CYTOLOGICAL DISTINCTION IN ALGAE 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 cus in the first divisions respectively of the antheridium and 
the oogonium.!. 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 emadion, though the 
demonstration is not yet quite convincing. 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 olysiphonia, a genus which bears tetraspores, is now 
before us, fully worked out by S. Yamanouchi.* 
He finds in 2 wiolacea 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’ 
1Strasburger, Pringsh. Jahrd., 1897; Farmer, Phil. Trans., B. 1898. 
2 Morphologie u. Biologie der Algen, 1904-5. i 
3 Wolfe, ‘Cytological studies on Nemalion,” Anuals of Botany, 1904, p. 607. 
4 Bot. Gazette, 1906, p. 401. 
