14 MOSSES AND FERNS chap. 



difference is observed in the sporophyte. This in the simplest 

 Liverworts, e.g. Riccia, is very much Hke 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 Anthoceroteae. 

 In such forms as Amura (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 

 root -hairs. In forms a little more advanced, i.e. Metss^eria. 

 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 thalloid 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 Marchantiaceae 

 (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 root-hairs 

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

 thallus of Aneura {Riccardid), two other characteristic types are 

 met with, the peculiar spiral thallus of Riella, and the leafy 

 axis of the more specialised Jungermanniaceae. Between the 

 latter and the strictly thallose forms are a number of interesting 

 intermediate 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 (Amphigastria) 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. 

 Occasionally {PreissicL) thickened sclerenchyma - like fibres 

 occur, but these are not common. Mucilage cells of various 

 kinds are common. The secreting cells may be hairs on the 

 ventral surface, and especially developed near the apex, where 

 the mucilaginous secretion serves to protect against drying up ; 

 or they may be isolated {Marchantia) or rows of cells {Cono- 

 cephalus) within the tissue of the thallus. In the Anthoceroteae 



