REPRODUCTION 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 
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