median line, and central and terminal nodules ; striæ obscure.
Length of frustule -0005". v.v.
Vi\ Sm. Ann. ser. 2. vol. xv. pi. i. 9. July 1854.
Fresh water. Lasswade near Edinburgh, Dr. Greville. (Vaucluse, May
1854, TV. Sm.) ^
Supp, Plate LXI. 381.
G e n u s 39. RH A BDO N EM A , Kütz.
-Filaments compressed, continuous, attached, or stipitate; frustules
annulate, indefinite ; annuli plane, cellulate on their circumference,
forming internal septa ; valves elliptical, striate, with a median line,
striæ moniliform.
The present genus first presents us with a mode of increase in the
dimensions of the frustule, which we shall afterwards find occurring
in other filamentous forms, and which has caused no little perplexity
to the student of these organisms. As this phænomenon has hitherto
frequently escaped the notice of writers on the Diatomaceæ, and the
appearances it produces been misunderstood, it will be necessary to
enter into a somewhat lengthened explanation.
In the species in which this mode of increase occurs, the valves are
similar in character to those of the other Diatomaceæ with which we
are familiar, and are formed during self-division in the manner already
described (Introduction, vol. i. Section V.) ; but instead of the usual
repetition of the process of valve-formation, there detailed, we are
here presented with a subsequent intervalvular development, which,
not confined to the exterior of the frustule, projects a plate of silex
into its interior, forming a septum or partition extending towards,
but not reaching, the centre of the cell, and appearing as a compressed
rim, or annulus of silex, whose outer or larger circumference
follows the exterior outline of the frustule, and whose inner edge
bounds the free space, which serves as a channel of communication
between the chambers into which the cell is thus divided. This
process is either simultaneous, and the frustule definite, or successive,
and the frustule indefinite. In the first case, the annuli of silex
are formed during the production of the valves in the progress of
self-division, and on every repetition of such production ; while in
the second case, the formation of the annuli is continued after the
production of the valves, and is repeated an uncertain number of
times before the recurrence of a new valve-production. The interior
of the frustules in both cases is divided by the annuli into chambers,
communicating with each other by the free spaces left between the
inner margins of the annuli, or by perforations in the septa ; consequently
the frustules, although apparently complex in structure,
still remain unicellular as in the more ordinary forms. This may
easily be seen by the distribution of the endochrome in the living, or
its aggregation into a simple mass in the dead frustule. I t is to be
noted, however, that in the case of those species in which this increase
is indefinite, we do not find the regularity of operation which characterizes
self-division in other species ; the formation of the new valves
taking place with apparent indifference at any part of the elongated
frustule, that is to say, between any two of the annuli. This gives a
want of symmetry to the appearance of these forms, and enables
the observer to detect their true character. On a front view, the
septa, which project into the cell, are seen as costæ, extending from
the margin towards the centre of the frustule ; and on a side view,
the spaces intervening between the convex interior edges of the septa,
or the perforations in the septa when such exist, appear as circular
or elliptical outlines traced upon the inner surface of the valve. These
appearances have received the name of “ vittæ ” from Kützing, and
“ canals ” from Meneghini, but either term appears to be unsuitable if
their true nature is considered. Ralfs, who was cognizant of the true
character of the septa, but who does not appear to have apprehended
the mode of their formation, accurately describes the appearances to
which they give rise (Transactions of the Botanical Society of Edinburgh,
vol. ii. p. 22), as “ produced by siliceous plates arising internally
from the margin of the filament, and extending towards, but
not reaching the centre. The interior is thus divided into chambers
opening into a central space. When viewed laterally, the central space
has the appearance of a canal, especially as the inner edge of each plate
has a concave outline. This appearance is more striking in Tabellaria,
Striatella toeniæformis [= Grammatophora marina, Kütz.), and
Tetraeyclus lacustris, where all the plates are nearly equal ; but in
Striatella arcuata ( = Rhabdonema arcuatum, Kütz.) and Tessella Catena