358 Bower . — On antithetic as distinct from 
Vancheria , shows their inconstancy, and if the ‘spores’ are 
of inconstant occurrence within the Siphoneae , how can we 
draw secure comparisons between those which are only 
occasionally present in the Siphoneae and those in the re- 
motely distant Mosses and Ferns? Accordingly the sporal 
stage comparable to that in Mosses or Ferns must be aban- 
doned as a fixed point in the life-cycle of such Thallophytes 
as Botrydinm , and the zygote alone remains as a point of fixed 
homology for comparative purposes 1 . It follows necessarily 
that the tracing of an alternation comparable to that of the 
Archegoniatae in such a plant as Botrydinm is a mere mental 
fiction. Such alternation as there is appears in this plant to 
be very directly dependent upon external conditions, and is 
to be recognised as a complicated form of alternation of 
homologous gametophytes, brought about by repeated and 
varied gametophytic budding. What has now been said of 
Botrydinm will apply in all essential points equally well to 
Ace tabular ia. 
Other groups of Algae also display phenomena comparable 
to these : but it will, I think, be unnecessary to follow the 
similar lines of reasoning out for all the main groups : it will 
suffice to remark that gametophytic budding appears to be 
absent, or at least rare, in certain cases (e.g. Fucus), but it 
is usually present ; thus in Oedogonium and Coleochaete 
swarm-spores are formed, in Volvox the special cells of the 
coenobium which give rise to new coenobia, in Florideae the 
tetraspores, &c. : in all these cases the vegetative reproduction 
is a gametophytic budding, and such alternation as occurs 
is an alternation of homologous generations , which may fre- 
quently be correlated directly with season, or other external 
circumstances. 
Turning to the Fungi, similar reasons to those above stated 
in the case of certain Algae may be found for recognising in 
them an alternation of homologous generations, derived one 
from another by a propagative process which may be styled 
gametophytic budding : reference will be made to certain 
1 Compare De Bary, Fungi, Eng. Ed., p. 121. 
