States and British 

 America* by Edward Tuckerman. 



LICHENES. 



Perennial, aerial Alga), 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 gonimous ; 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), ov foliaceous (becoming sometimes in 

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

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

 filainentous 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 ; L 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 



* This enumeration, originally prepared for Dr. Gray's Manual of the Botany 

 of the Northern States, has been enlarged by the addition of many species from 

 Arctic America, and from the Pacific coast, and is now published in the hope that 

 it may open the way to a more complete and satisfactory account hereafter. The 

 system is that of Fries, as presented in his Lichenographia Europma Reformata, 

 ■with some emendations derived from his later works. The characters of the sec- 

 tions and genera in the Lichenographia have been throughout the basis of those 

 here given, and in part are adopted entire. In the citation of authorities for 

 specific names, the common usage has been followed ; but the writer has else- 

 where adopted what appears the preferable one (Enum. Lick. JV. ^mer. 1845), 

 where will also be found some account of the Friesian System. 



