I02 MORPHOLOGY 



liverworts (fig. 231). They consist of dichotomously 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; (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 were said to 

 escape from cells of the thallus, but this statement has been disproved ; 

 (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. 



