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 l 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).- The nuclei 

 of the conjugating cells remain ap.irt throughout the winter in the resting 

 zygote (Fig. 40. i ), and only coalesce when germination begins in the 

 spring : the contents escape from the thin-walled zygote, and division of 



i Rednktwns-theilung, etc., 1900, p. S }. - Pringsh. [ahrb., vol xxii 



