I
The new valves formed during the process of self-division ordinarily
appear exact counterparts in form and size of the valves
of the original frustule; but a careful examination of the process
in the filamentous species has led me to conclude that a
slight enlargement occasionally takes place in the new valves,
thus causing a widening of the filament.
This increase is however so small, that in a filament of many
hnndi'ed frustules, the enlargement is scarcely appreciable.
The rapid attenuation represented by some authors in the filaments
of the Fragilarics must therefore be attributed to the
deceptive appearance presented by a compressed band when
slightly twisted, the semblance of attenuation being thus given
to the portions which are presented in an oblique direction to
the eye of the observer.
The increase in the new valves, although slight, will however
sufficiently account for the varying breadth of the bands in the
filamentous species, and the diversity of size in the frustules of
the free forms, without obliging us to suppose that a growth or
aggregation takes place in the siliceous valve when once formed.
Starting from a single frustule, it will be at once apparent, that
if its valves remain unaltered in size, while the cell-membrane
experiences repeated self-division, we shall have two frustules
constantly retaining their original dimensions, four slightly
increased, eight somewhat larger, and so on, in a geometrical
ratio, which will soon present us with an innumerable multitude
containing individuals in every stage, but in which the larger
sizes preponderate over the smaller; and such are the circumstances
ordinarily found to attend the presence of large numbers
of these organisms.
It is hardly necessary to remind the student acquainted with
the general laws of the vegetable kingdom, that this multiplication
of the Diatomaceous frustule is not a true reproduction
of the species, but merely a growth or extension of the original
frustule with which we may suppose self-division to originate;
and as the myriads of cells which constitute the entirety of the
largest sea-weed, are regarded in their aggregation as forming
but one plant, so may we consider the thousand millions into
which a single month’s growth has multiplied the original frustule,
as belonging to the individuality of the first organism in
the Diatomaceous chain of being.
With the free species of the Diatomaceæ the analogy may
seem a forced one ; but its correctness is at once apparent, if
we ascend to more complex combinations in the filamentous
and frondose genera, no very obvious difference existing between
the union of cells in Fmgilaria and those of many species
of the Conferva ; or their aggregation in Schizonema, and
various forms of the larger Algæ. The most important distinction
between the combination of cells in the two cases, is the greater
readiness with which the connection is dissolved in the Diatomaceæ
and the retention of independent life in each separated
cell ; but as- we descend in tbe scale to the lower Algæ, and
approach the Ulvacea or Palmellacea, even this difference disappears,
and the smallest portion of the frond, even a simple
cell, in these forms enjoys the same independent vitality and
power of increase as the Diatomaceous frustule.
The increase by self-division in the Diatomaceæ is therefore
not a renewal, but a mere extension of individual life ; and the
process has been justly defined by Mr. Thwaites, in the analogous
case of the Palmellacea, as an act of Gemmation rather
than Beproduction.
“ If we duly consider this fact,” remarks this acute and zealous
student of nature, “ how much does it exalt the lower tribes
of plants in our estimation ! since we may contemplate an individual
plant of theni not as the single phyton—not as the single
frond—not as the single cell— but as the aggregate, it may be,
of thousands of these view it occupying as much space, and
exercising as great an influence in the oeconomy of nature, as
the largest forest tree.” (Annals of Natm-al History, 2nd Series
voL ii. p. 315.)
S e c t io n VI.
T e r m in o l o g y a n d C l a s s i f ic a t io n .
The circumstances attendant upon the self-division of the
Diatom fmmish means for fixing the terminology applied to the