THE BKYOPHYTES 



271 



encounters what is known as alternation of generations} 

 By this is meant the descent of a spore-bearing plant, or 

 sporophyte, from a sexual plant, or gametophyte, then the 

 descent of a gametophyte from the sporophyte, and so on 

 indefinitely (see Sect. 357).2 In Marchantia the sporophyte 

 is a minute organism, incapable of separate existence and 

 living much like a parasite 

 on nourishment drawn from 

 the female receptacle. 



350. Summary of the Liv- 

 erworts. — The liverworts 

 show a distinct advance in 

 complexity over the algse. 

 This appears particularly in 

 the excellent facilities pos- 

 sessed by such forms as 3Iar- 

 chantia for photosynthesis 

 and respiration, as land 

 plants. It is also shown by 

 the complicated process of 

 sexual reproduction, matur- 

 ing the egg in a many-celled 

 archegonium and not in a 

 one-celled organ such as is 

 found among the thallophytes 

 {e.cf. Fig. 163). Alternation of generations is another in- 

 dication of an advanced position in the plant world. It 

 is important to note that while the majority of liverworts 

 are terrestrial they all have ciliated sperms, a characteristic 



1 This also occurs in a less evident form in red algse and in some sac 

 fnngi. 



2 The student may he asked to make a diagram of the life history of J/ar- 

 chantia on the plan of Fig. 203, or an illustrated one somewhat like Fig. 208. 



Fig. 196. A Small Portion of 

 Fig. 195 enlarged. 



s, the stalked spore-capsule. 



