114 STRUCTURAL BOTANY 



2. THE SEXUAL ORGANS 



a. The Antheridia 



Pellia is usually monoecious, the thallus producing 

 antheridia at first, and then beginning to form the 

 archegonia. Although our plant bears a general 

 resemblance to the prothallus of a Fern, we must not 

 expect to find an exact agreement. In the position of 

 the reproductive organs there is an important difference ; 

 in the Fern-prothallus they are usually limited to the 

 lower surface, while in Pellia and the Liverworts gener- 

 ally it is always the upper side which bears them. The 

 antheridia are easily seen with the naked eye, dotted 

 over the upper surface on either side of the midrib (see 

 Fig. 50). 



The antheridia when mature are globular bodies, 

 reaching 0'3 mm. in diameter, attached to the thallus 

 below by a very short multicellular stalk. Each anther- 

 idium is enclosed singly in a flask-like sheath, leaving 

 only a very narrow opening at the top (see Fig. 52). 

 This sheath is formed by the gradual growing up of 

 the thallus-tissue around the young antheridium. The 

 development takes place in the following way : 



The antheridium arises from a single superficial cell 

 situated on the upper side of the thallus, immediately 

 behind the growing-point. This cell rises above the 

 general level of the thallus, and divides by a transverse 

 wall ; the lower cell thus formed, after undergoing a 

 few further divisions, forms the short stalk. The upper 

 cell divides by a longitudinal wall into two cells, and 

 these rapidly subdivide in such a manner as to form a 

 single superficial layer enclosing a few central cells (see 



