BEPKODUCTION 435 



plants to produce both gonidia and gametes, it is more 

 usual for them to bear the former only. So for a long 

 series of individuals reproduction is brought about asexually 

 by gonidia. Then for some reason an individual produces 

 gametes, and the series is interrupted by the occurrence 

 of sexual reproduction. This is generally followed by a 

 further series like the first. We have here an instance of a 

 kind of alternation of generations, which is, however, 

 irregular and intermittent. As all the members of the 

 series, whether producing gonidia or gametes, are essentially 

 similar or homologous, this is often spoken of as homologous 

 alternation of generations. 



The forms which we have discussed appear all to be 

 capable of producing gametes if conditions require them. 

 They are accordingly termed gametophytes, and are dis- 

 tinguished as actual or potential as they do or do not give 

 rise to sexual cells. 



In plants which are higher in the scale the production 

 of both sexual and asexual reproductive cells ceases to be 

 possible upon the same individual, and we find consequently 

 that the plant exhibits two phases in its life cycle, one of 

 which is characterised by the production of sexual and the 

 other of asexual cells. How this sharply marked separation 

 arose is still a matter of controversy which we need not 

 here enter into. The two forms, however, differ essentially, 

 one being capable normally of producing only gametes, 

 the other of giving rise only to spores. The zygote arising 

 on the gametophyte from the sexual fusion is only capable 

 of originating a form which bears spores, while the spore can 

 only develop a form on, which sexual cells arise. The 

 asexual form in the life cycle is known as the sporophyte. 

 The occurrence of gametophyte and sporophyte regularly 

 in turn, as described, is known as antithetic alternation of 

 generations. It is of constant and regular occurrence in all 

 the groups of plants above the Thallophytes. 



The existence of a sporophyte, or form which is never 

 capable of bearing gametes, is still a matter of discussion 



28* 



