142 ASCOLICHENES. 



starch (Lichenin), the Iceland-Lichen and Manna-Lichen (Lecanora 

 esculenta) are used as food ; the latter grows on stones, in the 

 deserts of Asia and North Africa, and is often torn loose in large 

 masses and carried away by the wind. The Reindeer-Lichen is 

 not only the principal food of the reindeer, but it is also used in 

 the manufacture of Danish brandy. Cetraria islandica (Lichen 

 islandicus) is OFFICINAL. Colouring materials (lacmus, orseille, 

 persio) are made from several species, especially from Roccella 

 tinctoria (from the rocky coasts of the Mediterranean). Parmelia 

 saxatilis and particularly Lecanora tartarea are used for colouring 

 purposes in the Northern countries. 



About 2,000 species of Lichens have been described. If we 

 disregard the Basidiolichenes, which will be considered on page 

 176, the remaining Lichens (Ascolichenes) may be divided into 

 the two following orders according to the structure of the fruit- 

 bodies : 



Order 1. Pyrenolichenes. The ascocarps (apothecia) are 

 spherical or flask-shaped, as in the Pyrenomycetes, more rarely 

 linear (Grapliis) 



According to the nature of the thallus, these Lichens may be divided into : 



a. Thallus homoiomerous, but not gelatinous, branching according to the 

 mode of growth of the Algte : Ephcbe (Fig. 133), with Algae of the genus 

 Stigonema. 



b. Thallus homoiomerous, gelatinous : Lichina. 



c. Thallus heteromerous, crustaceous : Verrucaria, Pyrenula; Graphis (Fig. 

 139), which may be considered as Hysteriacese with gonidia; several species of 

 Graphis are common on bark. 



d. Thallus heteromerous, foliaceous : Endocarpon. 

 c. Thallus heteromerous, fruticose : Sphierophorus. 



Order 2. Discolichenes. These, as in the Discomycetes, 

 have open apothecia, which, as a rule, are cnpular, more rarely 

 hemispherical (Cladonia). 



According to the nature of the thallus, these Lichens may be divided into : 



a. Thallus homoiomerous, but not gelatinous, branching according to the 

 mode of growth of the Algae : Ccenogoniiim. 



b. Thallus homoiomerous, gelatinous : Collema (Fig. 132), with Algas of the 

 genus Nostoc ; Leptogium. 



c. Thallus heteromerous, crustaceous: Pertusaria (Fig. 140), Lecidca, with 

 apothecia open from the beginning ; Lecanora, with apothecia, which in the 

 beginning are closed, later on open, but with a rim formed by the thallus 

 (Fig. 138) ; Bceomyces, whose apothecia are borne on a stem formed by the 

 thallus. 



