68 ALTERNATION IN THE THALLOPHYTES 
phase to be continued in the tetrasporic plant. Tetraspore-formation 
terminates the sporophytic phase with typical reduction-phenomena, so 
that the tetraspores are prepared to develop the gametophyte generation. 
There is thus an alternation of a haploid, gametophytic phase with a 
diploid, sporophytic phase in the life-history of Polysiphonia, the cysto- 
carp being included as an early part of the latter. 
It appears, then, from the two types in which alone the cytological 
details are as yet available, that there is a want of uniformity of the cycle 
within the Florideae, not unlike that already noted for the Phaeophyceae. 
The alternation in /Vemalion, where there are no tetraspores, is of a more 
restricted type than that in Polysiphonia; for in the former reduction 
appears to follow comparatively soon after the fertilisation, but in the 
latter the event is deferred till the diploid plant produces tetraspores. 
Yamanouchi suggests that the tetrasporic plant may ‘have arisen by a 
suppression of the reduction-phenomena in connection with the carpospore, 
so that it germinates still with the sporophytic number of chromosomes, 
producing a diploid plant, and that the first tetraspore-mother-cells probably 
corresponded to monospores produced on the sexual plant of the simpler 
type, since such reproductive cells would very naturally become the seat 
of the delayed reduction-phenomena. This is a possible, though a some- 
what bold hypothesis. It may be anticipated that as the details become 
more fully known for the Florideae, a comparative basis, illustrated by 
intermediate steps, may provide more certain knowledge of the relation 
of these extreme types of cytological difference. At the moment it is 
interesting to see how great these differences are in the Florideae, as they 
have also been found to be in the Phaeophyceae: moreover, they are 
marked by no corresponding differences of external form: there is no 
haploid type of plant distinct from the diploid. This fact is probably 
referable to the uniformity of the conditions under which both generations 
live; but it also has its own interest in relation with what has been 
seen in the Archegoniatae ; for there it has been shown that a gametophyte 
may be either haploid or diploid without any modification of form. 
In certain Fungi also there has long been a suspicion that there is a 
somewhat similar alternation, and recent observations have tended to 
demonstrate that here also a cytological basis exists in some cases. The 
records of nuclear fusion in Fungi are rapidly increasing: in some cases 
in which such fusion may properly be held to be of a sexual nature, a 
doubling of the chromosomes has been observed in the _post-sexual 
divisions; but it has been found more difficult to locate the necessary 
reduction exactly: among the Fungi there seems indeed to be the same 
want of general uniformity in this as in the Algae. For instance, in the 
Peronosporeae, though the observations on Peronospora and on Adlbugo 
(Cystopus) are somewhat divergent,’ there are several records of nuclear 
1Wager, Annals of Botany, iv., p. 127, x., p. 295, xiv., p. 263; Berlese, Jahr. f. wiss. 
Bot., xxxi., p. 159; Stevens, Bot. Gaz., xxviii, p. 149. 
