THAIXOGENJS. 717 



Artificial Analysis of the Natural Orders in the Sub-class 

 AcnoGKSM. 



(Tbe Numbers refer to the Orders.) 



1. "With a distinct axis or stem. 



A. Leaf y plants. 



a. Sporangia on the back or margin of the fronds 



or leaves, or on metamorphosed leaves . Filices. 286. 



b. Six)rangia arranged in or near the axils of 



leaves or bracts, or immersed in their sub- 

 stance. 



1. Not enclosed in sporocarps. 



Sporangia sessUe, without a calyptra . Lycopodiaceae. 289. 

 Sporangia sessile, with a calyptra . . Musci. 290. 

 Sporangia stalked, with a calyptra . . Musci. 290. 



2. Enclosed in sporocarps. 

 Vernation circinate. 



Spores not mixed with elaters . . Marsileacece. 288. 

 Vernation not circinate. 

 Spores mixed with elaters . . . Hepaticacece. 291. 



B. Leafless plants. 



Stem simple, or with whorled branches. 



Fructification terminal, in club-shaped or 



cone-like masses Equisetaceoe. 287. 



Stem always branched in a whorled manner. 



Fructification at the base of the branches Characece. 292. 



2. With no distinct stem or axis. 



No true leaves, but forming a green thalloid 

 expansion Eepatkaceos. 291. 



Sub-class II. Thallogencs. 



Natural Order 293. Lichenes. — The Lichen Order. — 

 Character. — Perennial plants, composed of parenchymatous 

 cells, arranged so as to form a foliaceous {fig. 822), somewhat 

 woody, scaly, crustaceous, or leprous thallus ; living and fructify- 

 ing in the air, and growing on the bark of trees, or on old palings, 

 walls, &c., or on stones, or on the exposed surface of rocks ; usu- 

 ally epiph}-tic, but sometimes parasitic, and commonly presenting 

 a dry, shrivelled, more or less lifeless appearance. Eeproductio7i 

 either vegetative hjgonidia (see page 377) ; or hj trne fructifica- 

 tion, consisting of, 1. apothccia, which are sessile or stalked, 

 and generally of a rouncled {fig. 822) or linear form {fig. 821), 

 and composed of asci or theccs {figs. 823 and 82-t), enclosing 

 4, 8, 16, or numerous spores; 2. oi spermagonia contaAmng sper- 

 matia {figs. 825 and 826) ; and 3. of very rarely, ^yc;?i"c?irt enclo- 

 sing styJospores. (For detailed account of the fructification of 

 lichens, see pages 374 — 377.) 



Distribution, <^'c. — Lichens are distributed in all parts of the 

 world. The pulverulent species " are the first plants that 

 clothe the bare rocks of newly formed islands in the midst of the 

 ocean, foliaceous lichens follow these, and then Mosses and 

 Liverworts." They also form a considerable proportion of the 



