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 Poly sip honia, 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 Nemalion, 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 Albugo 

 (Cystopus) are somewhat divergent, 1 there are several records of nuclear 



1 Wager, Annals of Botany, iv., p. 127, x., p. 295, xiv., p. 263 ; Berlese, Jahr. f. wi'ss. 

 Bot., xxxi. , p. 159; Stevens, Bot, Gaz., xxviii., p. 149. 





