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 develops a papil- 



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 (periclinal) 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). 



