F r i t s e li , Studies on Cyauophyceae. 207 



(e) Tolijpothri.r. The species of Tolijpotltrix are always pro- 

 videcl witli a well-developed external sheath of the same tough 

 consistency, as in Lijiifibya: and at first sight one very easily 

 oveiiooks tliis and tlie cell-slieath as distinet structnres. As soon 

 liowever as the lilaments are treated with lodine no doubt can 

 remain, for the lilanient of celLs with their immediate investments 

 contracts away from the external sheath (fig. 13), at the same 

 time becoming stained so that inner invostment and cell-sheath 

 show up quite well. There is liowever one point, which distin- 

 gnishes these contracted filaments from those of an O.sciUaria or 

 a Ljjiigbya] the cell-sheath is much more prononncedly monili- 

 form tlian in either of the latter genera (iig. 13, c. s.) and if the 

 lihiments 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 iilament (i. e. independent of the external 

 sheath) in ToJypothr'ix recalls that of Anabaena or Nosioc to some 

 extent, and, in that the cell-sheath is not entirely coherent, is 

 less specialised than in O-sciUaria and Lynghya. 



The effects of different reagents on hlaments of ToJypothrix 

 are similar to those in Lynghya and it only remains to draw 

 attention to the fact, that in the former genus the heterocysts 

 are included in the general external sheath, — a point of dif- 

 ference from AiiabaeHa and from the genus, next mentioned. 



(fj Rividarla fm-nishes a particularly interesting case; I 

 examined Gloeotricliia natans (Hedwig) Eabenh. from the Plankton 

 of Ceylon. Here the basal end of the iilament is almost invari- 

 ably occu])ied by a heterocyst and if this is absent the lowest 

 cell exhibits distinet modifications, as evidenced by its behaviour 

 towards reagents ^) ; the other end of the filament is produced 

 into a longer or shorter, generally much - attenuated hair-like 

 structure. The base of the iilament, exclusive of the heterocyst-), 

 is surrounded bv a mucilaginous sheath, the external limits of 

 which are sometimes well-marked even in unstained material, 

 although just as often invisible; emanating from the })roximal 

 portion of the cell, immediately adjacent to the heterocyst, the 

 limit of the sheath arches outwards aud thus comes to be sepa- 

 rated from the following cells by a considerablc interspace. The 

 sheath can generally only be followed uji a little way and is 

 unrecognisable in the upper portion of the iilament; this is 

 undoubtedly due to the fact, that it is only excreted by the 



^) Wlierea.s all tho c.olls of tlic lilaiiient take oii a browii colonr with 

 Todine the 1)asal lieterocyst or, if this is absent. the lowerniost «•eil reiiiiiiiis 

 unstained (cf. Fritsch 04. p. 9t)). 



-) The heterocysts of this species (Iig. 10h) are ver}' peculiar. Uiider 

 a low power one can disting-ui.sh the following structure. On the exterior 

 of tlie heterocyst is a thin iiH'ml)riine. which encloses a rounded sliglitlv 

 flask-shaped cell, which is provided with inner investnient and cell-sheath. 

 Between the cell-sheath and tlie al)Ove-nientioned thin memhraue is a clear 

 Space of consideral)le width , which is appareiitly eniptv. The actual cell 

 on the other hand is occupied by deep blue-green honiogeneous contents. 

 which Uli its entire lumen. I shail publish further details subsequentlj'. 



