Fritsch., Studies 011 Cyanopkyceae. 
205 
may be due to colouration of tlie cell-contents. There is no 
trace of external mncilage. When placed in a 33 °/o solution of 
cliromic acid tlie cell-contents, as in otlier Cyanophyceae , are 
slowly dissolved. For some time tlie slieatli still remains visible 
as a faint line outside tlie inner investment, but ultimately it 
disappears completely and tbere only remain tlie cavities of tlie 
protoplast, siirrounded by tlie inner investment. 
The usiial distinction between Oscillaria and Lyngbya depends 
on tlie absence of a slieath in tlie former and its presence in 
tlie latter, but it lias long been asserted that tliis is a difference. 
Avhich is scarcely tenable. *Äs Gromont (loc. cit. p. 222 foot- 
note) points out, practically all species of Oscillaria are provi- 
ded with a delicate sheatli and it may be questionecl whether 
by suitable conditions of cultivation tlie few exceptions miglit 
not also be shown to liave a very delicate one, for tlie demon- 
stration of the cell-sheath is always a matter of difficulty in 
small-celled species of Anabaena or Kostoc for instance, In tlie 
present state of our knowledge of species of Oscillaria it also 
seems very probable, that some of fliese naked forms may be 
merely young stages of sheathed species. There is no doubt 
ho weiter, that there is a series of forms, in which the sheatli 
is a prominent feature and in which it is markedly thickened. 
but tliis sheatli does not correspond to the cell-sheath of an 
Anabaena , nor to the above-described slieath of Oscillaria Fröh- 
lichii , but finds its homologue in the external mucilage of the 
former. In a marine species of Lyngbya (L. salina Ktz. ?), 
which I collected recently on the coast of Brittany, a conside- 
rable number of filanients (diam. 12 g) merely present the struc- 
ture above described for Oscillaria , but tlie majority liave a 
further envelope outside the (cell-) slieath (Big. 16, c. s.): tliis 
external sheatli (fig. 16, e. s.) is limited towards the exterior by 
a well-marked line, wdiich is separated from the filament by a 
narrower or wider space, fi lier] witli in visible mucilage and it 
quite eviclently corresponds to the external mucilage of an 
Anabaena. When tliis slieath is of considerable thickness (it 
offen attains 30 g in cliameter) the outer limit is itself thick 
(diam. 3 g about), ivliilst the remaincler of the slieath presents 
numerous layers of stratiiication. Again in a species of Lyng¬ 
bya from near Trincomalie in Ceylon all the fdaments are sur- 
rounded by a ivell-developed and consistent slieath, which is 
more uniform than in the last-cliscussecl species and encloses a 
filament with a thin coherent cell-sheath. resemblino’ the struc- 
/ O 
ture of an Oscillaria in all respects. External slieath and cell- 
sheatli are liere in close apposition and the two miglit easily be 
overlooked as clistinct structures. 
We thus see, that in "one series of forms the external 
mucilage of Anabaena , Nostoc etc. has been discarded altogether 
and the onlv im r estnient common to the whole filament is the 
coherent cell-sheath: these are the species of Oscillaria , wliicli 
are thus capable of movement during the whole of tkeir life. 
