14 MOSSES AND FERNS chap. 



Hepaticse with the Mosses one is at once struck with the very- 

 much greater diversity of structure shown by the former group, 

 although the number of species is several times greater in the 

 latter. On the one hand, the Hepaticae approach the Algae, 

 the thallus of the simpler forms being but little more compli- 

 cated than that of many of the higher green Algae. On the 

 other hand, these same simpler Liverworts resemble in a most 

 striking manner the gametophyte of the Ferns. The same 

 difference is observed in the sporophyte. This in the simplest 

 Liverworts, e. g., Riccia, is very much like the spore-fruit of 

 ColeochcEte, one of the confervoid green Algae; on the other 

 hand, the sporogonium of Anthoceros shows some most 

 significant structural affinities with the lower Pteridophytes. 

 The simplest form of the gametophyte among the Hepaticae 

 is found in the thallose Jungermanniaceae and Anthocerotes. 

 In such forms as Aneura (Fig. 38) and Anthoceros (Fig. 55) 

 the thallus is made up of almost perfectly uniform chlorophyll- 

 bearing tissue, fastened to the earth by means of simple 

 rhizoids. In forms a little more advanced, e. g., Metzgeria, 

 Pallavicinia (Fig. 38), there is a definite midrib present. 

 From this stage there has been a divergence in two directions. 

 In one series, the Marchantiaceae, there has been a specialisa- 

 tion of the tissues, with a retention of the thallose form of 

 the plant. In Riccia (Figs. 1-9) we find two clearly marked 

 regions, a dorsal green tissue, with numerous air-spaces, and a 

 ventral compact colourless tissue. In the higher Marchantia- 

 ceae (Fig. 16) this is carried still further, and the air-chambers 

 often assume a definite form, and a distinct epidermis with 

 characteristic pores is formed. In the Marchantiaceae also 

 ventral scales or leaf-like lamellae are developed, and rhizoids 

 of two kinds are present. Starting again from the flat, simple 

 thallus of Aneura the^e has been developed the leafy axis of the 

 more specialised Jungermanniaceae. Between the latter and 

 the strictly thallose forms are a number of interesting inter- 

 mediate forms, like Blasia and Fossomhronia, where the first 

 indication of the two dorsal rows of leaves is met with ; and in 

 Blasia at least the rudiments of the ventral row of small leaves 

 (amphigastra) usually found in the foliose forms are present. 

 The tissues of the Liverworts are very simple, and consist 

 for the most part of but slightly modified parenchyma. Occa- 

 sionally (Preissia) thickened sclerenchyma-like fibres occur, 



