206 
Prit-scli, Stadies on Cyanopliyceae. 
In anotber series of forms bowever an external sbeatb is present 
as well as the colierent cell-slieatli — dnring a part of tlie life- 
history at least — in consequence of which motion bas disap- 
peared except in tlie liormogonial stage; tliese are tbe species of 
Lyngbya. If this difference is kept in mind tliere is no diffi- 
cnlty in keeping tlie two genera clistinct except in tlie liormo- 
gonial stage. Of conrse it still remains to be seen wketlier all 
species of Oscillaria cannot linder certain conditions excrete 
external mncilage and so acquire tlie cbaracters of a Lyngbya : 
for in tlie case of Oscillaria caldariorum Hanck Oomont describes 
liow (contrary to berbarium - specimens at bis disposal) tbe 
filaments sbowed no trace of a sbeatb (XB. tlie cell-sbeatb was 
surely present), wben first collected, bnt af'ter some weeks culti- 
vation. on sable de riviere sterilise et simplement bumecte 
tbey acquired, de gaines solides ne differant en rien de celles 
cpie presentent les ecbantillons places par les auteurs dans le 
genre Lyngbya'. IVbat I liave endeavoured to empbasize is 
tbat tbe sbeatb of an Oscillaria is qnite a different tbing to tbe 
sbeatli of a typical Lyngbya. 
As in Oscillaria , tbe tr ans verse portion of tbe inner invest- 
ment in a Lyngbya (and tbe remarks of tliis and tbe ensuing 
paragrapb apply also to Tolypothrix) vary in tbickness and may 
be very mticb obscnred, by tbe granulär cell-contents. Inside 
tbe external sbeatb tbe cell-sbeatb is often not well-developecl. 
bnt as soon as tlie filament is liberated as a bormogonium tbe 
cell-sbeatb begins to tbicken laterally: in fact wbenever. tbe fila- 
ment comes into contact witli tbe exterior stieb tbickening takes 
place. 1t often liappens tbat a bormogonium is partly liber¬ 
ated from a filament. wben tlie process of liberation c-eases: tbe 
free portion tben not only forms a well-marked cell-sbeatb. bnt 
outside tbis forms a fresfi strip of external sbeatb (fig. II): tbis 
may bappen repeatedlyand tbus we get appearances like fig. II. 
Tbe tendency to produce a tbickened Investment, as soon as 
exposed, is also illustrated by fig. 12; bere a small part of tbe 
filament bas died away, baving left tbe transverse septa still 
persistent: tlie two ends of tbe filaments tbus exposed. are 
covered by a very mucli tbickened portion of tbe cell-sbeatb. 
In tbe sanie way, in an exposed termination of a filament in 
Änabaena , we always find tbe cell-sbeatb, extending riglit round 
tbe exposed surface (fig. 5). 
Tbe contraction of tbe immediate cellular envelope is well 
illustrated, wben a typical Lyngbya is treated witb cfiromic acid: 
after some time we find a row of more or less emptied cells 
(i. e. witb tbe cell-contents dissolved away) hing loosely witliin 
tbe outer sbeatb (cf. fig. 1, PI. II . Gromont. loc. cit. and mv 
fig. 15). ULtimately bowever tbe sbeatb becomes entirely dis¬ 
solved away and tliere onlv remains a row of emptv cells witb 
tbe inner Investments (cf. Oscillaria abovei. II lien treated witb 
Yesuvin tbe sbeatb of a Lyngbya of comse develops a well- 
marked brown colour (cf. Änabaena). 
