CH. IX] ALTERNATE GENERATION. 119 



and not to insist in any detail on the connection between 

 it and the reproduction of Phanerogams. 



The fern exists under two quite distinct forms, as 

 different in appearance as the caterpillar and butterfly; 

 these two forms may for the moment be called $ and 0. 

 The alternation of generation consists in this : both S and 

 have reproductive organs, but S only produces 0, 

 and only produces S. So that the pedigree of a g 

 fern would be represented by 



The process is further remarkable for the working ^ 

 together of two types of reproduction, sexual and 

 asexual. The essence of sexual generation is that a cell, 

 the ovum or egg-cell, is fertilised by a male cell which 

 unites or melts up with it, and the fertilised egg-cell 

 thus becomes capable of developing into an embryo 

 or young plant. The essence of asexual reproduction 

 is that the parent produces a cell which developes into 

 a young plant without being stimulated by a process of 

 fertilisation. In the fern, the form bears an egg-cell 

 which when fertilised developes into $; S produces, 

 without a sexual process, certain reproductive cells called 

 spores, and these produce the form 0. The 

 pedigree may thus be amplified by adding the 

 letter ra to express the union of the male 

 element. 



The diagram shows that the act of fertilisa- i "* 

 tion is confined to the form 0, so that instead of S 

 there being (as in quadrupeds) a regular series .L 

 of sexually produced generations, there are alter- 

 nate sexual and asexual generations. 



