426 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. Itis 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 
