MU80I. 351 



are sometimes very sliorl, bear flattened discs in which the antlieridia 

 are immersed ; the fertile (ir arcliegonial branches bear spreading 

 discs, upon the under side of whidi the dependent arcUegnnia are clus- 

 tered. The ripe sporogonium (capsule) is enclosed in a perianth ; it 

 opens by splitting part way down fnim the top into several segments, 

 and contains two-fibred elaters mixed with the spores. 



Marehantia polymorpha, a common species, is used by quacks as a 

 medicine. 



Miirchantia occurs in the Tertiary (Eocene) of Europe, but has not 

 been detected in North America. 



Order Jungertuanniacese. — Plants composed of a thallus, a thalloid 

 stem, or a stem with two or three rows of leaves ; when there are three 

 rows the third row is on the under side (constituting the amphigastria). 

 The sexual organs are distributed monoeciously or dioeciously ; in the 

 thalloid species they occur much as in the Marchantiacem ; in tlie 

 foliose forms the antlieridia "are usually in the axils of the leaves, 

 either singly or in groups," and the archegonia are most frequently 

 clustered upon the summits of the shoots, and are generally concealed 

 by the leaves. The ripe sporogonium (capsule), which is usually long 

 stalked, opens by splitting into four parts from the apex to the base ; 

 it contains one- or two-fibred elaters mixed with the spores. Many 

 species are common on rocks and the bark of trees. 



The modern genera Jungermannia, PruUania, and L'jeunia were 

 represented in the Tertiary (Miocene). 



§ II. Class Musci. 



457. — The adult plant-body in this class, which includes, 

 besides the Sphagnums, all the true Mosses, is always a leafy 

 stem, which is rarely bilateral. It is fixed to the soil or other 

 substratum by means of articulated root-hairs, or rhizoids, 

 which grow out from the sides of the stem. The leaves are 

 sessile, usually composed of a single layer of cells, and either 

 nerveless, or traversed longitudinally by a single rib, rarely 

 by two ; they are arranged in two or three straight or spiral 

 rows^ and are usually inserted more or less obliquely to the 

 stem. 



458. — The tissues of the Mosses present a considerable 

 advance upon those of the Liverworts. In the stem there is 

 usually a considerable thickening of the outer layer, or layers, 

 of cells, constituting a kind of imperfect sclerenchyma. In 

 some cases {Leucobryum, Barlula, etc.) the remainder of 

 the stem is composed of thin-walled tissue (parenchyma), 



