392 A TEXTBOOK OF THEORETICAL BOTANY 



is marked by the longitudinal division of the apical cell into two, after which 

 both apical cells continue to cut off segments in the same way as before. 



Reproduction 



Reproduction in Pellia is both sexual and asexual, but the two follow 

 one another so that an immutable alternation of generations obtains. 



As we have seen, the sporophyte is relatively small and lives parasitically 

 upon the gametophyte. In Pellio, like all other Bryophyta, the thallus is 

 the gametophyte, and upon it are borne the sex organs. They consist of 

 antheridia, the male organs, and archegonia, the female organs. 



The Antheridium 



The antheridium arises from a single superficial cell which lies on the 

 upper surface of the thallus immediately behind the growing point. It 

 divides transversely forming a lower cell, which, after a few further divisions, 

 forms the stalk, and an upper cell, which divides by a longitudinal septum 

 into two, then into four and then into eight. By periclinal divisions there is 

 formed an outer layer of cells enclosing a small central mass of cells. This 

 layer constitutes the wall of the antheridium, whilst the central cells, after a 

 considerable number of divisions, constitute antherozoid mother cells. 

 Whilst development of this antheridium has been going on, growth has also 

 taken place in the rest of the thallus so that the mature antheridium lies 

 some distance behind the growing point, and a tissue has grown up completely 

 enclosing it, except for a small opening at the top (Fig. 373). 



From each antherozoid mother cell a single antherozoid is produced, 

 which consists almost entirely of the nucleus together with two long flagella 



(Fig. 374). At the last division 

 of the mother cells two plastid- 

 like bodies are present, which 

 are placed on each side of 

 the nucleus and function as 



■^■*% 



Fig. 272— Pellia epiphylla. Vertical section of 

 antheridium showing mature antherozoids. 



Fig. 374. — Pellia epiphylla. Mature 

 antherozoid. {After Showalter.) 



