70 ALTERNATION IN THE THALLOPHYTES 
uredospores, and the teleutospores: these collectively are compared with a 
sporophyte-generation, and all show in their cells the paired nuclei, which 
divide in close association together, showing on division four chromatin- 
masses. The final nuclear fusion takes place in the maturing teleutospore, 
while the subsequent division of the fusion-nucleus shows changes which 
correspond to synapsis: at the same time there is a reduction of the 
chromatin-masses from the four characteristic of the paired nuclei 
collectively, to two. It seems thus clear that an alternation of phases, 
the one with ‘n,” the other with ‘“2n” chromatin-masses exists. It is 
known to be obligatory in those Uredineae which show the full cycle, 
and the limits of the two generations coincide respectively with a process 
of fertilisation (with suspended nuclear fusion), and a process of reduction. 
It is therefore comparable in its broad cytological features with the 
obligatory alternation seen elsewhere. The analogy with the Florideae is 
here again so obvious as to have led to the suggestion of some phyletic 
relation of the Uredineae with that group. As a corollary on these 
observations and conclusions, it has been further suggested that the 
absence of sexuality in the Basidiomycetes may be due to an apogamous 
shortening of the life-cycle, so as to omit the sexual stage altogether. 
There remain for consideration certain of the Algae, which show 
post-sexual complications of an obligatory nature: they have been reserved 
to the last because they have long been singled out as those Thallophytes 
which most naturally suggest the manner in which the alternation in the 
Archegoniatae may have originated. An important feature in them is, 
that in close relation to the sexual fusion, rearrangements of nuclear 
condition occur; in some, these precede the act of fusion, though commonly 
they follow it; but in either alternative an apparently obligatory phase 
is associated with sexual fusion in the life-cycle, and there is good reason 
to think that its existence is bound up with the post-sexual reduction. This 
has been specially remarked by Strasburger in connection with the germination 
of zygotes in the Conjugatae! and in various Chlorophyceae. The actual 
fact of post-sexual reduction has not yet been established in them by 
chromosome-counting; but the fact that the post-sexual divisions of the 
nuclei are commonly into. four, shows a pregnant analogy with tetrad- 
division, while in some cases the four nuclei are formed notwithstanding 
that only two are eventually required. This would hardly have been the 
case unless there were some importance attached to the division into 
four. Examples will now be given illustrating these points. 
In the unicellular Desmids, where no somatic complications arise, 
conjugation and germination of the zygote have been studied by Klebahn, 
whose drawings of Closterium are here reproduced (Fig. 40).2 The nuclei 
of the conjugating cells remain apart throughout the winter in the resting 
zygote (Fig. 40. 1), and only coalesce when germination begins in the 
spring: the contents escape from the thin-walled zygote, and division of 
1 Ceber Keduktions-theilung, etc., 1900, p. 83. 2 Pringsh. Jahrb., vol. xxii. 
