the shells.” (Eroin Sillimaii’s Journal for May 1851, quoted
in Ann. Nat. Hist. 2nd Scr. vol. viii. p. 157.)
The usual colour of the living frustule I have stated to be a
yellow, with a brown or greenisli shade of a greater or less intensity
; this colour is found to arise from the contents of the
cell, which consist of a mucilaginous fluid, in which float numerous
minute grannies, generally accumulated in thin layers towards
the internal coll-walls. MTien the frustule is so tiu-ncd that this
layer of endochrome is presented edgeways to the eye, the gra-
nides appear to be chiefly aggregated into two plates, applied to
the opposite sides of the frustule; and when self-division is in
progress, and the cell-contents are divided into two portions,
such a separation or temporary aggregation of the endochrome
must necessarily ensue; but in the simplest condition of the
frustule the contents are diffused over the entire surface of
the cell-walls, precisely as may be seen in the cells of many
of the lai-ger Algae, or of some water plants of a higher order,
as in the leaves of Hydrocharis Morsics-ranm and others.
The endochrome of the Diatomaceous frustule is not, however,
spread with uninterrupted regularity over the entire wall of the
cell; at a spot exactly in the centre of the frustule it is usually
absent, being aggregated in a narrow line or ring around
a circular space occupied by a transparent vesicle, the nucleus
or cytoblast of the cell, which frequently encloses an evident
nucleolus. Nor are we to regard the endochrome as being
always confined to the surface of the cell-wall. It is often diffused
throughout the entire cavity, in lines radiating from the nucleus.
Such a disposition frequently occurs in Pleurosigma
and others, and is invariably found in Navicula elliptica. (See
Frontispiece, fig. CLH.)
In most species several smaller globules, in addition to the
central vesicle, are found occupying certain fixed positions within
the cell; these are probably special secretions from the cell-
contents, of an oily nature ; and I am disposed to attribute to
them the origin of the peculiar odour exhaled from the frustules
in the process of burning. The number of these globules is
frequently four, often placed near the extremities, or, more rarely,
clustered around the central vesicle. Professor Kiitzing has examined
these globules with the greatest care, and has convinced
himself of their oily nature. He regards them as representing
the amylaceous secretions of the Desmidieæ and Confervæ, and
the starch-granules of the cotyledons of the higher vegetables.
(Bacillarien, p. 23.)
A distinct movement of the granular particles of the endochrome,
closely resembling the circulation of the cell-contents in
Closterium Lunula, noticed by Mr. Ralfs (British Desmidieæ, Preface,
p. xxi.), and which I have frequently detected in the same
species, has occasionally fallen under my notice in some of the
larger species of Diatomaceæ.
In Surirella biseriata, Plate VIII. fig. 57, this motion has
been more especially apparent ; but I have also observed it take
place in NitzscMa scalaris, Plate XIV. fig. 115, and Campylo-
discus spiralis, Plate VII. fig. 54.
This circulation has not, however, the regularity of movement
so conspicuous in the Desmidieæ, and is of too ambiguous
a character to fiumish data for any very certain conclusions,
save one, viz. that the Diatom must be a single cell, and
cannot contain a number of separate organs, such as have been
alleged to occupy its interior; since the endochrome moves
freely from one portion of the frustule to another, approaching
and receding from the central nucleus unimpeded by any intervening
obstacle.
Of the chemical composition of the endochrome, I am at present
unable to offer a satisfactory analysis. Professor Frankland,
of Manchester, who kindly consented to direct his attention to
the chemical constituents of these organisms, but whose engagements
have hitherto prevented him from completing the necessary
investigations, informs me that a large amount of u'on exists
in the state of a silicate or protoxide in the siliceous epiderm ;
and he attributes to its presence the brown colour which is
assumed upon exposing the Diatoms to the influence of a
moderate heat, the protoxide of iron, by the gradual absorption
of oxygen, being converted into brown peroxide of iron, which
assumes a redder tinge upon being more strongly heated.
Of the behaviour of the endochrome under the influence of
certain reagents, I may merely remark at present, that dilute