22 



MOSSES AND FERNS chap. 



always sessile ; sporogonium with a short stalk, producing 

 besides the spores sterile cells, which may have the form of 



very simple elaters. 



Fam. 3. MarcJiantiece 



Air-chambers (with exception of Dtimo7'tierd) highly 

 developed, and the chambers in most cases containing a loose 

 filamentous assimilative tissue. Pores upon the dorsal surface 

 always present (except in Duviortiera) and highly developed, 

 ring-shaped or cylindrical. Sexual organs always in groups, 

 usually upon special long-stalked receptacles. Sporogonium 

 stalked and when ripe breaking through the calyptra, opening 

 by teeth or a circular cleft, more seldom by four or eight valves. 

 The archesporium develops sterile cells, usually in the form of 

 elaters, as well as spores. 



Tlie MarchantiacecE 



The Marchantiaceai constitute a very natural order of plants, 

 all of whose members agree very closely in their fundamental 

 structure. The separation of the Ricciacese as a group co- 

 ordinate with the Jungermanniaceae and Anthocerotese is not 

 warranted, as more recent investigations, especially those of 

 Leitgeb,^ have shown that the two groups of the Marchantiaceae 

 and Ricciacese merge almost insensibly into each other. 



They are all of them strictly thallose forms, the thallus being 

 unusually thick and fleshy, and range in size from a few 

 millimetres in some of the smaller species of Riccia, to 10 to 20 

 centimetres in some of the larger species of Duinortiera and 

 ConocepJialus. In most of them branching is prevailingly 

 dichotomous, and as this is rapidly repeated, it often causes the 

 thallus to assume an orbicular outline. Some forms, however, 

 e.g. Targionia (Fig. i, E), fork comparatively seldom, and the 

 new branches are for the most part lateral. The thallus is 

 fastened to the substratum by rhizoids, which are unicellular 

 and usually of two kinds, those with smooth walls and those 

 with peculiar papillate thickenings or teeth that project inward 

 (Fig. 11). The cells of the lower layers of tissue are usually 

 nearly or quite destitute of chloroplasts, which, however, occur 



^ Leitgeb (7), vol. iv. 



