102 MORPHOLOGY 
liverworts (fig. 231). They consist of dichotomousty branching plates 
of similar cells, all containing chloroplasts, and with no differentiation 
of tissues into dorsal and ventral regions. 
In developing from the spore, only two stages are to be observed: (i) the fila- 
mentous stage, followed by (2) the stage of the apical cell with two cutting faces. 
Among the Marchantiales it was noted that these two stages are followed by an- 
other, which is accompanied by a distinct differentiation of tissues. The adult 
thallus of Aneura and Pellia, therefore, represents an embryonic stage of the 
thallus of Marchantiales. 
If the sporophyte of Riccia were combined with the gametophyte of 
Pellia in a single life history, the result would be the simplest hypothetical 
liverwort. Among the more highly developed Anacrogynae (Fossom- 
bronia, Blasia, Symphyogyna, etc.) the gametophyte becomes more 
complex, beginning to lobe and to develop indistinct leaves (fig. 232), 
and in some cases the body becomes distinctly leafy. These leafy forms 
are classed among the Anacrogynae, however, because the archegonia 
are dorsal and not terminal. 
The gametophytes may be multiplied vegetatively in various ways: 
(i) by the death of main axes, thus isolating branches which become 
independent individuals; (2) by gemmae, which are many-celled bodies 
formed on the surface of the thallus or cut off from the margin; in 
Aneura two-celled gemmae have been seen to escape from cells of the 
thallus ; (3) by tubers, which are special subterranean branches formed 
at the end of the growing season and which remain dormant until the 
return of favorable conditions. 
Sex organs. The sex organs occur singly or in groups on the 
dorsal surface of the thallus (figs. 231, 232) or of special branches. 
In Aneura, for example, the antheridia are sunk singly in pits or 
chambers formed by the overgrowth of the surrounding tissue (as in 
Marchantia). 
The development of the antheridium is very different from that described for 
the Marchantiales. The projecting papillate cell, derived from the antheridium 
initial, divides transversely, the lower cell by successive divisions building up a 
stalk of varying length. The outer cell, which is to form the somewhat globular 
body of the antheridium, divides by a vertical wall, and this is followed by four 
other vertical and intersecting walls, so directed as to cut off four peripheral cells, 
which inclose two central cells. The peripheral cells form the wall of the antherid- 
ium, a single layer of cells in thickness ; while the two central cells give rise to the 
sperm mother cells. The sperm has a more or less coiled body and the usual 
pair of long terminal cilia. 
