2 INTRODUCTION. 



sometimes frondose that is to say, the stem and 

 leaves are combined in a flat prostrate frond, which 

 lies flat upon the matrix. The foliaceous Hepaticae 

 moreover have their leaves of a thinner and more 

 delicate substance, and are destitute of a distinct 

 central midrib. Finally, the Hepaticse usually 

 contain within the fruit capsule, intermixed with 

 the spores, elongated spiral fibres, called elaters, 

 which are absent from true Mosses. These are the 

 most evident general distinctions which are relied 

 upon in the discrimination of the two groups. 



The entire group of Hepaticae are subdivided 

 into four natural orders, which have each their 

 peculiar character. These are the Marchantiacea, 

 which are frondose, and have the sporangia disposed 

 on the under side of stalked receptacles, the spores 

 being mixed with elaters ; this is accepted generally 

 as the highest order of Liverworts. Then follow 

 the Jungermanniacea, in which the fruit is pedun- 

 culate and solitary, the sporangium when mature 

 splitting into four equal valves, and the spores 

 mixed with elaters. After these are the Ricciacece, 

 the species of which are frondose, with the sporangia 

 sunk into the substance of the frond, or borne upon 

 the surface, and the spores unmixed with elaters. 

 Finally the Anthocerotacece, in which, although the 

 vegetation is frondose, the sporangium is exserted 

 and erect, splitting longitudinally on one or both 

 sides,and exhibiting a thread-like central columella. 

 The elaters, when present, elementary and imperfect. 

 Of all these the most abundant in species is the 

 Jungermanniacece> which includes far more than the 



