94 PLANT STRUCTURES 



sperm, the organs producing the two being known as oogo- 

 nium and antheridium respectively. 



(5) Algae the main line. The Algae, aquatic in habit, 

 appear to be the Thallophytes which lead to the Bryophytes 

 and higher groups, the Fungi being regarded as their de- 

 generate descendants ; and among the Algae the Chloro- 

 phyceae seem to be most probable ancestors of higher forms. 

 It should be remembered that among these Green Algae the 

 ciliated swimming spore (zoospore) is the characteristic 

 asexual spore, and the sexual spore (zygote or oospore) is 

 the resting stage of the plant, to carry it over from one 

 growing season to the next. 



61. General characters of Bryophytes. The name given 

 to the group means " moss plants," and the Mosses may be 

 regarded as the most representative forms. Associated 

 with them in the group, however, are the Liverworts, and 

 these two groups are plainly distinguished from the Thallo- 

 phytes below, and from the Pteridophytes above. Starting 

 with the structures that the Algae have worked out, the 

 Bryophytes modify them still further, and make their own 

 contributions to the evolution of the plant kingdom, so 

 that Bryophytes become much more complex than Thallo- 

 phytes. 



62. Alternation of generations. Probably the most im- 

 portant fact connected with the Bryophytes is the distinct 

 alternation of generations which they exhibit. So impor- 

 tant is this fact in connection with the development of the 

 plant kingdom that its general nature must be clearly under- 

 stood. Probably the clearest definition may be obtained by 

 tracing in bare outline the life history of an ordinary moss. 



Beginning with the asexual spore, which is not ciliated, 

 as there is no water in which it can swim, we may imagine 

 that it has been carried by the wind to some spot suitable 

 for its germination. It develops a branching filamentous 

 growth which resembles some of the Conferva forms among 

 the Green Algae (Fig. 81). It is prostrate, and is a regu- 



