M archantiales. 
103 
apical growth being arrested by the formation of the archegonia, so 
that further growth is only possible by means of adventitious 
branches. 
In 1 argionia (Figs. 11, 12, 13) the air-chambers, arranged in a 
Fig. 12. Targionia hypophylla. Transverse section of thallus, with an 
antheridial branch on the right. 
single layer, contain assimilating filaments, and the pore is sur¬ 
rounded by several concentric rings of cells. Each of the male 
branches, arising from the sides of the midrib, consists of a cylin¬ 
drical basal portion and an expanded terminal receptacle. The 
upper surface of the latter is studded with little conical outgrowths, 
each traversed by a canal leading down to an antheridium-con- 
taining chamber (Fig. 12). The archegonia, formed just behind the 
growing-point of the thallus, are arranged in two longitudinal rows, 
and from either side of the group there arises a scale which grows 
forward and, meeting the opposite scale, forms a complete sheath 
over the developing sporogonium, the edges of the two scales 
becoming united by interlocking processes (Figs. 11, 13). The 
sporogonium consists of a spherical capsule, separated from the 
bulbous foot by a seta which eventually elongates, rupturing the 
calyptra and pushing apart the two halves of the involucre, exposing 
the upper portion of the ripe capsule in order that the spores may 
be dispersed. The cells forming the apical part of the capsule-wall 
are larger than the rest and have short elater-like cells attached to 
them, so that in this region the wall is two-layered, and at dehiscence 
