The individuals--each known as a _frustule_--assume very various
forms and may be unicellular or multicellular, but present no
differentiation; many similar cells may be connected in chains,
embedded in mucilaginous masses, or attached to mucilaginous
stalks. The cells are bilateral or centric, often asymmetrical,
slightly dorsiventral and have no cilia; those living in the free
condition have the power of sliding upon a firm substratum. The cell
contains 1 cell-nucleus and 1–2 plate-shaped or several disc-shaped
chromatophores. The colouring material “_Melinophyl_” contains, in
addition to a modification of chlorophyl, a brown colouring matter,
_diatomin_. 1 or 2 pyrenoids sometimes occur. Starch is wanting and the
first product of assimilation appears to be a kind of oil (?).
[Illustration: FIG. 14.--_Pinnularia_: _B_, from the edge, shows
the valves fitting together; _A_, a valve.]
[Illustration: FIG. 15.--Various Diatomaceæ. A _Diatoma vulgare_.
B _Tabellaria flocculosa_. C _Navicula tumida_ (lateral views).
D _Gomphonema constrictum_ (lateral views). E _Navicula
west[=i][=i]_ (lateral views).]
The cell-walls are _impregnated with silica_ to such a degree that
they are imperishable and are therefore able to contribute in a great
measure to the formation of the earth’s crust. The structure of their
cell-wall is most peculiar and _differs from all other plants_ (except
certain Desmidiaceæ); it does not consist of a single piece but is
made up of two--the “shells”--(compare _Exuviella_ and _Prorocentrum_
among the Dinoflagellata) which are fitted into each other, one being
a little larger than the other and embracing its edge, like a box with
its lid (Fig. 14 _B_). The two parts which correspond to the bottom
and lid of the box are known as _valves_. Along the central line of
the valves a longitudinal _rib_ may often be found, interrupted at
its centre by a small cleft (perhaps homologous with the cilia-cleft
of the Dinoflagellata), through which the protoplasm is enabled to
communicate with the exterior (Fig. 14 _A_). It is principally by
reason of the valves, which bear numerous fine, transverse ribs, striæ
or warts, etc. (Figs. 14, 15, 17), that the Diatomeæ have become so
well known and employed as test objects in microscopical science. When
the division takes place, the two shells are separated a little from
each other, and after the cell-contents have divided into two masses,
two new shells are formed, one fitting into the larger valve, the
other one into the smaller valve of the original frustule. The latter
cell (frustule) is thus, upon the whole, smaller than the mother-cell,
and as the cells do not increase in size, some frustules are smaller
than the ones from which they are derived, and thus, by repeated
divisions, it follows that smaller and smaller frustules are produced.
This continued diminution in size is, however, compensated for by the
formation, when the cells have been reduced to a certain minimum, of
_auxospores_, 2–3 times larger. These may either be formed _asexually_
by the protoplasm of a cell increasing, rounding off and surrounding
itself with a new wall (_e.g. Melosira_) or after _conjugation_,
which may take place with various modifications: 1. Two individuals
unite after the secretion of a quantity of mucilage, and the valves
then commence to separate from each other, on the side which the two
individuals turn towards each other. The protoplasmic bodies now
release themselves from their cell-wall, and each rounds off to form an
ellipsoidal mass; these two protoplasmic masses (gametes) coalesce to
form a zygote, the cell-nuclei and chromatophores also fusing together.
The zygote increases in size, and surrounds itself with a firm, smooth,
siliceous wall--the _perizonium_. The auxospores, whichever way they
arise, are not resting stages. The germination of the zygote commences
by the protoplasm withdrawing itself slightly from the cell-wall and
constructing first the larger valve, and later on the smaller one;
finally the membrane of the zygote bursts (_e.g. Himantidium_). 2.
The conjugation occurs in a similar manner, but the protoplasm of the
cells divides transversely before conjugation into two daughter-cells.
Those lying opposite one another conjugate (Fig. 16) and form two
zygotes. The formation of the perizonium, and germination take place
as in the preceding instance (_e.g. Epithemia_). 3. Two cells place
themselves parallel to each other, and each of the two cell-contents,
without coalescing, becomes an auxospore. The formation of the wall
takes place as in the preceding case. This is found in the Naviculeæ,
Cymbelleæ, the Gomphonemeæ (_e.g. Frustulia_, _Cocconema_).
[Illustration: FIG. 16.--Conjugation of _Cymbella variabilis_.
_A_, The protoplasm in the two cells has divided into two masses;
_B_ these masses coalesce in pairs; the cells (_B C_) enclosed
in a mucilaginous matrix. _C D_ Auxospores and their formation.]
The Diatomaceæ may be found in salt as well as in fresh water (often
in such masses that the colour of the water or mud becomes yellow or
brown; in the same manner the genera _Chætoceros_, _Rhizosolenia_,
_Coscinodiscus_, and several others, form large slime-masses,
“Plankton” on the surface of the sea), on damp soil and in dust blown
by the wind. They occur as fossils in the recent formations, often
in large deposits (siliceous earth, mountain meal), as in the cement
lime in Jutland, the alluvial deposits beneath Berlin, in clay strata
beneath peat bogs, in guano, etc. These accumulations of fossilized
diatoms are used in the manufacture of dynamite and in various
manufactures.
The Diatomaceæ appear nearest to, and must be placed as a group
co-ordinate with the Dinoflagellata, as they doubtless may be supposed
to derive their origin from forms resembling _Exuviella_, and to have
lost the cilia. The resemblances to the Desmidiaceæ which are striking
in many respects, can only be conceived as analogies, and cannot be
founded upon homologies, and it is therefore impossible to regard them
as proof of genetic relationship. The family contains only one order.
[Illustration: FIG. 17.--Various Diatomeæ. _A Synedra radians._
_B Epithemia turgida_ (from the two different sides). _C Cymbella
cuspidata. D Cocconeis pediculus_ (on the right several
situated on a portion of a plant, on the left a single one more
highly magnified).]
Order 1. =Diatomaceæ.= This order may be divided into two
sub-orders, viz.--
Sub-Order 1. =Placochromaticæ.= The chromatophores are discoid, large,
1 or 2 in each cell; the structure of the valves is bilateral and
always without reticulate markings. The following groups belong to
this sub-order: _Gomphonemeæ_, _Cymbelleæ_, _Amphoreæ_, _Achnantheæ_,
_Cocconeideæ_, _Naviculeæ_, _Amphipleureæ_, _Plagiotropideæ_,
_Amphitropideæ_, _Nitzchieæ_, _Surirayeæ_, and _Eunotieæ_.
Sub-Order 2. =Coccochromaticæ.= The chromatophores are granular, small
and many in each cell. The structure of the cells is zygomorphic
or centric, often with reticulate markings. The following groups
belong to this sub-order: _Fragilarieæ_, _Meridieæ_, _Tabellarieæ_,
_Licmophoreæ_, _Biddulphieæ_, _Anguliferæ_, _Eupodisceæ_,
_Coscinodisceæ_, and _Melosireæ_.