II MUSCINEAi—IIIiPATICJl—MARCIIANTIAIJlS 71 



acteristic lacunar tissue of these forms. In tlie latter resj)ect 

 Monoclca closely resembles DitJiiorficra, and as in that ^enus, 

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

 aquatic habit of the plant. Monoclca evidently Ijclonos to the 

 lower series of Marchantiacea?, and may perhaps ])e com])ared 

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



Resume of the Mar chant ialcs 



Comparing the different members of this order, one is struck 

 by the almost imperce])tiljle gradations in structure between the 

 different families, and this accounts for the dift'erence of opinion 

 as to where certain genera belong. That the Ricciace?e cannot 

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

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

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

 the other Marchantiaceas. The gradual increase in complexitv 

 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 lamellre, 

 through Ricciocarpus and Tcssalina, where definite air-cham- 

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

 the lower Marchantiese, 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 Tcssalina, with a similar absence of a stalk, 

 but the first indication of sterile cells, through the Corsiniccu, 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 Finibriaria. 



