LICHENES. 761 
Peronospora (Phytophthora) infestans is the fungus which causes the 
potato disease. 
Polyporus.—P. destructor is one of the Fungi found in the Dry Rot of 
wood. (See Merulius.) Thin slices of P. igniarius and P. fomentarius, 
when softened by beating with a mallet, are sometimes employed externally 
to restrain hemorrhage. Similarly prepared slices soaked in a solution of 
nitre, and dried, constitute Amadou or German Tinder. When impregnated 
also with gunpowder, they form Black Amadou. Amadou has been sometimes 
used to give support and pressure in certain surgical affections, and as a 
moxa.—P, squamosus and P. betulinus, when pressed, sliced, and prepared 
by rubbing with pumice, &c., are used to make razor strops. <P. officinalis, 
Larch or White Agaric, has been employed externally as an astringent; and 
internally, to check perspiration, and as an emetic, cathartic, &e. It was 
formerly employed as an anthelmintic, but its action is frequently violent. 
Larch Agaric is now imported from the northern part of Russia, where 
it grows on the stems of Larix sibirica—P. anthelminticus, a native 
of Tavoy in the Tenasserim provinces of Burmah, is known as Shan-mo 
(Worm Mushroom), being there highly esteemed as an anthelmintic.—P. 
( Boleius) Laricis canadens's, Canadian Agaric, is reputed tobe a valuable 
remedy in acute rhoumatism.—A species of Polyporus, believed by Berkeley 
tobe P. Pini canadensis, Schweimtz, a native of Canada, is said to bea 
tonic bitter, and is recommended as an application to wounds. 
Puceinia graminis is the fungus which produces the Mildew of Wheat. 
Saccharomyces (Torula). —The so-called Yeast plant is a mycelial form 
of S. cerevisiw ; and the so-called Vinegar plant is also a more developed 
form of the mycelium ot the same fungus. The ferment obtained in brewing 
beer is produced by Saccharomyces cerevisiw; it is official in the British 
Pharmacopeia. 
Tuber, the Truffle-——The species of Truffle, several of which oceur in 
Britain, are subterranean. ‘They are highly esteemed as seasoning or 
flavouring agents. The best are imported from France, Algeria, and Italy ; 
they are commonly preserved in oil. J. exstivum, T. cibarium, and T.. 
melanosporum are the more frequently used species. 
Order 2. Licuenes, the Lichen Order.—Character.— 
Perennial plants, composed of hyphal tissue resembling that of 
Fungi, but its constituent cells are firm and dry, and enclose 
the cells known as gonidia (figs. 853, gon, and 855, gon), which 
contain chlorophyll, and are now frequently regarded as minute 
Algze, upon which an Ascomycetous Fungus is parasitic. (See 
page 388.) The whole is arranged so as to form a foliaceous, 
somewhat woody, scaly, crustaceous, or leprous thallus ( figs. 851 
and 852) ; living and fructifying in the air, and growing on the 
bark of trees, or on old palings, walls, &e., or on stones, or on 
the exposed surface of rocks; usually epiphytic, but sometimes 
parasitic, and commonly presenting a dry, shrivelled, more or 
less lifeless appearance. Reproduction either vegetative by 
means of soredia (see page 390); or by true fructification, con- 
sisting of, (1) apothecia, which are sessile or stalked, and gene- 
rally of a rounded (fiy. 852, ap) or linear form (fig. 851), and 
composed of asci or thecx ( Jig. 853, as), enclosing 4, 8, or 16 
spores ; (2) of spermogonia containing spermatia (figs. 852, sp, 
and 854, sp); and (3) of, very rarely, pycnidia enclosing stylu- 
spores. (For detailed account of the fructification of Lichens, 
see pages 388-390.) 
