F r i t s c h. , Stiidies ou CVanophj^ceae. 205 



may be due to colouration oi' the cell-contents. There is no 

 trace of external niucilage. ■ "When placecl in a 33 °'o Solution of 

 cliromic acicl tlie cell-contents, as in otlier Cijanoplnjccac, are 

 slowly cli-ssolved. For some time the slieath still remains visible 

 as a faint line outside tlie inner Investment, but ultimately it 

 disappears completely and there only remain the cavities of the 

 protoplast, suiTounded by the inner Investment. 



The usual distinction between OsciUaria and Lynghja depends 

 (jn the absence of a slieath in the fornier and its presence in 

 the latter, but it has long been asserted that this is a difference, 

 which is scarcely tenable. As Gromont (loc. cit. p. 222 foot- 

 note) points out, practically all species of OsciUaria are provi- 

 ded with a delicate sheath and it may be questioned whether 

 by suitable conditions of cultivation the few exceptions might 

 not also be shown to have a very dehcate one, for the demon- 

 stration of the cell-sheath is always a matter of difficulty in 

 small-celled species of Änahaena or Nostoc for instance. In the 

 present state of our knowledge of species of OsciUaria it also 

 seems very probable, that some of these naked forms may be 

 merely young stages of sheathed species. There is no doubt 

 however, that there is a series of forms, in which the sheath 

 is a prominent feature and in which it is markedly thickened. 

 but this sheath does not correspond to the cell-sheath of an 

 Anahaena^ nor to the above-described sheath of OsciUaria Fröh- 

 licliii. but finds its homologue in the external mucilage of the 

 former. In a marine species of Lynghya iL. xalina Ivtz. '?). 

 which I collected recently on the coast of Brittany, a consido- 

 rable number of hlaments (diam. 12 u) merely present the struc- 

 ture above described for OsciUaria, but the majority have a 

 fm-ther envelope outside the (ceU-) sheath (Fig. 16, c. s.); this 

 external sheath (fig. 16, e. s.) is limited towards the exterior b}^ 

 a well-marked line, which is separated from the lilament by a 

 narrower or wider space, hUed with invisible mucilage and it 

 quite evidently corresponds to the external mucilage of an 

 Anahaena. When this sheath is of considerable thickness (it 

 often attain s 30 // in diameter) the outer limit is itself thick 

 (diam. 3 // about), whilst the remainder of the sheath presents 

 numerous layers of stratilication. Again in a species of Lyn;/- 

 hya from near Trincomalie in Ceylon all the filaments are sur- 

 rounded by a well - developed and consistent sheath, which is 

 more imiform than in the last-discussed species and encloses a 

 hlament with a thin coherent cell-sheath, resembling the struc- 

 turo of an OsciUaria in all respects. External sheath and cell- 

 sheath are here in dose apposition and the two might easily be 

 overlooked as distinct structures. 



Wo tlms see, that in one series of forms the external 

 mucilage of Anahaena, Nostoc etc. has been discarded altogether 

 and the only Investment common to the whole tilament is the 

 coherent cell-sheath; these are the species of OsciUaria^ which 

 are tlnis capaljlo of movement dm-ing the whole of their lifo. 



