20() Fritsch. Stiulies on Cyanophyceae. 



In anotlicr series of fonns liowever an external slieath is present 

 as \\v\\ as tho rolierent cell-slieath — during a part of tiio lifo- 

 liistory at least — in consequence of which motion has disap- 

 p('ared ext-opt in tho hormogonial stage; these are tlie speeics of 

 Lyughija. If lliis difference is kept in mind tlicre is no diffi- 

 culty in koeping tlie two genera distinct exccpt in the hormo- 

 gonial stage. Of course it still remains to be seen whethcr all 

 species of 0,scilIaria cannot under certain conditions excrete 

 external mucilage and so acqnire the characters of a Ljitif/hya^ 

 for in tlio case of 0.sciUaria raldariorum Hauck Gomont describes 

 how (contrary to herbarium - specimens at his disposal) the 

 iilaments showed. no trace of a shcath (NB. tlie cell-sheath was 

 surel}^ })resent), when iirst coUected, bnt after some weeks culti- 

 vation, (^n sable de riviere sterilise et simplement hum(>cte' 

 tliey acquired, de gaines solides ne differant en rien de celles 

 que presentent les echantillons places par les auteiu's dans le 

 genre Lynghya. What I have endeavoiired to emphasize is 

 that the sheath of an Osrillaria is quite a different thing to the 

 sheath of a typical Lynghya. 



As in Oscillaria, the transverse portion of the inner Invest- 

 ment in a Lynghya (and the remarks of this and the ensning 

 Paragraph apply also to Tolypotlirix) vary in thickness and may 

 be veiy mucli obscnred by the granulär cell-contents. Inside 

 the external sheath the cell-sheath is often not well-developed, 

 but as soon as the hlament is liberated as a hormogonium the 

 cell-sheath begins to thicken laterally; in fact whenever, the hla- 

 ment comes into contact with the exterior such thickening takes 

 place. It often happens that a hormogonium is partly liber- 

 ated. from a filament, when the process of liberation ceases; the 

 free portion then not only forms a well-marked cell-sheath, but 

 outside this forms a fresh strip of external sheath (fig. 14); this 

 may happen repeatedlyand thus we get appearances like fig. 14. 

 The tendency to produce a thickened Investment, as soon as 

 exposed, is also illustrated by hg. 12; here a small part of the 

 filament has died away, having left the transverse septa still 

 persistent; the two ends of the filaments thus exposed, are 

 covered by a very much thickened portion of the cell-sheath. 

 In the same way, in an exposed termination of a filament in 

 Äiiabaena, we always find tlie cell-sheath, extending riglit round 

 the exjDosed. surface (fig. 5). 



The contraction of the immediate cellular envelope is well 

 illustrated, when a typical Lynghya is treated with chromic acid; 

 after some time we find a row of more or less emptied cells 

 (i. 6. with the cell-contents dissolved away) lying loosely within 

 the out er sheath (cf. fig. 4, PI. IV. Gomont, loc. cit. and my 

 fig. 15). Ultimately however the sheath becomes entirely dis- 

 solved away and there only remains a row of empty cells with 

 the inner Investments (cf. OsciUarla above). AVhen treated with 

 Vesuvin the sheath of a Lynghya of coui'se develops a well- 

 marked brown colour (cf. Auahaena). 



