CRYPTOGAMS 



381 



occurs as felted patches of fine filaments on the bark of trees, with the hymenium 

 on the parts of the thallus which are 

 turned away from the light. 



The Lichens are everywhere widely 

 distributed, growing on the ground, on 

 rocks, and on tree-trunks. They occur 

 in localities unfavourable for every 

 other kind of vegetation, and can 

 endure the greatest heat or cold or 

 prolonged drought without injury. On 

 the recurrence of sufficient moisture 

 and the proper temperature, their vital 

 activity asserts itself anew ; they are 

 thus enabled to inhabit even the peaks 

 of the highest mountains. In conjunc- 

 tion with the Mosses, they characterise 

 by their abundant development the 

 vegetation of the polar regions, particu- 

 larly that of the so-called tundras, the 

 chief vegetation of which is represented 

 by Cladonia rangiferina. 



Officinal. — The only representative 

 (Lichen islandicus). 



Fio. 309.— Cora p. 

 B, from below ; 



onia. A, Viewed from above ; 

 ym, hymenium. (Nat. size.) 



of the Lichens is Cetraria islandica, 



II. BRYOPHYTA (MOSSES) 



The Bryophyta or Muscineae comprise two classes, tile Hepaticae 



.. , or Liverworts, and the 



\ \ Musci or Mosses. They 



are distinguished from 

 the Thallophyta by the 

 characteristic structure 

 of their sexual organs, 



ANTHERIDIA and ARCHE- 



GONIA, which are simi- 

 lar to those of the 

 Pteridophyta, the most 

 highly developed of the 

 Cryptogams. The Bryo- 

 phyta and Pteridophyta 

 are accordingly regarded 

 as having been derived 

 from a common ancestor, 

 and, in contrast to the 



Thallophyta, they are referred to collectively as Archegoniatae. 



The antheridia or male sexual organs are stalked, ellipsoidal, 



spherical, or club-shaped, with thin walls formed of one layer of cells 



Fig. 310. — Marchantia polymorpjia. A, Nearly ripe antheridium 

 in optical section ; p, paraphyses. B, Spermatozoids fixed 

 with 1 per cent perosmic acid. (^1 x 90, B x 600.) 



