in MARCHANTIE^ 69 



All of the other forms have the archegonia borne upon a 

 special receptacle, which, as the sporogonia develoi^, is raised 

 upon a stalk. Here belong, according to Schiffncr,^ sixteen 

 genera with about 150 species. The receptacle may be, as we 

 have seen, strictly dorsal in origin, or it may include the 

 growing point of the archegonial branch, or finally it may be 

 a branch system arising from the repeated dichotomy of the 

 original growing point. 



Resume of the MarcJiantiacece 



Comparing the different members of this order, one is 

 struck by the almost imperceptible gradations in structure 

 between the different families, and this accounts for the 

 difference of opinion as to where certain genera belong. 

 That the Ricciaceae 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 Corsinieae and 

 Targioniese, 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-chambers 

 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 

 development 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 



1 Schiffner (i), p. 25. 



