4 26 VEGETABLE PHYSIOLOGY 



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 fol- 

 lowed 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 are not homolo- 

 gous, one being capable normally of producing only gametes, 

 the other of giving rise only to spores. A further develop- 

 ment also makes itself evident in that the zygote arising 

 on the gametophyte 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 is from 

 this point upwards known as the sporophyte. The occur- 

 rence of them 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 



