CHAPTER. V. 



LIVERWORTS AND MOSSES. 



55. Alternation of generations. — In the liverworts and 

 mosses, as in all the plants higher in the scale, there occur 

 two well-marked phases in the course of their lives. One of 

 these phases is marked by the formation of sexual reproduc- 

 tive cells, or gametes (see ^[ 369), the egg and sperm, 

 whence it is called the sexual phase, or the gametophyte. The 

 other is characterized by the formation of non-sexual repro- 

 ductive cells, the spores (see T 3°4-)> whence it is called the 

 non-sexual phase, or sporophyte. These two phases alternate 

 with each other, the sexual reproductive cells of the game- 

 tophyte producing, under suitable conditions, the sporophyte, 

 whose non-sexual reproductive cells give rise to the game- 

 tophyte. To this regular sequence of the two phases the 

 phrase alternation of generations has been applied.* 



In the higher liverworts and mosses both phases have 

 nutritive work to do, but in many this is confined to the 

 gametophyte, and in all the gametophyte carries on the 

 greater part of it. To this phase, therefore, attention is 

 first given. 



Liverworts. 



56. The thallus. — The form and structure of the vegeta- 

 tive body of the simplest liverworts is scarcely different from 



* Rather obscure suggestions of the alternation of generations are to be 

 found among the algse and fungi, but they are not definite enough to 

 warrant discussion in this book. Let the student notice, however, that 

 this feature does not appear suddenly in plant life, though introduced 

 abruptly into the account of it. 



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