CHARACEÆ.
[Illustration: Fig. 40. Nitella flexilis.]
THE Characeæ are submerged annual water plants, growing in stagnant
pools and ditches rather than in running streams. It is a small order
containing but three genera, but the numerous species are dispersed all
over the world, especially in temperate climates. The genera found in
this country are Nitella and Chara. The Nitella flexilis (fig. 40) may
be taken as a representative of the order. Its stem or axis is formed of
very long cylindrical transparent tubes, joined by their flat ends, and
surrounded at each junction by a whorl of long tubes which are forked or
trifid at their extremities. In some species the branches are jointed,
and have whorls precisely like those on the main axis. On the internal
surface of the tubes, which are sometimes several inches in length,
there are four longitudinal bands parallel to the axis of the tubes,
which are occasionally twisted: two of these bands are broad and covered
with oval green particles; while the other two are narrow, transparent,
and colourless. Each tube is filled with a limpid semifluid liquid, in
which pale green particles and jelly-like globules of a starchy nature
float; and when these particles are watched with a microscope, they show
that, in every tube of the plant, a continual current of that liquid,
with its particles, ascends one of the green bands and descends by the
other, even when the stem and branches are twisted; but they never flow
in the colourless bands, though there is nothing to hinder them.
According to the observations of microscopists there is probably a
gyration of an azotized viscid fluid in all plants originating in, and
maintained by, vital contractility of structure, but in none is it so
evident as in the Characeæ. Its rate is increased by heat and diminished
by cold, like the circulation of the blood in animals, because the
activity of the vital energy bears a precise relation to the quantity of
heat received. The gyration is instantly arrested by a shock of
electricity.
The reproductive organs of the Characeæ are of two kinds, both growing
in the axils of the branchlets, namely, dark-red globules, which are
antheridia, and nucules or pistillidia, which contain germ cells.
Sometimes they are found in different individuals, but in most of the
Nitellas they are in the same individual, the globules being placed
closely below the nucules, as in fig. 41, A, B. The envelope of the
nearly spherical globules is formed of eight spherico-triangular valves.
From the middle of the interior surface of these valves, a perpendicular
orange-coloured column extends to the centre of the globule, where its
summit is crowned with a mass of confervoid filaments, which are formed
of a linear succession of minute cells; while from the base of the
column, bands of orange-coloured spherules imbedded in gelatine radiate
along the interior surface of the valve to its margin as shown at C, in
the same figure. After successive changes in the matter within the
confervoid filaments, (fig. 42, D-G), the microscope shows that ‘in
every one of the cells there is formed a spiral thread of two or three
coils, which, at first motionless, after a time begins to move and
revolve within the cell; at last the cell wall gives way, and the spiral
thread makes its way out, partially lengthens itself, and moves actively
through the water in a tolerably determinate direction, by the lashing
action of two long and very delicate filaments with which it is
furnished’[65] (fig. 42 H).
[Illustration: Fig. 41. Antheridia of Chara fragilis:—A, antheridium
developed at base of nucule; B, do., the nucule enlarged, and the
antheridium laid open by the separation of its valves; C, one of the
valves, with its group of antheridial filaments.]
The nucule is an ovoid sac with five long cells spirally twisted round
it, the sac being full of a viscous fluid containing globules of starch
and oil. This nucule falls off when fertilized by the spirally-coiled
ciliated bodies, and then germinates.
The Characeæ may also be artificially reproduced by cuttings; while
under favourable circumstances they are reproduced by nodular bodies
rich in starch grains, which arise from the creeping root, and also by
clusters of cells called bulbils filled with starch grains, which arise
from a modification of the nodes.
The Charas, properly so called, are monœcious or diœcious, more or less
opaque and brittle, their many-jointed tubular stems bearing whorls of
long slender awl-shaped branchlets. The fruit, accompanied by a cluster
of short bracts or ramuli, is placed at intervals in the hollow side of
the branchlets, one at each joint. In the bristly Chara the awl-shaped
branchlets are simple, pointed at the extremity, and composed of about
seven joints, with a whorl of from four to seven short bracts, and
having fruit at each articulation. Many of the species of Characeæ are
thickly incrusted with lime, and the whole order is exceedingly fetid.
[Illustration: Fig. 42. Antheridia of Chara fragilis:—D, E, F,
successive stages of formation of spermatozoids in the linear cells of
the antheridial filaments; G, escape of mature spermatozoids, which are
shown detached at H.]