MU8GI. 351 



are sometimes very*sliort, bear flattened discs in which the antlieridia 

 are immersed ; the fertile or archegonial branches bear spreading 

 discs, upon the under side of which the dependent archegonia are clus- 

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

 opens by splittin;)' part way down from the top into several segments, 

 and contains two-fibred elaters mixed with tlie spores. 



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

 medicine. 



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

 been detected in North America. 



Order Jungermanmacese. — 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 nionceciously or dioeciously ; in the 

 thalloid species they occur much as in the MarchantiiicecB ; in the 

 foliose forms the antlieridia "are usually in the axils of tlie 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, Prullania, and L-jnunia 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 (^Leucolryum, Barhula, etc.) the remainder of 

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



