II MUSCINE^— HEPATIC^— MARCHANTIALES n 



acteristic lacunar tissue of these forms. In the latter respect 

 Monoclea closely resembles Dumortiera, and as in that genus, 

 the absence of the air-chambers may be attributed to the semi- 

 aquatic habit of the plant. Monoclea evidently belongs to the 

 lower series of Marchantiaceae, and may perhaps be compared 

 to Targionia. See Ruge (i). Cavers (7), Campbell (19). 



Resume of the Marchantiales 



Comparing the different members of this order, one is struck 

 by the almost imperceptible gradations in structure between the 

 dififerent families, and this accounts for the difference of opinion 

 as to where certain genera belong. That the Ricciacese cannot 

 be looked upon as a distinct order is plain, and they may perhaps 

 be best regarded as simply a family co-ordinate with the Cor- 

 sinieae and Targionieae, and not a special group opposed to all 

 the other Marchantiaceae. The gradual increase in complexity 

 of structure is evident in all directions. First the thallus passes 

 by all gradations from Riccia — with its poorly defined air- 

 chambers with no true pores and single ventral lamellae, 

 through Ricciocarpus and Tessalina, where definite air-cham- 

 bers are present, opening by pores of the same form as those of 

 the lower Marchantieae, and separate ventral scales occur — to 

 forms like Marchantia, where the air-chambers are very definite 

 and contain a special assimilating tissue, and the pores are of 

 the cylindrical type. With this differentiation of the thallus 

 is connected the segregation of the sexual organs and the devel- 

 opment of special receptacles upon which they are borne. 

 Finally, in the development of the sporogonium, while there is 

 almost absolute uniformity in the earlier stages, we find a 

 complete series of forms, beginning with Riccia, where no stalk 

 is developed and all the cells of the archesporium develop spores, 

 ascending through Tessalina, with a similar absence of a stalk, 

 but the first indication of sterile cells, through the CorsiniecE, to 

 forms with a massive foot and elaters fully developed. It 

 may be said, however, that there is no absolute parallelism be- 

 tween the development of the gametophyte and that of the 

 sporophyte; for in Marchantia, the most specialised genus as 

 to the gametophyte, the sporogonium is less developed than in 

 the otherwise simpler Targionia and Fimbriaria. 



