CRYPTOGAMS 



379 



thallus, which is of a light bluish green or brown colour, whitish on the under side, 

 and bears the apothecia obliquely on its margin. This Lichen is found in 

 mountainous regions in the northern part of the Northern Hemisphere, and also at 

 Cape Horn ; it has an officinal value as a demulcent. Xanthoma parietina 

 (Fig. 303) may be taken as an example of one of the commonest of the foliaceous 

 Lichens. The thallus is orange-yellow in colour, and bears numerous apothecia on 

 its central portions. Graphis scripta may be cited as a well-known example of the 



Fig. 304. — Usnea barbata. ap, Apothecium. 

 (Nat. size.) 



Fig. 305. — Cetraria islandica. ap, Apothecium. 

 (Nat. size. Officwal.) 



&J&G* 



crustaceous Lichens ; its grayish white thallus occurs on the bark of trees, par- 

 ticularly of the Beech, on whose surface the apothecia are disposed as narrow, black 

 furrows resembling writing. To the crustaceous Lichens belongs also Sphaero- 

 thallia esculenta, growing on rocks in the steppes and deserts of North Africa and 

 Asia. The thallus falls into small pieces the size of a pea ; scattered by the wind 

 they are utilised by the Tartars in the preparation of earth-bread. The North 

 European crustaceous Lichen OchrolecMa tartarea affords, like Moeeella, litmus and 

 red indigo. 



A peculiar mode of development is exhibited by the genus Cladonia, whose 

 primary thallus consists of small horizontal scales 

 attached directly to the ground, from which rises an 

 erect portion, the podetitjm, of varying form and struc- 

 ture in the different species. In some cases the podetia 

 are stalked and funnel-shaped, bearing on the margin 

 or on outgrowths from it knob -like apothecia, which 

 in 0. pyxidata are brown, in C. cocci/era (Fig. 306) 

 bright red. In other species the erect podetia are 

 slender and cylindrical, simple or forked ; in 0. rangi- 

 ferina, Reindeer Moss, which has a world-wide distri- 

 bution, particularly in the tundras of the North, the 

 podetia are finely branched (Fig. 307), and bear the 

 small brown apothecia at the ends of the branches. 

 Frequently the podetia of this species and often also of the others remain sterile, 



Fig. 306. — Cladonia cocci/era. 

 t, Scales of primary thallus. 

 (Nat. size.) 



