6 INTRODUCTION. 



central nerve, more or less defined, which is the axis 

 of the frond (fig. i). These 

 fronds lie flat upon the ma- 

 trix, or in a few cases are 

 floating, and are attached by 

 delicate radicles proceeding 

 from the under surface, which 

 'may be quite smooth, or 

 scaly, or more or less hairy. 

 Sometimes the fronds are 

 deeply and intricately lobed, 

 and at others nearly 

 simple, or notched at the apex. Occasionally 

 solitary, but more usually radiating, overlapping, 

 and forming imbricated tufts. In rare instances 

 the upper surface of the frond is punctate with 

 minute pores, which are the analogues of stomata 

 in the higher plants. 



The foliaceous Hepatics have a thin thread-like 

 stem, which is so weak that the plants are only 

 erect when growing in dense tufts, or mixed with 

 Sphagnum or other mosses, the lower portion being 

 mostly naked or only furnished with radicles. 

 Upwards the stem is forked or branched, and the 

 branches themselves sometimes pinnate ; occasion- 

 ally it is normally simple. The stems being so 

 often prostrate the arrangement of the leaves is in 

 two rows, on opposite sides of the stem, but in- 

 serted more or less obliquely, so as to lie nearly 

 flat, in prostrate forms (fig. 2). The leaves are ex- 

 ceedingly variable in outline, seldom so simple as in 

 mosses, and without any mid-rib or nerve. In 



