Fritscli, Studies 011 Cyanophyceae. 
207 
(e) Tolypothrix. The species of Tölypotlirix are always pro- 
vided witli a well-developed external sheath of the same tough 
consistency, as in Lyngbya ; and at first sight one very easily 
overlooks this and the cell-sheath as distinct structures. As soon 
however as the filaments are treated with lodine no doubt can 
remain, for the filament of cells with tlieir immediate investments 
contracts away froni the external sheath (fig. 13), at the same 
time becoming stained so that inner investment and cell-sheath 
show up quite well. Tliere is however one point, which distin- 
guish.es these contracted filaments from those of an Oscillaria or 
a Lyngbya ; the cell-sheath is much more prononncedly ntonili- 
form than in eitlier of the latter genera (fig. 13, c. s.) and if the 
filaments are examined carefully, it will be found that here and 
there it is split between adjacent cells. That is to say the 
structure of the actual filament (i. e. independent of the external 
sheath) in Tolypothrix recalls that of Anabaena or Nostoc to some 
extent, and, in that the cell-sheath is not entirely coherent, is 
less specialised than in Oscillaria and Lyngbya. 
The effects of different reagents on filaments of Tolypothrix 
are similar to those in Lyngbya and it only remains to draw 
attention to the fact, tliat in the former genus the heterocysts 
are included in the general external sheath, — a point of clif- 
ference from Anabaena and froni the genus, next mentioned. 
(f) Rivularia furnishes a particularly interesting case: I 
examined Gloeotrichia natans (Hedwig) liabenh. from the Plankton 
of Ceylon. Here the basal end of the filament is almost invari- 
ably occupied by a heterocyst and if this is absent the loivest 
cell exliibits distinct modifications, as evidenced by its behaviour 
towards reagents 1 ); the other end of the filament is produced 
into a longer or shorter, generally much-attenuated liair-like 
structure. The base of the filament, exclusive of the heterocyst 2 ), 
is surrounded by a mucilaginous sheath, the external limits of 
which are sometimes well-marked even in unstained material, 
although just as often in visible; emanating from the proximal 
portion of the cell, immediately adjacent to the heterocyst, the 
limit of the sheath arches outwards and thus comes to be sepä- 
rated from the following cells by a considerable interspace. The 
sheath can generally only be followed up a little way and is 
unrecognisable in the upper portion of the filament; this is 
undoubtedly due to the fact, that- it is only excreted by the 
b Whereas all tlie cells of the filament take on a brown colonr with 
Iodine the basal heterocyst or, if this is absent, the lowermost cell remains 
unstained (cf. Fritsch 04. p. 90). 
2 ) The heterocysts of this species (fig. 10 h) are very peculiar. Under 
a low power one can distinguish the following structure. On tlie exterior 
of the heterocyst is a thin membrane, which encloses a rounded slightly 
flask-sliaped cell, which is provided with inner investment and cell-sheath. 
Between the cell-sheath and the above-mentioned thin membrane is a clear 
space of considerable width, which is apparently empty. The actual cell 
on the other liand is occupied by deep blue-green homogeneous contents. 
which fill its entire lumen. I sliaii publisli further details subsequently. 
