42 MOSSES AND FERNS chap. 



once apparent, but the former are more highly developed in all 

 respects. The development of definite air-chambers in the 

 green tissue, and a continuous epidermis with the characteristic 

 pores, is common to all of them with the exception of the 

 peculiar genera- Dumortiera and Monodea, where the develop- 

 ment of the air-chambers is partially or completely suppressed. 

 The genera Ricciocarpus and Tessalina on the one hand, and 

 Corsinia and Boschia on the other, connect perfectly Riccia 

 with the Marchantiaceas as regards the structure of air-spaces 

 and epidermis, as they do in other respects. The epidermal 

 pores in the Marchantiaceas are sometimes simple pores sur- 

 rounded by more or less symmetrically arranged guard cells 

 (Fig. 1 1, D), or they are, especially upon the female receptacles, 

 of a most peculiar cylindrical form, which arises by a series of 

 transverse walls in the primary guard cells (Fig. ii, C). 

 There is a good deal of difference in the character of the air- 

 chambers in different genera. In Reboulia and Fimbriaria, 

 for instance, they resemble a good deal those of Ricciocarpus, 

 a more or less complete division of the primary chambers being 

 produced by the formation of diaphragms or laminae, which 

 give the green tissue an irregular honey-combed appearance, 

 and in .these forms there is not a sharp separation of the 

 green tissue from the ventral colourless tissue. In other 

 genera, Marchantia, Targionia (Fig. i8), Conocephalus, the 

 dorsal part of the thallus is occupied by a single layer of very 

 definite air-chambers, each opening at the surface by a single 

 central pore. Seen from the surface the boundaries of these 

 spaces form a definite network which in Conocephalus (Fig. i, 

 D) is especially conspicuous. The bottom of these chambers is 

 sharply defined by the colourless cells that lie below, and the 

 space within the chamber is filled by a mass of short, branching, 

 conferva-like filaments, which in the centre of the chamber have 

 free terminal cells, but toward the sides are attached to the 

 epidermal cells and are more or less confluent with the adjacent 

 filaments. 



As in Riccia rhizoids of two kinds are present, but the 

 thickenings to the tuberculate rhizoids (Fig. 12) are much 

 more pronounced, and these are not infrequently branched, and 

 may extend nearly across the cavity of the hair. The ventral 

 scales are not produced by the splitting of a single lamella, as 

 in Riccia, but are separate from the first and usually arranged 



