comes everted and protruded into the cavity of the other.
(PI. X V I I . fig. 5.)
“ The cause of this protrusion, and the reason why it only
occurs at the precise period of the reproduction of the cells,
are easily accounted for, and both arise from unequal internal
pressure of the contiguous cells on each other, which inequality
of pressure is produced hy the emission of the endo-
clrrome of one cell into a neighbouring cell either in the same
or different filaments; thus, when a cell has discharged its
contents, its cavity is empty, and no resistance can be offered
by it to the protrusion of the inverted portion of the adjacent
cell or cells, replete as it or they may be with fluid and endochrome.
This explanation applies likewise to the fact, that
Avhen a number of cells have either emptied themselves of
their contents, or have been the recipients of those of other
cells at the same time, no eversión takes place, for in this case
there is no inequality of internal pressure.
“ But while a correct exposition may be given of the cause
of this protrusion and intromission, it is not so easy to offer
a satisfactory explanation of the purpose to be attained by it.
The eversión, doubtless, assists In effecting the dislocation of
the cells, and thus, reproduction being perfected, hastening the
destruction of the species and dispersion of the spores; processes,
which, from the greater length of the cells and consequent
continuity of the enveloping sheath, would possibly
occupy, were it not for some special provision of the nature
indicated, a much longer time than in the short-celled species.
A subordinate and not unimportant use of this provision is, the
assistance which it affords in the determination of allied species.
“ I t is remarkable th a t no similar conformation presents
itself to our notice In the genera Tyndaridea and Mougeotia,
so closely allied to Zygnema, for In these the cells invariably
terminate by plane surfaces, which, however, may be either
everted or inverted to a slight extent.
“ This peculiar formation of the cells of some Zygnemata
was first noticed by me in the spring of 1842, but its true
nature only became apparent to me in the early portion of
the present year. “When viewed through a low power of the
microscope, and in a Zygnema whose filaments are as yet
separate, it exhibits the appearance of two curved knife-
blades slightly approximating to each other at their apices,
near to which usually lies the divided spiral thread, and
strongly impressing the superficial observer, from the position
and aspect of these blades, with the idea that they are the
instruments which effect its separation, and reminding him of
the beautiful provision whereby the section of pollen granules
is accomplished.
“ On transmitting a short time since a specimen of the
Zygnema quadratum, in a state of reproduction, to the Rev.
M. J . Berkeley and Mr. Ralfs, hut unaccompanied hy any
remarks in reference to the structure of the cells, both these
gentlemen noticed their peculiar conformation, and from the
former I received correct sketches of their appearances.
“ The structure of the joints in Zygnema was long since
noticed in one species of the genus by Mohl, who thus describes
it in his paper upon the multlphcation of cells by
division, inserted in the ‘ Flora ’ : — ‘ In elongatum Ag.,
the dissepiments have a very peculiar structure, which I have
found in no other species. The terminal surface of each cell
is not even, but elongated into a blunt conical process. This
process can only be observed in its true state when two joints
are separated one from the other ; when, on the contrary, the
threads are unbroken the process is generally introverted like
the finger of a glove, and exhibits the form represented at
PI. I. fig. 8. «, 5, c. This is the common condition, and in
most threads no joint is found otherwise constructed. But I
have now met with a single thread in which a part of the
articulations has the ordinary length, while another part has
joints only half as long. In these shorter articulations it
was normal that only the alternate dissepiments had the
structure peculiar to this species (so th a t by these dissepiments
the thread was divided into articulations of the ordinary
length), while, on the contrary, the intermediate dissepiments.
exhibited the form usual in Confervoe.”
“ The observation, th a t ‘ this process can only be observed
in its true state (that is, everted) when two joints are separated,
the one from the other,’ is inaccurate, for the cells may
be separated and yet the processes inverted, the eversión of