412 A TEXTBOOK OF THEORETICAL BOTANY 



meanwhile divides and grows while the surrounding cells shrink away, so 

 that the antheridia lie in a cavity and are only exposed, when mature, by the 

 breaking open of the roof. The structure of the individual antheridium is 

 closely similar to that in other Liverworts, but this endogenous formation 

 is unique and has led to some controversy as to whether the male organs 

 in this plant are really homologous with antheridia elsewhere. The general 

 opinion is in favour of their homology and regards their present position as 

 derived from a superficial position in former times. 

 The antherozoids are like those of other Hepaticae. 



The Archegonium 



The archegonia differ from those of other Liverworts, and from those 

 of the Ferns in several particulars (Fig. 395). They originate from a single 



Neck.^ 



Oosp/iere 



•in 



Fig. 395. — Anthoceros laevis. Longitudinal section of 

 thallus through an archegonium. 



superficial cell. This divides vertically to form three peripheral cells and a 

 primary central cell. The peripheral cells become the archegonial wall, 

 but as they are completely sunk in the thallus tissue their further development 

 is indistinguishable from that of the neighbouring cells. The primary central 

 cell now divides horizontally. The lower portion divides again to form the 

 oosphere and the ventral canal cell, while the upper portion, or cover 

 cell, forms the six neck canal cells and the four cells that form a group or 

 mouth at the top of the neck. These latter project above the general thallus 

 surface. All the cells in the neck above the oosphere gelatinize and disappear, 

 leaving a clear opening down to it. 



The Sporogonium 



The zygote divides into four tiers of four cells each. From the upper- 

 most the sporogonium proper develops, while the lower three make up the 

 foot. Periclinal divisions of the four top cells cut off an inner group of 



