CHAPTER VI. 



LIVERWORTS AND MOSSES. 



50. 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 repro- 

 ductive cells, or gametes, the egg and sperm (see ^ 304), 

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

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

 ductive cells, the spores (see ^ 263), whence it is called the 

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

 with each other ; i.e., the eggs produced by the gametophyte 

 do not form a new gametophyte but a sporophyte ; and the 

 spores of the sporophyte do not form a new sporophyte but a 

 gametophyte. Representing the gametophyte by G and the 

 sporophyte by ^ the sequence is G-»-^S-»-^G-m-^S-^^G'»^S, 

 and so on, generation after generation. Often the gameto- 

 phyte forms other gametophytes repeatedly, but usually the 

 succession is interrupted, sooner or later, by the formation of 

 fertile eggs and from these a sporophyte. In such cases the 

 sequence may be represented thus: GGGG'^-^Sm-^GGGm-^S 

 ^-^GG, etc. The sporophyte of these plants wecer propagates 

 its own form. To this regular sequence of the two phases 

 the phrase alternaHon of generations has been applied.* 



* Rather obscure suggestions of the alternation of generations are to 

 be found among the alg« 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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