94 
MORPHOLOGY 
an internal atmosphere. The superficial cells (epidermis) of the 
dorsal region, discontinuous on account of the numerous clefts, may 
or may not contain chloroplasts. The cells of the ventral region, 
against the substratum, do not contain chloroplasts, and the super- 
ficial cells often give rise to simple, hairlike rhizoids that serve as 
holdfasts. 
Antheridium. The antheridia occur in discoid areas slightly raised 
above the general surface of the thallus, each antheridium standing at 
the bottom of a deep pit formed by the overgrowth of the surrounding 
tissues (fig. 204). 
The antheridium 
develops from a 
single superficial 
cell (antheridium 
initial), and con- 
sists of a superficial 
layer (wall) of ster- 
ile cells investing a 
compact mass of 
sperm mother cells 
FIG. 204. Riccia: section through the thallus along one (fig- 2Io). 
of the dorsal grooves, showing the tissue at the bottom of the rp, ., .,. 
groove bearing antheridia (to the right) and archegonia (to the . . . 
left) ; the rhizoids are shown arising from the lower surface l! 
of the thallus. late protrusion, which 
is cut off by a trans- 
verse wall as a projecting cell. A series of transverse divisions transforms this 
projecting cell into a row of cells. Then vertical (periclinaf) walls cut off central 
cells, which by successive divisions produce the mass of sperm mother cells (figs. 
205-210). In each mother cell two sperms are formed, the oblique spindles for 
which are shown in two regions of fig. 210. 
Archegonium. The archegonia are sunk in deep pits or furrows of 
the thallus by the overgrowth of the surrounding tissues. The arche- 
gonium develops just as the antheridium as far as the projecting cell. 
This cell, however, divides by three vertical walls that surround an inner 
cell on all sides. A transverse division of this cell then completes the 
investment of a central cell. This central cell, by transverse divi- 
sions, develops the axial row, consisting of four neck canal cells, a 
ventral canal cell, and the egg. The investing sterile cells develop the 
venter and neck, the latter consisting of six vertical rows of cells (figs. 
212-219). 
