CYTOLOGICAL DISTINCTION IN ALGAK 67 



which the thallus itself is diploid, but no alternation is known to exist. 

 The cytological observations would indeed seem to exclude it ; for the 

 doubling of the chromosomes which follows on fertilisation is maintained 

 throughout the somatic divisions, and reduction has been found to take 

 place in Fucus in the first divisions respectively of the antheridium and 

 the oogonium. 1 Such examples as these, taken from the group of the 

 Brown Seaweeds, show that an obligatory alternation, though present in 

 some of them in a type comparable cytologically with that of the Archegoniatae, 

 is not a constant feature for them all, in the same sense as it is in the 

 Mosses and Ferns. 



In the Red Seaweeds the probability has long been contemplated that 

 the peculiar developments following on fertilisation consist in the formation 

 of a phase of the nature of a sporophyte. This position and a corresponding 

 terminology have been accepted and developed for the Florideae generally 

 by Oltmanns, in his work on Algae.' 2 Until quite recently the necessary 

 cytological details have only been observed in Nemalion, though the 

 demonstration is not yet quite convincing. 3 It is stated that on fertilisa- 

 tion of the procarp by the spermatium a nuclear fusion takes place : 

 this results in a doubling of the chromosome-number from eight, 

 which is the number in the somatic divisions of the thallus, to sixteen in 

 those post-sexual divisions of the cystocarp which precede the maturing 

 of the spores. On the other hand, though no tetrad-division occurs, a 

 reduction-division is stated to be immediately associated with the pro- 

 duction of the carpospores. If this be so, then the post-sexual stage, 

 being diploid, will be cytologically comparable with the sporophyte-stage, 

 and the carpospore on germination will initiate again the haploid or 

 gametophyte stage. It is, however, to be borne in mind that neither 

 Nemalion nor the genera allied with it bear tetraspores, which are so 

 marked a feature in most members of the family. Fortunately the cyto- 

 logical history of Polysiphonia, a genus which bears tetraspores, is now 

 before us, fully worked out by S. Yamanouchi. 4 



He finds in P. violacea that the carpospore on germination shows 

 40 chromosomes, and that the same number appear in the vegetative 

 mitoses of the tetrasporic plant: so that it may be inferred that the tetrasporic 

 plants come from carpospores. The tetraspore on germination shows 

 20 chromosomes, and the same number appear in the vegetative mitoses 

 of the sexual plant : so it may be inferred that the sexual plants come 

 from tetraspores. The nuclei of the gametes contain each 20 chromo- 

 somes : the fusion-nucleus in the fertilised carpogonium has 40 chromosomes, 

 and gives rise to a series of nuclei in the central cell : some of these 

 enter the carpospores, which are consequently a part of the sporophytic 



1 Strasburger, Pringsh. Ja/trb., 1897; Farmer, Phil. Trans., B. 1898. 



2 Morpfwlogie u. Biologic der Algen, 1904-5. 



3 Wolfe, "Cytological studies on Xemalion" Annals of Botany, 1904, p. 607. 

 4 Bot. Gazette, 1906, p. 401. 



