r
true Nostoc, whether young or old, present one uniform diameter
; there are no gradations of size. I t cannot therefore be
supposed that the threads are increased in number by the
effusion of the minute contents of the cells.
The multiplication of the threads in a frond having been
as it seems to me satisfactorily accounted for, the manner of
the formation of new fronds remains to he determined. When
a Nostoc has arrived at the full and last period of its developement,
the pellicle formed by the inspissation of the mucous matrix
bursts: the mucous contents and the filaments are effused:
these last become disarticulated, so as to form short fragments,
each of which retains about it a portion of mucus, so that in
this state it corresponds with a Nostoc in the first period of its
developement. In this mode of multiplication, remarkable as
it is, there is nothing generically peculiar. A Conferva multiplies
itself occasionally by the disarticulation of the filaments.
The only difference between the case of the Nostoc and the
Conferva is, that the process in the first is natural, and in the
second artificial. While, however, the separation of the primary
filaments accounts amply and satisfactorily for the multiplication,
not only of the threads of a frond, but also for
the increase of a number of the fronds themselves, it falls
short of explaining the first developement of the first thread.
The species of the genus Nostoc, like the freshAvater Algce in
general, are short-lived: in the course of a few months they
pass through the stages of their developement; they die, disappear
; the filaments themselves are destroyed; and then are
seen no more until the advent of another season. I t is plain,
therefore, that the true mode of reproduction of the species of
this family is something more than a mere separation of the
filaments into fragments. I t is now, tha t in my vicAV Ave
learn the real nature of the enlarged vesicles: these are the
true reproductive bodies, at the season not proper for their
developement, lying concealed in the earth, awaiting their
appointed time to start into groAvth, activity, and life^—•
all else of the plant, the mucus and the filaments, utterly
perishing.
In the “ Annales des Sciences Naturelles,” third series.
tome ii., there are some interesting observations on the reproduction
of Nostoc verrucosum by M. Gustave Thuret, of
whose accurate and skilful researches I have already had occasion
to avail myself in the Introduction.
“ When the plant has arrived at its full developement, the
external pellicle, formed by the thickened mucilage, breaks,
and permits the escape of the green jelly, which is composed
of mucilage and threads. These last scatter themselves in the
water the more easily, th a t they are endowed at tha t time with
a spontaneous motion, analogous to that of the Diatomacea.
This curious phenomenon has been already observed by
Vaucher, Avho believed he had found it, although much more
evident in all the other Nostocs. In spite of the assertion of
that conscientious observer — in spite of the earnestness with
which he sought to generalise this fact (Hist, des Tremelles,
p. 215. et suiv.), it is difficult to compare movements at least
equivocal, with mobility so evidently spontaneous, which the
threads of the Nostoc verrucosum possess. Perhaps it is not
without some Interest to remark on this subject, that the
spontaneous movements, the locomotion, are again met with
under different forms, in the Algce, which, like the Nostocs,
live in running waters — in the Oscillatoria, in the spores of
Vaucheria, the Confervea, &c., and tha t this faculty appears
to be a condition of their station.
“ To observe well this phenomenon in the Nostoc verrucosum,
the most simple way is to place in a plate filled with
water some fine specimens freshly gathered. At the end of
two or three days the external skin breaks, the threads expand
in the water, and form at the bottom of the plate, or
on the surface of the liquid, a green scum, nearly like that
of the Oscillatoria. I f then you have recourse to the microscope,
you obserA'e that the threads, originally very long,
and winding in a thousand ways, are divided into a number
of fragments of unequal lengths, all nearly straight, of
scarcely bent, which move in the direction of their lengths,
and seem to creep upon the surface of the object glass. The
larger globules are detached and immoveable; no increase
of magnifying power or mode of clearing, no re-agent, no
to