40 Myr.J. Ralfs on the mode of growth in Oscillatoria. 
marked merely by faint transverse strie. Still that the cells 
divide as in the other simple Algz will scarcely be contested, if 
indeed the fact be not sufficiently proved in those species which » 
have some of the striz about twice the ordinary distance apart, 
as is always the case when cells are dividing. . 
In general the cells are indicated, as I have just stated, by 
more or less evident transverse, straight strie; but at certain — 
intervals the junction margins become rounded during division 
and the filament separates into distinct portions. All the Osei- 
latoriee have the filaments inclosed in sheaths. When the sheath 
divides together with the cell, the original filament at once forms 
two; and as this process is continually going on, we can easily 
conceive the rapid extension of the stratum consequent upon the 
progressive increase in the number of filaments. 
It may be necessary to mention, that it is easy to distinguish 
between a natural separation and a fracture. In the latter case 
the ends formed by violence are abrupt ; in the former they are 
usually rounded. 
When, as in some species, there is a complete separation of the 
internal filament unaccompanied by simultaneous division of its 
sheath, the latter retains the portions in connection. Lyngbya 
ferruginea affords a good example of this kind, and as its fila- 
ments are stouter than those of most species of Oscillatoria,; no 
better plant can be selected for observation. If a portion of the 
stratum be examined, filaments of various lengths may be seen 
mingled together ; but they are all of the same breadth, although 
some of them are not longer than broad. : 
When separated portions are thus held together by the sheath, 
there is generally a short interval between them. Whether this 
results from an elongation of the sheath or the mutual repulsion 
of the inclosed portions is doubtful. The latter I consider as the 
more likely cause. May it not be produced by an electric cur- 
rent developed at the instant of partition? Perhaps the radia- 
tion of the filaments from the stratum, in some species of Oscil- 
latoria, may be similarly accounted for. 
Microcoleus is known by its numerous, short, simple Oseilla- 
toria-like filaments being contained within cither a simple or a 
slightly branched, inflated sheath or frond. The presence indeed 
of this common covering is the character which separates Micro- 
coleus from Oscillatoria; for the filaments and their manner of 
division are alike in both. 
In Oscillatoria the parted filaments are retained together 
merely by the common mucus which permits a comparatively 
wider range, and allows them to diverge in various directions. — 
In Microcoleus, on the contrary, their freedom is restricted ; the 
frond by its form and size keeps them parallel and binds them 
