ALGÆ.
THE principal objects in the study of plant-life are the organs by means
of which they obtain and assimilate substances that are essential for
their nourishment and growth, and those by which the perpetuity of their
race is maintained and their type transmitted from age to age. In the
lowest group of plants, represented by the Algæ, which come first into
consideration, the two properties are combined; in the highest they are
distinctly different, but the progress from one to the other may be
traced through an ascending series of vegetable structure. In the simple
grades of vegetables, the primordial cell frequently constitutes the
whole plant; it appears first, and then envelopes itself with a coat
either of cellulose or of a gelatinous substance.
Many instances of this are to be found amongst the Algæ, which are all
aquatic plants, and are found growing either attached to other bodies,
or floating independently, and live, some species in fresh water, and
others in the sea and its estuaries. The Algæ absorb carbonic acid and
give out oxygen, under the influence of sun-light, exactly as do the
flowering plants; and the quantity of oxygen disengaged by them is said
to be enormous.
Before proceeding to trace the structure and development of the Algæ, it
may be desirable to indicate something of the classification of this
curious group of plants. As already stated, they are without exception
aquatic plants. They comprise three distinct orders, the Chlorospermeæ,
having green spores; the Rhodospermeæ, having red spores; and the
Melanospermeæ, having olive-coloured spores. These groups embrace all
the varied plants known as sea-weeds, as well as the cellular plants
which are developed in fresh water.
The Chlorospermeæ are separable into three groups, namely, those which
are simply cellular, including the Palmelleæ, the green Desmidiaceæ, and
the yellow-brown silicious-coated Diatomaceæ; those which are
filamentous, called generally confervas, and including the true
Confervaceæ, in which the threads have no compound axis, the
Batrachospermeæ, in which the threads are partially incorporated with an
axis, the Nostochineæ, in which the slender moniliform threads are
invested with a mucous or gelatinous mass, the Oscillatoriæ, and some
others; and those which are foliaceous, comprising the Ulvaceæ. All
these are monœcious plants, whose reproductive bodies are zoospores
provided with ciliary appendages, or motionless cysts filled with
endochrome, true spermatozoids being rarely present.
The Rhodospermeæ divide primarily into two groups defined by the nature
and position of their spores: one having the spores indefinite, produced
within mother cells; the other having the spores single in the upper
joints of the threads of the nucleus. The first group includes the
Ceramiaceæ, which are filiform articulate plants, with the nucleus
naked, and the Rhodymeniaceæ, which are compound inarticulate plants,
with the spores generated within the cells of moniliform threads. The
second group includes, amongst others, the Rhodomeliaceæ and the
Laurenciaceæ, the former articulate, the latter inarticulate, and both
bearing terminal spores, and having the nucleus conceptacular. To this
group also belong the calcareous Corallinaceæ and the cartilaginous or
membranaceous Sphærococcoideæ. The plants of this group are diœcious,
with two kinds of fruit, spores and tetraspores, and they bear
antheridia filled with active spermatozoids.
The Melanospermeæ divide into two series, the articulate and
inarticulate. The former comprise the Ectocarpeæ, which are filiform
plants with external cysts, and the Chordariæ, which are interlaced
cylindrical plants with immersed cysts. The latter include the
Laminariæ, flat, often strap-shaped, sometimes gigantic plants, having
the spores superficial and indefinite, and the Fucaceæ, which constitute
a large proportion of the shore-weeds of our seas and estuaries, and
which bear their spores in elliptic or spherical conceptacles sunk in
the frond. The Melanospermeæ are either monœcious or diœcious, and
spermatozoids are general amongst them, though occasionally propagation
is effected by means of zoospores resembling the spermatozoids.
Having thus indicated the several groups of the great Algal family,
their structure and development will now be traced, commencing with the
most simple forms, which occur among the CHLOROSPERMEÆ.
Spring water absorbs oxygen, nitrogen, and a large proportion of
carbonic acid gas from the earth and the atmosphere, without losing its
limpidity, but notwithstanding this apparent purity, if exposed for a
time to the sun, green slime appears, and this the microscope shows to
be full of globules or vesicles filled with green matter—the primordial
cell in its earliest form. No green slime is formed in spring water if
kept in darkness, so solar light is the principal agent in this growth,
which is by no means a spontaneous birth; it is merely the development
of one or more of the many kinds of germs, invisible to the naked eye,
that exist in the earth, air, and water in myriads, waiting till
favourable circumstances enable them to germinate.
[Illustration: Fig. 6. Palmoglœa macrococca:—A, full grown cell; B-E,
successive stages of binary division; F, row of cells produced by a
succession of subdivisions; G-I, cells treated by iodine; K-M, cells in
conjugation.]
The slime that covers damp walls or stones, and moist cliffs or rocks in
the sea, also the slime or mucus that sometimes swims on the surface of
water, are said by M. Bory de St. Vincent to be provisional creations
waiting to be organized. Of this the conferva, Palmoglœa macrococca
(fig. 6), is an example. It is a green slime covering damp places,
consisting of microscopic primordial cells, each of which is surrounded
by a gelatinous envelope, and filled with green granular matter
occasionally concentrated into a nucleus. This singular plant is
propagated in two different ways. The endochrome or green matter within
the cell spontaneously divides into two equal parts, the thin coat of
the cell bends round the two ends, separates them, then each half takes
a globular or ovoid form, and secretes a gelatinous substance round
itself which completes the separation, so that they form two distinct
and independent plants, in every respect similar to that from whence
they were derived. After a little time, each of these plants undergoes a
similar bisection, so that four new plants are formed with their
gelatinous envelopes; by the same process eight are produced and so on
indefinitely, the organ of nutrition being the same with that of
reproduction. Again, the membrane or film that covers each of these
primordial cells is so thin and soft, that occasionally two adjacent
cells of the series unite into one mass by a fusion of their sides and
internal matter, which is then coated by a membrane, and after various
internal changes becomes a spore which terminates a generation. By and
by the spore germinates, produces a green primordial cell which secretes
a gelatinous coat, and becomes by the process of bisection the parent of
a new generation, which terminates by the union of two adjacent cells to
produce a spore, a cycle of alternate modes of reproduction that may be
continued till ended by some external circumstance, as the cold of
winter.
When the matter in two adjacent cells joins to form a spore, it becomes
granular, and mixed with minute particles of oil, which unite in a drop;
and the spore, which is at first green, gradually assumes a yellow brown
colour; conversely when the spore begins to grow, the oil disappears,
and the green matter takes its place. This is a frequent occurrence
during the formation of spores in this class of plants, for the
endochrome or internal matter,—which consists of a small variety of
elements probably in a state of unstable equilibrium or change,—is
easily decomposed and recombined into new substances by chemical action,
but the bisection of the cells of the Palmoglœa so as to form new
individuals is probably owing to heat alone. There is no apparent
difference between the cells selected to produce spores by their union,
and the others. It seems that in every plant certain cells are reserved
for certain purposes. Professor Karsten conceives the nucleated cells to
be reserved for reproduction, while those destitute of nuclei are
designed for secretion.
The Protococcus pluvialis (fig. 7), one of the unicellular Confervæ, is
frequently met with in rain-water cisterns. The spore of the plant (fig.
7 A) is a globular primordial cell invested with a double coat of
cellulose, sometimes separated by an aqueous fluid, sometimes not. The
cell is filled with protoplasm, a colourless watery liquid in which red
and green particles are scattered. When this spore begins to grow, the
endochrome, or solid matter in the primordial cell, divides
spontaneously into two similar and equal parts, round at one end, and
tapering to a point or beak at the other, each being coated by a very
thin film of the transparent colourless protoplasm.
[Illustration: Fig. 7. Protococcus pluvialis:—A, encysted cell; B, C,
cells divided into two; D, cell divided into four; E, cell divided into
eight; F, cell divided into thirty-two; G, escaped motile gonidia; H-L,
primordial utricles furnished with cilia.]
After various changes, the green matter with several red spots is
condensed into the thick round half, while the tapering beaked part is
left transparent, being only filled with the watery liquid. Both bodies
are then coated with cellulose, and two vibratile filaments called
cilia, from their resemblance to eye-lashes, proceed from a point near
the beak. The whole of these changes take place while the two bodies are
still within the common cellulose covering; the moment they come out of
it, by a rupture in the cell-wall, they swim about with the greatest
velocity by means of their cilia, which lash the water so rapidly that
they are invisible even with a microscope. The activity of these
zoospores, as they are called, continues for about an hour and a half;
the motion then becomes gradually less rapid; the cilia may now be seen,
and soon fall off; then the bodies acquire a firmer coat of cellulose,
and sink to the bottom of the water, where they remain at rest as still,
or winter spores. There is great variety in the Protococcus, for the
matter in the primordial cell sometimes divides not only into two equal
and similar parts, but into 4, 8, 16, 32 equal and similar parts
consecutively; each brood is developed into zoospores, which ultimately
become resting spores.
When a spore is to be formed in a primordial cell, the starch and green
matter condense into a nucleus in its centre, and a membrane envelopes
the liquid and the nucleus within it, so that a spore in its first stage
is a free and independent cell containing azotized matter swimming in a
formative liquid. If the spore is to be motile it remains of a green
colour, and gets cilia; but if it is to be a winter spore, the internal
matter forms into granules, mixed with particles of red oil, which
coalesce into a drop, and it generally undergoes the same
transformations as those which take place after the conjugation or union
of two adjacent cells into one as already described. The zoospores may
lose their cilia, fall to the bottom of the water as green spores, and
reproduce a facsimile of the parent plant as buds do, or they may
acquire a cellulose coat, undergo the transformations and change of
colour mentioned, and sink to the bottom of the water as red winter
spores.
Under certain circumstances which do not seem to be perfectly known, it
happens that during the formation of some of the zoospores the green
matter is gradually changed into a red oily substance; they lose their
cilia, acquire by secretion their cell-walls and a mucous envelope, and
float on the water as winter spores. Should they be left dry, they may
remain in that state for an indefinite length of time without losing
their vitality, and as they are extremely small, they are carried by
currents of air into the atmosphere, from whence they are brought down
in the rain, and having fallen occasionally in places where they were
never seen before, have given rise to the idea of spontaneous
generation.
Many cycles may be accomplished from the still cell to the zoospores,
and back again, producing numerous generations from the same plant
before it returns to the red thick-walled cell, which may again be
dormant for an unlimited time. These cycles, however, do not finish the
history of the plant, for there can be little doubt that, in some stage
of its existence, a conjugation of two cells occurs, as in the
Palmoglœa.
Sometimes when the division of the endochrome of the spore of the
Protococcus is successively divided into sixteen parts, or even sooner,
the new cells thus produced get two long cilia, as in fig. 7 H, and are
liberated before they acquire their cellulose coat. This motile
primordial cell soon acquires a bag-like investment (fig. 7 I, K, L,) of
cellulose, through which the cilia pass, and thread-like extensions of
the protoplasm are not unfrequently seen to radiate from the primordial
cell to the surrounding bag, as in fig. 7 I, showing that the
transparent space is only occupied by a watery liquid. The varieties of
this plant are very numerous, and all related to one another. Sometimes
the whole of the matter within the primordial cell of the spore divides
at once into 4, 8, or 16 parts, giving rise to as many minute primordial
cells.
The cilia are extensions of the colourless transparent film which covers
the zoospores, and their vibrations are generally believed to be a
consequence of the vital contractibility of that film, and intimately
related to the changes taking place in the cell on which they are borne.
The persistence of their motions after a cell is detached from a
compound body covered with them, being like the persistence of the
contractibility of muscle fibre after being detached from a living
animal, proves that we must look to a contractile energy in the film of
protoplasm for the maintenance of these curious operations.
It appears that a cell cannot perform two functions at the same time,
and that one must either precede or follow the other. Thus, the
zoospores have two distinct periods of action; the first is that of
mechanical motion alone, which is followed by one of growth and
multiplication, manifestations which, though very dissimilar, are really
modes of action of the same vital energy that formed these bodies while
they were yet in their parent cell. In fact, it seems to be a general
law, that each cell is endowed, altogether or for a time, with its own
mode of action, and is incapable of any other.
Of the Volvocineæ, by some regarded as fresh-water microscopic plants,
the Stephanosphæra pluvialis may be taken as a type. This plant consists
of a colourless transparent globe not more than the 48/1000th of an inch
in diameter, containing eight green primordial cells arranged in a
circle in its equator. Each primordial cell is furnished with a pair of
cilia; these 16 cilia pierce through the hyaline globe, and by their
vibrations, they make it rotate about an axis perpendicular to the plane
of its equator, and move actively through the water. Each of the
primordial cells, which are green with a spot of red in the centre,
secretes a cellular covering, and they swim about in the interior of the
globe as free cells. Eventually they escape either by fissure of the
globe, or by its gradual dissolution. After swimming about for a short
time they become motionless, lose their cilia, and sink to the bottom as
green still spores.
If, after being dried, water be poured on one of these green still
spores, it takes up the water, its contents become closely granular, and
fill the whole membrane of the spore. Then it divides, first into
halves, then into quadrants or heart-shaped segments, meeting in a point
in the centre of the membrane. These quadrants are ultimately divided
into 8 wedge-shaped segments, whose contour lines, like the spokes of a
wheel, meet in the centre, and each gets a pair of cilia. The coloured
matter is driven back in each individual towards the thick end of the
wedge as if by centrifugal force, and a colourless plasm remains in the
points or beak. These disappear, a cavity is formed in the centre of the
disc, the eight bodies assume the form of a wreath in close contact, and
the original cilia, which continue to vibrate, cause the rotatory and
progressive motion of the whole organism.
Sometimes the eight globular bodies have been seen to divide into a
number of extremely minute motile cells, while yet within the parent
globe. These gonidia, as they are called, are, with a few exceptions
which may reproduce the plant, believed to perish when they come into
the water.
The division of the primordial cell of this plant is confined to a
certain time of day; it begins towards evening, and is completed the
following morning, and according to Mr. F. Currey, the exact time is the
same in Lapland, where there is no night, and at Berlin in spring when
the day and night are almost equal. The fertility is enormous. It is
calculated that in eight days, under favourable circumstances,
16,777,216 families of the Stephanosphæra pluvialis may be formed from
one resting spore.
The transmutation of chlorophyll in the Protococcus and Volvocineæ, from
green to red and vice versâ, which so frequently occurs in the lowest
class of plants, shows that its molecules must be united by very feeble
affinities, and easily converted into new combinations either by direct
chemical action, or by other substances also in a state of change.
[Illustration: Fig. 8. Volvox globator.]
The Volvocineæ consist of various species according as the internal
matter of the primordial cell divides into 2, 4, 8, 32, or a greater
number of equal parts, forming respectively as many free cells which
ultimately become ciliated spores, by means of which the globe either
rotates on the spot, or in straight lines. The Volvox globator found in
fresh-water pools is one of the most remarkable of these, both for its
peculiarity and beauty of structure and for its comparatively large
size, since in some lights it is visible to the naked eye while swimming
in a drop of water. When viewed with a microscope, it is a pellucid
sphere whose surface is studded with green spots, often connected by
green threads; each of the spots has two cilia, so that the surface is
bristled with these filaments, whose vibrations give the sphere either a
rolling or smooth motion, or make it spin like a top in the same place.
In the interior of the sphere there are from two to twenty dark green
globes of different sizes; the smaller are attached to the internal
surface, while the larger rotate freely by their cilia in the internal
cavity. After a time the sphere bursts open and its inhabitants swim
forth, and soon assume the form and character of that which gave them
birth.
The growth and development of the Volvox globator are peculiar, for in
the primordial cell the red and green endochrome breaks up into numerous
angular masses, and a central globe rather larger than the rest. The
angular masses are connected by green threads, the interstices between
all the bodies are filled with a hyaline substance secreted from their
surfaces, and the whole is enclosed in a distinctly membranous globular
envelope.
As this young Volvox increases gradually in size, the hyaline matter is
increased, the green threads lengthen, and the angular masses assume the
form of a flask the 1/3000th of an inch in diameter exactly as in the
Protococcus; for the green matter with a few red spots is collected in
the thick end, while the hyaline beak is turned towards the
circumference of the sphere, which is pierced by their long cilia. Each
of them is invested with a pellucid envelope of considerable thickness,
the borders of which are flattened against those of similar envelopes.
While these ciliated bodies are approaching maturity their endochrome
exhibits vacuoles or apparently empty cavities of a spherical form about
one-third of its own diameter. Mr. G. Busk discovered that these
vacuoles expand and contract at regular intervals of about forty
seconds. The contraction, which almost obliterates the cavity of the
vacuole, is rapid and sudden; the dilatation is slow and gradual. This
action ceases when the body comes to maturity.
When this mass of zoospores connected by green threads is immature and
begins to expand into a hollow sphere, then the central globe is
continually bisected so as to form 4, 8, 16, 32, 64, or a greater number
of equal and similar parts, each of which is ultimately developed into a
zoospore exactly the same with the matured green zoospores on the
surface of the primary sphere, so that the ‘Volvox globator is a
composite fabric made up of a repetition of organisms in all respects
similar to each other,’ which Professor Ehrenberg the first to discover,
though he did not investigate the development of the plant.
It appears that certain spheres of the Volvox are monœcious, that is,
each sphere contains male and female cells, though the greater number of
cells are neutral. The germ or female cells are larger and of a deeper
green than the others; the male cells resemble them, but the endochrome
within them breaks up symmetrically into a multitude of linear particles
aggregated into discoid bundles beset with vibratile cilia, which move
about within their cells and soon become decomposed into their component
corpuscles. Each of these corpuscles has a linear body, thicker at its
posterior end, and furnished with two long cilia. The female cell, when
fertilized, gets a smooth envelope, and then a thicker one, beset with
conical-pointed processes, and the contained chlorophyll gives place, as
in Palmoglœa, to starch and a red or orange coloured oil. It appears
that the Volvox stellatus and V. aureus are only phases of the Volvox
globator.
The Desmidiaceæ are minute green algæ inhabiting fresh-water pools or
slow running streams, never those that are muddy. They are free
unicellular plants, sometimes triangular, sometimes cylindrical,
crescent or bow-shaped, smooth or spined. So varied are their
microscopic forms that a description would be tedious. In plants of such
extreme minuteness, the only means of ascertaining the nature of their
component materials is by chemical tests. A solution of iodine turns
starch blue, and cellulose brown, and thus it is found that the interior
of the Desmidiaceæ is occupied by a mass of starch granules, covered
with chlorophyll, and mixed with a formative fluid. This mass, enclosed
in a delicate membrane, constitutes the primordial cell; it has an
exterior coat of firm cellulose, and the whole is more or less enveloped
in a gelatinous substance. Like other plants, when in bright sunshine,
the Desmidiaceæ decompose carbonic acid gas, give off the oxygen, and
assimilate the carbon into chlorophyll.
[Illustration: Fig. 9. Various species of Staurastrum:—A, vestitum; B,
aculeatum; C, paradoxum; D, E, brachiatum.]
These plants are frequently distinguished by projections from their
cellulose coat above their surface, these being sometimes short and
conspicuous, but often projected in spines, which form a beautiful
symmetrical hyaline border round the green internal cell, as shown in
fig. 9. Another peculiarity of the Desmidiaceæ is the appearance of
their being divided into two symmetrical parts by a satural line, as the
name implies, though there is no real division.
Many of the Desmidiaceæ, but more especially the genus Closterium, are
remarkable for having a double circulation of the internal fluid in
opposite directions, maintained by a vital contractile energy. One
current flows between the cellulose horny coat, and the thin film
covering the chlorophyll, while the other spreads in a broad stream in
the contrary direction between the thin film and the chlorophyll mass,
carrying from the latter some of its coloured particles to the
extremities of the frond, where there seems to be a connection between
the two streams.
[Illustration: Fig. 10. Economy of Closterium Lunula:—A, frond showing
central separation; D, frond in a state of self-division.]
The type of the Desmidiaceæ is continued by various modes of bisection,
depending upon the genus and species of the plant. In the Closterium
Lunula, which has an elongated crescent shape, as in fig. 10 A, the
endochrome or internal matter divides into two equal parts, which
retreat from one another at the middle line; and a constriction of the
cellulose coat takes place between them, which increases till it closes
entirely round the extremities, as in fig. 10 D; then one of the halves
remains at rest while the other moves from side to side, and finally
detaches itself from the other with a jerk. In each of these halves a
constriction of the endochrome may be seen, dividing it into an obtuse
and an elongated part, and for some time the circulating fluid flows
round the obtuse end, but the latter gradually assumes the form of the
elongated end, the regular circulation of the fluid is established, and
in five or six hours after the separation, two young desmids are formed
precisely similar to their parent, the Closterium Lunula.
The Cosmarium, another Desmid, consists of a cell of two lobes united by
a narrow isthmus. When about to multiply, the isthmus swells into two
globular expansions, separated from each other and from the two lobes of
the cell, by a narrow neck. These enlargements increase and assume the
appearance of half segments of the original cell. In this state the
plant consists of four segments lying end to end, the two old ones
forming the extremes, with the two new ones in the middle. At last, each
of the middle segments gets a new half, which soon acquires the full
size and characteristics of the old one. This process, which is
accomplished in twenty-four hours, is repeated ere long, and being
continued indefinitely, the extreme lobes of the row are thrust farther
and farther asunder, and the whole constricted thread or chain of
Cosmaria is enclosed in a gelatinous sheath. The last two central lobes
contain no portion of the original frond or plant, and may thus be
considered to be entirely new individuals.
Many of the Desmidiaceæ multiply by the subdivision of their endochrome
into a multitude of granular particles called gonidia, which are set
free by the rupture of the cell wall, and of which every one may develop
itself into a new cell. The gonidia may be zoospores with cilia and
active locomotion, or they may be enclosed in a firm envelope, and
become resting spores. The movement of the zoospores at first within the
cavity of the cell which gave them their origin, and afterwards
externally to it, has frequently been observed in the varied species of
the genus Cosmarium, and has been described under the name of the
‘swarming of the granules,’ from the resemblance of the moving mass to a
swarm of bees. Their subsequent history is unknown.
In the Pediastrum, a plant consisting of a cluster of cells, the
zoospores are not emitted separately, but those formed by the
subdivision of the endochrome of one cell into 4, 8, 16, 32, or 64
parts, escape from the parent plant still enclosed in the inner tunic of
the cell, and it is within this that they develop themselves into a
cluster resembling that in which they originated.
Mr. Thwaites discovered that the Desmidiaceæ are also propagated by
conjugation, which would be impossible if the hard coat of the adjacent
cells about to unite did not split open; then the whole endochrome in
one cell passes into and blends with that in the other cell, so as to
form one mass, which soon acquires a delicate membranaceous envelope. At
first the mass consists of granular green matter, but when the membrane
becomes thicker, it changes to brown or red. This body, which is called
a sporangium, is sometimes smooth, sometimes granular, covered with
tubercles or rough with spines, according to the nature of the original
plants. The filamental species are propagated by conjugation, but the
subsequent history of the produce is still obscure, though there is
reason to believe that they give rise to plants of different forms,
while all the other modes of increase only reproduce a facsimile of the
parent.
Desmidiaceæ exist in America, but their distribution is little known. In
Europe, their maximum seems to be in the south of England. They abound
in small shallow pools that do not dry up in summer, and also on boggy
moors. The larger kinds are spread out as a thin gelatinous stratum at
the bottom of water, or collected in little tufts; others form a dirty
cloud upon the stems and leaves of aquatic plants. They have been found
in a fossil state in flint, their spores have been discovered in the
grey chalk at Folkestone, and the cells of various species of Closterium
and Euastrum are imbedded in the marls of the United States of North
America.
The Diatomaceæ, or Brittleworts, are unicellular microscopic plants so
numerous that there is hardly a spot on the face of the earth, from
Spitzbergen to Victoria Land, where they may not be found. They abound
in the ocean, in still and running fresh water, and even on the surface
of the bare ground. They extend in latitude beyond the limits of all
other plants, and can endure extremes of temperature, being able to
exist in thermal springs, and in the pancake ice in the south polar
latitudes. Though much too small to be visible to the naked eye, they
occur in such countless myriads as to stain the berg and pancake ice
wherever they are washed by the swell of the sea; and when enclosed in
the congealing surface of the water they impart to the brash and pancake
ice a pale ochreous colour.
Although the diatoms have a vast variety of forms, they all consist of a
simple primordial cell whose external coat of cellulose is so deeply
interpenetrated with silex that it is indestructible, a structure which
constitutes the peculiar characteristic of the tribe. This primordial
cell, as in other plants, contains organizable liquid or protoplasm,
through which golden-brown granules are pretty regularly distributed,
except in the centre, where they are collected into a nucleus. Round
this nucleus they commonly form a ring from which radiating lines of
granules diverge to the interior wall of the cell. In each of these
there is a double current of granules, similar to the circulation in the
Desmidiaceæ; it was discovered by Professor Smith in some of the
comparatively large diatoms. At times oil globules are seen in the
protoplasm. The golden-brown matter is supposed to be chlorophyll, whose
green tint has been changed by the presence of iron, which is
assimilated in this group. Such is the internal structure of a race of
plants altogether invisible to the naked eye. Their external forms,
reproduction and movements, are no less wonderful.
The silicious envelope of the simple cell of a Diatom or frustule, as a
single plant is usually called, consists of two valves or plates,
commonly of the most perfect symmetry, closely applied to each other
along a line of junction like the two valves of a bivalve shell, and
each valve being more or less concavo-convex, a cavity is left between
the two which is occupied by the golden-brown cell described above. The
form of the cavity differs greatly, for sometimes each valve is
hemispherical, so that the cavity is globular; sometimes it is a small
segment of a sphere, resembling a watch-glass, so that the cavity is
lenticular; in short, the form of the cavity depends upon that of the
valves, which may be heart-shaped, or much elongated, square,
triangular, boat-shaped, or furnished with outgrowths, which, however,
is rare.
[Illustration:
Fig. 11.—A, Diatoma vulgare:—_a_, side view of frustule; _b_, frustule
undergoing self-division.
B. Grammatophora serpentina:—_a_, front and side view of single
frustule; _b_, front and end view of divided frustule; _c_, frustule
about to undergo division; _d_, frustule completely divided.
]
The diatom or frustule is considered to present its front view when the
joint or suture of the valves is turned to the eye, as in fig. 11 B,
_b_, whilst the side view is seen when the centre of either valve is
directly beneath the eye, as in fig. 11 A, _a_. When the diatoms are
young the valves are in close contact, but as they increase in size by a
secretion round their edges, the valves separate from one another, and
the cell membrane which is left exposed is immediately consolidated by
silex, and forms a kind of hoop between the valves, as in fig. 12. This
hoop increases in breadth as the cell increases in length. When the two
valves are circular discs, they are separated by a circular hoop, round
the edges of which water is admitted to nourish the plant; but when the
diatom has an elongated form, the water enters through depressed points
in its extremities which are free from silex.
[Illustration: Fig. 12. Biddulphia pulchella.]
[Illustration: Fig. 13. Pleurosigma angulatum.—A, entire frustule; B,
its hexagonal areolation; C, the same more highly magnified.]
Numerous as these plants are, the valves of each genus have their own
peculiar ornaments, consisting of the most beautiful and symmetrical
designs, which are impressed upon the young valves when they are in a
plastic state. The genus Navicula and others are marked with the finest
striæ, some diagonally, others transversely. Rows of round or oval spots
disposed in parallel lines are peculiar to some; the valves of others
are covered with hexagonal forms of the most perfect structure, as those
of the Pleurosigma angulatum, fig. 13, where A is the magnified diatom,
and B and C its hexagonal areolations, seen under higher and higher
microscopic powers; but the figures on the discoid genera are the most
beautiful of all. There is generally a small ornamented circular space
in the centre of the valves, from whence rays extend to the
circumference, dividing the surface of the valves into eight, ten, or
more equal parts, the alternate segments being differently and highly
ornamented, as in the Actinocyclus undulatus (fig. 14), where A is the
side view, and B is the front view. The Arachnoidiscus Ehrenbergii takes
its name from the likeness of the figures on its circular valves to a
spider’s web. According to the observations of Mr. Shadbolt, each valve
is formed of two superposed layers; on the uppermost of these, which is
a thin horny transparent substance, the spider’s web is engraven; and
the undermost silicious layer, which forms the supporting frame-work, is
like a circular Gothic window. The genus Triceratium, nearly allied to
the preceding in general characters, though differing in having a
triangular shape, has many species in a fossil state, while others are
still existing in the ocean, and in tidal rivers. The Triceratium favus,
one of the largest and most beautifully marked, occurs in the mud of the
Thames, and that of the estuaries of other rivers on our coasts; it is
also frequently found on the surface of uncleaned shells.[32] From the
few examples given, a faint idea only can be formed of the variety and
beauty of the engravings on the diatoms. It had long been doubted
whether those on the valves of Coscinodiscus, Triceratium and others,
were elevations or depressions, but Professor Rood of New York, United
States, has proved them to be depressions by an optical arrangement
which will be useful for the investigation of microscopic forms.
[Illustration: Fig. 14. Actinocyclus undulatus.]
Diatoms increase by spontaneous bisection, by conjugation, and by the
resolution of their endochrome into minute spores, called gonidia. When
bisection is about to take place the cell elongates, the hoop increases
in breadth, the endochrome divides into two equal parts, and the coating
of the cell bends in between them, which gives the diatom the appearance
of an hour-glass. At last they separate, and upon each of the new
surfaces a new silicious half is formed, usually the exact counterpart
of the old one, so that there are two diatoms instead of one; and the
process may be continued indefinitely. In most cases, the new diatoms
thus produced are free and independent. Sometimes, however, they adhere
to one another by a fragment or connecting membrane, and if they happen
to be slender and rectangular, and attached side by side, they form a
slender filament, or if attached by alternate angles they form a zigzag
chain, as in fig. 11.
The Meridion circulare (fig. 15) is a diatom of exquisite beauty,
millions and millions of which cover every submerged stone, twig, or
blade of grass, and even form the mud at the bottom of the streams at
West Point, in the United States of North America. Its frustule or
single diatom is long, slender, and rectilinear, but being broader at
one end than at the other, by continued bisection and adhering to one
another they form a circular, spiral, or flattened helical screw of
several turns. The individual frustules of some marine diatoms have a
precisely similar form, being rectilinear and broader at one end than
the other, but each frustule is attached by its narrow end to the
extremity of branching cellulose stems fixed to sea-weeds or stones, and
by a continuous subdivision of which the stem does not partake, they are
spread out at their free ends like a fan.
[Illustration: Fig. 15. Meridion circulare.]
By continual bisection a diatom is propagated through many generations,
but at some stage or other, owing to an unknown cause, propagation by
conjugation takes place. When two frustules are near to each other, two
little swellings arise in one, which meet two little swellings in the
other opposite to it. These soon unite and elongate, the septum or
division between them is absorbed so that they form two tubes in which
the endochrome of the two frustules becomes mixed, and a spore is formed
in each of the two connecting tubes, which increase in size and change
in form till they resemble in every respect the parent, except in being
much larger. As these young diatoms swell, they split the two parent
frustules, become free, and lay the foundation of a twin series of
generations. In the Fragillaria only a single spore is formed.[33]
In Surirella and Epithemia the manner of conjugation is somewhat
different. In the former the valves of two free adjacent frustules
separate from each other at the suture or line of junction and the two
endochromes are discharged; they coalesce and form a single mass, which
becomes enclosed in a gelatinous envelope, and in time this mass shapes
itself into a frustule resembling that of its parent, but larger. In
Epithemia, however, the endochrome of each of the conjugating frustules
divides at the time of its discharge into two halves; each half of the
one coalesces with each half of the other, and two frustules are formed
which become invested with a gelatinous envelope and gradually assume
the form and markings of the parent frustules, but grow to a much larger
size, for the spore masses have the power of self-increase up to the
time that their envelopes are consolidated. This double conjugation
seems to be the ordinary type of the process among the diatoms.[34] But
these plants multiply also by gonidia. It is thought probable that as
long as the vegetative processes are in full activity diatoms multiply
by self-bisection, but when a deficiency of warmth, of moisture, or of
some other condition, gives a check to these, that they increase by
gonidia, some of which becoming encysted, possess a greater power of
resisting unfavourable circumstances, and thus the species is maintained
in a dormant state till a change enables them to germinate. It is even
thought they may be the origin of distinct species.
A peculiar spontaneous locomotion is exhibited by some diatoms of a long
narrow form, as the Naviculæ, which by a succession of jerks in the
direction of their length, go to a certain distance, and then return
nearly by the same path. The motion of the Bacillaria cursoria is still
more unprecedented. The frustules, which are narrow, lanceolate, and
acute, are joined end to end in a long line by some highly elastic
invisible medium. One of the terminal frustules remains at rest while
all the others slide over it till the line is so much stretched that
they are nearly detached from one another; then they all slide back
again in the same manner, and this alternate motion is continued
indefinitely at regular intervals of time. The velocity of the diatoms
at the free end of the row is very considerable; in the Bacillaria
paradoxa it is 1/200th of an inch in a second; the impetus of one has
been observed to upset and even to push aside a plant as much as three
times its size which obstructed its path. If the frustule at the free
end gets entangled, the fixed frustule takes the lead and continues the
motion till the other is free. Minute particles in the vicinity are
sometimes attracted and dragged after the frustules, sometimes they are
repelled, possibly by some invisible organs; but the whole motion of the
diatoms themselves may perhaps be attributed to the action of light and
heat upon the highly contractile substance, whatever it may be, which
connects their frustules, since their motion is exactly in proportion to
the quantity of light and heat received, for it ceases during darkness,
and is renewed on the return of light; ultimately it may disperse the
individual frustules, which are not more than between the 28/10,000th
and the 84/10,000th of an inch in length and the 4/10,000th of an inch
in breadth.
[Illustration: Fig. 16. Bacillaria paradoxa]
This Bacillaria paradoxa (fig. 16) differs from the preceding species in
its motion; each half of the row of frustules moves in an opposite
direction on each side of a central stationary frustule, and the
alternate motion is so regular as to time, that if in advancing, the
frustules meet with an impediment, they wait till the proper time comes
for their retreat. The jerking motions of the Naviculæ are ascribed by
Prof. W. Smith to forces acting within the plants, originating in the
vital operations of growth, by which the surrounding water is drawn in
at one end of the frustule, and expelled at the other.
Some species of diatoms are so universal that they are found in every
region of the globe; others are local, but the same species does not
inhabit both fresh and salt water, though some are found in brackish
pools. The ocean teems with them. Though invisible as individuals to the
naked eye, the living masses of the pelagic diatoms form coloured
fringes on larger plants, and cover stones or rocks in cushion-like
tufts; they spread over the surface as delicate velvet, in filamental
strata on the sand, or mixed with the scum of living or decayed
vegetable matter floating on the surface of the sea; and they exist in
immense profusion in the open ocean as free forms. The numbers in which
they exist in all latitudes, at all seasons, and at all depths—extending
from an inch to the lowest limit to which the most attenuated ray of
light can penetrate, or at which the pressure permits—are immeasurably
in excess of what we have been in the habit of assuming. Temperature has
little to do with the distribution of diatoms in the tropics; it
decreases with the depth at a tolerably fixed rate till it becomes
stationary. It increases in the polar regions with the depth, and
approaches the standard, which is probably universal, near the bed of
the ocean.
Nothing can exceed the vividness of colour or massiveness of the
endochrome or soft internal matter of the floating diatoms, that matter
which diminishes their specific gravity and makes the plant buoyant
which otherwise would be weighed down by its silicious coat. At those
periods in which the structural and reproductive phenomena proceed most
vigorously, their position in depth must be fluctuating; hence they
approach and vanish from the surface. Their growth is perfected by the
heat and light which penetrates the sea in calm weather.
Diatoms are social plants crowded together in vast multitudes. Dr.
Wallich met with an enormous assemblage of a filamental species of
Rhizoselenia, which is from six to twenty times as long as it is broad,
aggregated in tufted yellow masses, which covered the sea to the depth
of some feet, and extended with little interruption throughout six
degrees of longitude in the Indian Ocean. They were mixed with
glistening yellow cylindrical species of such comparatively gigantic
size as to be visible to the naked eye.
Other genera constitute the only vegetation in the high latitudes of the
Antarctic Ocean. Dr. Hooker observes, that without the universal
diffusion of diatoms in the South Polar Ocean, there would neither be
food for the aquatic animals, nor would the water be purified from the
carbonic acid which animal respiration, and the decomposition of matter,
produce. These small plants afford an abundant supply of food to the
herbivorous mollusca and other inhabitants of the sea, for they have
been found in the stomachs of oysters, whelks, crabs, lobsters,
scallops, &c. Even the Noctiluci, those luminous specks that make the
wake of a boat shine like silver in a warm summer night, live on the
floating pelagic diatoms, and countless myriads are devoured by the
enormous shoals of salpi and other social marine animals.
The silicious shells of the diatoms form extensive fossil deposits in
various parts of the globe, containing species which have long ceased to
exist, and others that are identical with those still alive even in
their most minute and delicate engravings. The polishing slate of Bilin
in Bohemia, which occurs in beds 14 feet thick, and the Tripoli and
Phonolite stones on the Rhine consist entirely of the silicious coats of
diatoms, while the city of Richmond in Virginia stands upon a marine
deposit of the debris of diatoms 13 feet thick, and of unknown extent.
Near the Mediterranean, very extensive strata, consisting almost
entirely of marine Diatomaceæ, alternate with calcareous strata chiefly
formed of Foraminifera, the latter being a race of microscopic mollusca.
The fossil Diatomaceæ at Oran in Algeria are particularly perfect and
beautiful. In many of these deposits existing species are found.
The trade winds bring over large quantities of dust mixed with diatoms,
which sinks through the upper into the lower current, blowing over
America, and at last falls in Europe. Professor Ehrenberg found that
this dust contained chiefly true American species, many of which were
identical with forms existing at the bottom of the Antarctic Ocean,
where an area of 4,800 square miles was discovered by Sir James Ross
skirting the volcanic coast of Victoria Land, consisting of the remains
of these microscopic plants, which have deposited their silicious valves
at death for countless generations, producing geological changes of
enormous magnitude; while a still greater area of sea-bed in the North
Atlantic is the perpetual grave-yard of myriads of microscopic mollusca.
Thus the Supreme Being, whose power is stupendously manifested in the
motions of the celestial bodies, creates generations of infinitesimal
creatures, adorns them with exquisite beauty, and makes them His agents
to form future continents.
The Confervaceæ are a numerous tribe of pretty little plants, usually of
a green colour, growing in fresh and salt water, on moist ground, wet
rocks, and thermal springs. There is scarcely a gently running stream in
which they may not be seen, like bunches of green threads, attached to
stones and waving in the current; some are so soft as to become almost a
mass of jelly when taken out of the water. They are sometimes branched,
but more frequently simple, formed of cylindrical cells, joined in a
single long row by their flat ends, and they increase in length by the
bisection of their terminal cells.
[Illustration: Cell multiplication in Conferva glomerata:—A, portion of
filament with incomplete separation at _a_, complete partition at _b_;
B, the separation completed; C. formation of additional layers of
cellulose wall.]
In unicellular plants bisection is an act of reproduction; in the
multicellular Confervæ it is an act of growth and extension which is
accomplished as follows:—The terminal cell of the plant grows to twice
its length, the matter within the primordial cell spontaneously divides
into two equal parts, and both the film and cellulose coat which cover
it, bend round, and form a double layer or cellulose division between
them. This cellulose layer extends over the whole exterior of the
primordial cell, so that the new cellulose division or septum becomes
continuous with a new layer which is formed throughout the interior of
the cellulose wall of the original cell. In this manner two perfect
cells are formed out of one, and as the extreme cell may undergo the
same process, the growth of the plant may be continued indefinitely.
Branches are sometimes formed by buds springing from any part of the
stem; though apparently so different, it results from the subdivision of
the cell which produces the bud.
[Illustration: Fig. 18. Zoospores.]
The Confervaceæ are generally reproduced by zoospores. In most cases the
endochrome within a cell divides itself into numerous segments, each of
which becomes a minute zoospore, and escapes into the water through a
rupture in the cell wall. This is the case in a very graceful genus of
Confervæ, of which the Chætophora elegans is a species. It consists of
filamental strings of cells, ending in a capillary bristle, with lateral
branches like narrow fronds. It is reproduced by zoospores. One half of
each zoospore is round, opaque, and full of matter; the other half
hyaline, and tapering to a beak furnished with four cilia. It frequently
happens in this genus of Confervaceæ, where the filaments are divided at
equal distances into little joints or compartments, that the zoospores
issue from the terminal cell first, then from the next, and so on in
succession till the upper part of the branch is left empty, while the
lower part is still forming zoospores. After moving in water for a time,
the zoospores retreat to a shady place, fix themselves to some
substance, and begin to grow. These plants rapidly cover a large surface
of water; for each individual cell may produce 100 zoospores, and as the
development and dissemination of them continues during the whole summer,
one plant may yield an enormous number.
The Sphæroplea annulina is a rare and very remarkable Conferva, whose
cinnabar-coloured spores make the surface of the water in which it
floats, like a pool of blood. It has no root, being merely a filament
with capillary extremities, formed of elongated cells joined end to end.
The spores only grow on the filaments that are exposed to the sun and
air; the filaments that are below the water are green and barren. The
spores are filled with red matter, grains of starch, and red oil, the
outer or cellulose coat being so plaited, that the spore looks like a
red star with white rays.
When a spore germinates, it produces a minute cell ending in capillary
fibres, which increases in length by the continual bisection of its
central cells, while the other Confervæ grow by the bisection of those
that are terminal. During this growth, the red contents of the spore are
so changed by a remarkable succession of chemical processes, that the
primordial cells in the filament of the young plant are filled with a
colourless viscous matter, an aqueous liquid, granules of starch, and
chlorophyll. In some of the cells the starch disappears, while the green
matter and the other materials arrange themselves into a series of
rings, alternating with empty spaces or vacuoles. After a time, the
green changes to a yellowish red, and then each ring in succession
resolves itself into a multitude of minute active particles, which move
with incredible velocity in the void spaces of the cell, till at last
the whole cell swarms with them. They are analogous to the pollen of
flowering plants, and thence are called spermatozoids. Their form is
cylindrical, thick, broad, and yellow at one end, sharp at the other,
with a colourless beak, and long cilia. The parent cell is at last
pierced by their united efforts, and out they rush in great confusion
into the water; some whirl round their centres, others swim in a circle,
many describe cycloidal curves by a series of leaps, and a few swim in
straight lines.
During the preceding changes another process is in progress, within what
may be called female cells. In these the starch, mixed with green matter
and a plastic substance, arrange themselves also into green and vacant
rings, and after various and complicated changes, each green ring forms
itself into a kind of plastic primordial free cell, which, after being
fertilized by the moving bodies, gets a stronger coat. The green matter
becomes first of a red-brown colour, then red; and after leaving the
parent cell it is invested with a plaited cellulose coat, and becomes a
star-like resting spore which may produce a new plant. No cryptogamic
plant exhibits a greater variety in the modes of action of the vital
forces, none more activity in the motile powers. In some of these
Confervæ, the moving male filaments, or spermatozoids, instead of
escaping singly from their prison cell in confusion into the water, are
discharged in a mass enclosed in a capsule furnished with cilia, which
moves with its lively burden like a zoospore, till a lid falls off which
sets them free.
Some pretty plants allied to the Confervæ are called Batrachospermeæ,
from the resemblance which their beaded filaments bear to the spawn of a
frog. They are all inhabitants of fresh water, chiefly of gently flowing
streams, and are so flexible that they yield to every movement of the
water, and when taken out of it are like a mass of jelly. Their colour
is usually a brownish-green, but sometimes it is of a reddish or bluish
purple. The central stem of the plant, though originally formed of a
single row of large cylindrical cells placed end to end, gets an
investment of cells, or rather branches, which ultimately becomes a
thick cylindrical stem, bearing, at nearly regular intervals, whorls of
short radiating branches, each composed of rounded cells, arranged in a
bead-like row, and sometimes branching again. Some of the radiating
branches grow out into transparent points, which may possibly be
antheridia, and contain motile bodies; for within certain cells in other
branches resting spores are found, which are agglomerated and form the
large dark globular masses that are seen in the midst of the whorls.
The Hydrodictyon utriculatum is another allied plant of singular
structure, which grows in fresh-water pools in the midland and southern
counties of England. It resembles a regularly reticulated green purse,
from four to six inches long, and is composed of a vast number of
tubular cylindrical cells, which adhere to one another by their rounded
extremities, the points of junction corresponding to the knots or
intersections of the network. Each of these cells may form within itself
from 7,000 to 20,000 gonidia, which at a certain stage of their
development are observed to be in active motion in its interior;
subsequently, by mutual adhesion, they form into groups which lay the
foundation of new net-plants, when set free by the dissolution of their
envelope. Besides these groups, there are certain cells which produce
from 30,000 to 100,000 more minute bodies of a longer shape, each of
which is furnished with four long cilia, and a red spot. These escape
from their cell in a swarm, move freely in the water for a time, then
come to rest, and sink to the bottom, where they remain, heaped together
in green masses. Their future fate is unknown, but they are believed to
be male filaments similar to those described, and are generally called
spermatozoids.
The Nostochineæ are either an assemblage of cells loosely united into
numerous green chaplets, or distinctly beaded filaments, generally
twisted, and occasionally branched; they are imbedded in a firm
gelatinous frond of different form, sometimes globular, sometimes
spreading in branched masses, often of considerable size. They are
frequently seen on damp shady walks in gardens: they shrink to a film in
dry weather, and reappear so suddenly in rain that they have been called
fallen stars. They are reproduced by spontaneous division of their
filaments; the segments escape from the gelatinous mass, move slowly in
the direction of their length, after a time come to rest, secrete a
gelatinous envelope, and not only grow in length by transverse
bisection, but split longitudinally into new filaments which are
separated by their gelatinous secretions. These movements, discovered by
M. Thuret, are evidently intended to disperse the plant.
Vesicular cells, destitute of endochrome, sometimes furnished with
cilia, and of a larger size than the others, are occasionally seen at
the end or middle of a filament of the Nostocs, sometimes situated at
intervals along their length; and near to these are sporangial cells, a
little larger than the ordinary cells. From analogy, it is believed that
the vesicular cells are antheridia, and that the sporangial cells
contain germs which, after being fertilized by the spermatozoids, are
set free and become resting spores. In some species, the sporangial
cells are oblong, and contain vividly green matter; in others, the cells
are elliptical and brown.
The species are widely distributed. Hormosiphon arcticus, a species
consisting of a modification of cellulose, abounds to such a degree in
the herbless polar regions, that it affords a welcome variety of food.
Each plant lies on a small depression of the snow, which covers the soft
and almost boggy slopes bordering the arctic seas, but it is carried by
the winds in every direction, rolling over the snow and ice to a
distance of several miles. Two northern species of Nostoc were found by
Dr. Hooker in Kerguelen’s Land, growing on wet rocks near the sea; one
of them was the common Nostoc commune. Other species occur in the warm
springs in India, as well as in the arctic and antarctic regions, and an
aquatic species is much used in China as a wholesome food. The genus
Monormia forms floating masses of jelly on the surface of brackish
water. The necklaces are of vast length, and, together with the jelly in
which they are imbedded, wave with the slightest motion of the water.
Floating masses grow on large ponds or lakes, which give the water a
green tint.
The structure of the Oscillatoriæ is microscopic. They are minute
filiform plants closely allied to the Nostocs; and consist of
transparent colourless tubular filaments containing colour cells of
various forms, more or less separated from each other, and visible
through their transparent tubes; the colour is usually some shade of
green, yellowish, or purple. In the genus Rivularia these tubular
filaments have a globular transparent cell at the base, and are closely
packed into little balls, either forming small groups, as in the
Rivularia nitida, or singly attached to stones and rocks. In Rivularia
nitida, the filaments radiate from a centre. Some Oscillatoriæ form
velvety cushion-like patches upon rocks, others are attached in tufts as
parasites to other sea weeds, while many are arranged in free or
attached stratified bundles. Lingbya furnishes a beautiful specimen of
the latter. The filaments in the stratified group are usually much
twisted and interwoven, and some of them exhibit singular oscillating
motions, as the Oscillatoria littoralis and spiralis, Spirulina
tenuissima and others; one end of the filaments remains at rest, while
the other extremity is in constant vibration. With a microscope the
movement in some species is seen to be from side to side like a
pendulum, in others it is spiral or twisting, and when a fragment of the
plant is set free when vibrating the movement is progressive. If a
fragment be put into a glass of water, its edge in a little time becomes
fringed with short filaments radiating from central points with their
tips outwards. They soon detach themselves from the fragment by their
oscillations, and as their vibrations continue after they are free, they
swim with a spiral motion to the edge of the water, and even ascend the
glass till arrested by the dry part above.[35] During these motions
there is a corresponding alteration in the form of the filamental tubes
believed to arise from rhythmical periods of vital contractibility,
which are affected by light and heat, because the motions are more rapid
in sunshine than in shade; besides, they are checked by strong chemical
agents. Some of the species have a tuft of delicate cilia at the
extremities of their filaments.
The free stratified bundles contain the simplest form of the
Oscillatoriæ. Each filament is a straight or slightly curved chain of
cells, full of coloured matter, and enclosed in a common transparent
colourless tube. Multiplication takes place in these by division; when
about to multiply, two adjacent coloured cells, or the two halves of a
divided cell, recede from one another, and the outer tube contracts at
the point of division, and separates them into two distinctly new
filaments. Sometimes the transparent outer tube does not yield, so that
the divided parts retain their places in the tube, which dilates when
these new parts are again divided. The manner of division varies with
the species, and the generic characters of the Oscillatoriæ depend upon
the different conditions of the external tube, and the form and
arrangement of the coloured cells within it. The tube often contracts to
the finest point during division, and frequently consists of distinct
coats, the number of which increases upwards, sometimes with such
regularity as to produce a beautiful streaked effect. Like their allies,
the Oscillatoriæ are reproduced by zoospores. While these parts are
growing, but especially during their dissolution, the endochrome
undergoes various changes of colour, staining the water they die in, and
rendering it putrid; some of the common kinds emit a strong odour of
sulphuretted hydrogen.
In the compound gelatinous Oscillatoriæ, the jelly is of very different
degrees of tenacity. The mass of the Dasyglœa is so slippery that it can
scarcely be taken hold of; Rivularia nitida (fig. 19) is equally so, its
tubes being so thick and tender. Many species of the genus Rivularia
have a peculiar mode of oblique alternate branching; species of that
genus grow on the stems of aquatic plants, on rocks in rapid streams, on
cliffs when washed by cataracts, or sometimes in calcareous water, in
consequence of which crystals of carbonate of lime are deposited on
their substance. The Rivularia nitida occurs among Algæ exposed at low
tides, and a species of another genus floats on fresh-water lakes like
green stars.
[Illustration: Fig. 19. Threads of Rivularia nitida.]
The Oscillatoriæ are found in every part of the world, most abundantly
in the temperate zones. They chiefly inhabit fresh water, but these
minute plants attain their greatest size in the sea. Numerous species
grow in warm springs, and one species, Trichodesmium erythræum (fig.
20), spreads for many square miles over the surface of the Indian seas
in faggots of red-brown threads, like fragments of chopped hay; the same
species is said to abound in the Red Sea also.[36]
The Conjugatæ are fresh-water plants of numerous species, which have
almost the same structure as the Confervæ, but the green endochrome
within the cells of their articulated threads is more highly organized,
and the manner of reproduction is altogether different and very
peculiar.
These plants consist of strings of cylindrical cells joined end to end
by their flat ends, and generally float freely on or near the surface of
still water, especially when buoyed up by the bubbles of gas which are
liberated from them by the heat and light of the sun. In the early stage
of their life, while as yet the cells are undergoing multiplication by
self-division, the endochrome is diffused pretty uniformly in each cell;
but as the plant approaches towards maturity, it undergoes various
modifications, according to the species. In some it consists of large
granules disposed in rows; in others it is formed into broad spiral
bands with large granules in binary or stellar groups placed at
intervals on it; and, in the œdogonium capillare and others, the
granules are united in spiral lines which cross one another and form a
network.[37]
[Illustration: Fig. 20. Trichodesmium erythræum.]
The act of conjugation by which spores are formed, usually takes place
between the cells of two distinct parallel filaments which happen to be
adjacent to each other, and all the cells of the two filaments generally
take part in it at once. The cells that are opposite to one another put
out little protuberances, which come into contact with each other; the
intervening partitions disappear, so that a tube is formed which
establishes a free communication or passage between the cavities of the
conjugating cells. In the genus Mesocarpus and others, the conjugating
cells pour their endochromes into a dilatation of the passage that has
been established between them, and it is there that the matter mingles
to form a spore or embryo cell. But in the Zygnema (fig. 21), which is
the commonest form of these plants, the endochrome of one cell passes
entirely over into the cavity of the other, and within the latter the
two endochromes coalesce into a single mass, round which a firm coat is
developed, and it becomes a spore. All the cells of one filament are
thus left empty, while spores are formed in all the cells of the
other.[38] Sometimes cells in the same filament conjugate, and
occasionally the endochrome in a cell divides into two parts, each of
which becomes a spore.
[Illustration: Fig. 21. Conjugation of Zygnema quininum:—A. two
filaments in the first stage of conjugation; B, completion of the act of
conjugation.]
Some of the spores are quiescent, others have cilia and are motile, but
both after a time become attached at one end by two or three root-like
fibres, and grow into filaments by repeated bisection. According to the
observations of M. Itzigsohn, the endochrome in certain filaments of
Spirogyra breaks up before conjugation into little spherical
aggregations, which are gradually converted into nearly colourless
spiral filaments, having an active spontaneous motion, and therefore
corresponding precisely to antherozoids. With the exception of South
America, the Conjugatæ are widely dispersed in warm and temperate
climates.
The genus Vaucheria may be assumed as a type of the Siphoneæ, whose
essential character is, that the plant consists of one single tubular
cell, however branched and complicated its form may be. The Vaucherias
form tufted masses of branching tubes, filled with bright green granular
matter, on mud and damp soil; they abound in fresh-water pools, and some
grow in the sea. When about to produce fruit, the extremities of some of
the tubes swell out in the shape of a club, in which a portion of the
green matter collects, takes a darker hue, and is separated from the
rest by a transparent space and a new envelope. After various changes,
the darker green matter forms itself into a zoospore, which is so active
that it breaks open the top of its club-shaped cell, and comes into the
water; sometimes several come, one after another. They are egg-shaped,
with a colourless beak, and as their whole body is bristled with cilia,
they leave a long current in their wake when they swim, which they do
with such impetus that they are flattened against any obstacle they meet
with, even to the discharge of their green endochrome. They escape from
their cell about eight in the morning, move for two hours, then come to
rest, and begin to grow into a new plant.