Anton Kerner von Marilaun.

The natural history of plants, their forms, growth, reproduction, and distribution: from the German of Anton Kerner von Marilaun (Volume 2) online

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V. Globator, and has rounded cells more widely separated and connected by very
delicate processes. But perhaps its most striking characteristic is the very great
variability in the number and distribution of the reproductive cells. The partheno-
gonidia, which vary in number from one to sixteen, may either occur alone or in
one colony with androgonidia or oospheres, or both. Most of the sexual colonies
are dioecious, though this is not always the case. The colonies containing androgo-
nidia unaccompanied by other reproductive cells often develop very numerous (up
to 1100) spermatozoid bundles, the androgonidia forming one-third of all the cells of
the colony. The spermatozoids differ from those of F. Globator by their larger size,
by their terminal flagella at the end of a shorter beak, and by the possession of a
well-developed leaf -green chromatophore. We must, therefore, consider F. aureus
as not so highly developed, in some respects at least, as F. Globator.

A Volvox-colony always swims in the direction of a given axis passing through
its body, and at the same time rotates to the right or left about an axis which is
inclined obliquely to the antero-posterior axis. The eye-spots of the vegetative cells
are much better developed in the anterior half of the colony, and are always
situated on the side of the cell nearest the anterior pole. These facts tend to
support the view of the function of eye-spots in general suggested in page 629.

Volvox stands at the head of the series of colonial (ccenobe-forming) organisms
which we have been tracing, a series diverging from a Chlamydomonas- or Sphm-
rella-likQ type, and whose successive forms gradually increase in size, complexity,
and sexual differentiation. Volvox itself has been well spoken of as " the culmina-
tion of Nature's attempt to evolve a higher organism out of a coenobe ". It was an
attempt which failed, or rather which could not be carried any further than Volvox
itself. A delicate, easily-ruptured Volvox-sphere could certainly not continue to
exist if it were much more than a millimetre in diameter. As it is, the wall is often
split, and all sorts of smaller organisms get inside, resulting in the more or less
speedy collapse of the Volvox-colouy.

But there are other series diverging from the Chlamydomonadege, and some at
least of them have followed lines on which it was possible for higher and more
varied plant-forms to be developed.

At the first stage along one of these lines of descent we find ourselves among
forms in which the dominant phase of the life-history falls in a resting stage, either
fixed or freely floating in the water. From this resting stage motile forms (zoo-
spores), corresponding with the free-swimming Chlamydomonas individuals, are
directly developed. These zoospores, after a short period of swarming, come to rest,
often fixing themselves by their anterior end to some solid object. With little or
no change in the constitution and appearance of the cell the main portion of the



636 THE SUBDIVISIONS OF THE VEGETABLE KINGDOM.

life cycle is passed in this fixed condition, and cell divisions take place, the products
eventually again developing flagella and being set free as zoospores. The genera
Chlorangium and Physocytium are examples of the simplest form of this type o:
life-history. Forms with a rather more complicated structure in the fixed stage are
found in the genera Mischococcus (a common form on the surface of threads of the
higher Algae), Euglenopsis (a newly-discovered American plant), and their allies
In these the protoplasm of the zoospore, after fixing itself and putting on a delicate
cell- wall, pushes out the surface of its membrane away from the substratum, thus
forming a tube of gradually increasing length, the apex of which is always occupie(
by the protoplasm. Division of the protoplasm and subsequent pushing out of the
wall of the tube in different directions by the daughter-cells results in a branching
of the hollow stalk, and in this way quite a considerable branching plant-body
may be produced. Eventually some or all of the cells occupying the apices of the
various branches of the tube acquire flagella and escape into the water as zoospores,
which again settle on solid objects and give rise to new plants.

Other forms in which the cell derived from a zoospore multiplies by division, the
products eventually again giving rise to zoospores, are Schizochlamys, Botryococcus,
Dictyosphcerium and Tetraspora. In these, however, the immotile phase is not fixed,
but forms floating colonies of various conformation. Into this topic we cannot
enter further, except to remark that Tetraspora forms flat colonies of cells arranged
in one plane and held together by the swollen mucilaginous cell-walls. Cell division
takes place in planes at right angles to that of the colony. This type of colony is
specially interesting, as it suggests the form of thallus found in Ulvacece, which in
turn appears to lead on to the higher forms Confervoidece.

Pleurococcacece. More or less closely allied to the above-mentioned genera are
others which do not form zoospores at all. These types with no motile phase in
their life-cycle may be conveniently classed together as Pleurococcacece. The type-
genus Pleurococcus contains some of the most widely-distributed algal forms known.
P.vulgaris forms the bulk of the green coating of damp earth, tree trunks, palings,
&c., in all regions of the globe. It consists of roundish cells, dividing in three direc-
tions in space and thus forming solid masses of cells hanging together in multiples of
two, and often flattened by lateral contact. Each cell contains several parietal chro-
matophores which may, however, fuse together to form a single one. Resting aki-
netes are formed by the cells ceasing to divide, becoming spherical, and thickening
their walls. At the same time oil appears in the protoplasm. It is probably mainly
in this phase that Pleurococcus gets distributed by the wind from one place of growth
to another. Owing to the resemblance of the akinetes of some of the confervoid
Algae to those of Pleurococcus, it has often been stated, and indeed is still held by
some algologists that Pleurococcus itself is merely a growth-phase of these higher
Algae. But recent culture- experiments leave little room for doubt that Pleurococcus
is a perfectly autonomous form, although it may often be associated with pleurococcoid
stages of other Algae. Eremosphcera is a pretty form, common in fresh water, with
single floating spherical cells. Each cell contains numerous separate chlorophyll-



THALLOPHYTA. 637

bodies, embedded in a parietal layer of protoplasm, and a nucleus suspended by
protoplasmic strands in the centre of the cell. Multiplication is effected by division
of the protoplasm into two daughter-cells which escape by rupture of the mother-
cell membrane. Scenedesmus is another motionless floating fresh-water form. It
consists of oblong cells united into groups of two, four, or eight, which lie side by
side, palisade fashion. Some or all of the cells often possess straight or horn-like
projections of their walls, which give the cell groups a very characteristic appear-
ance. The single solid chromatophore occupies nearly the whole cell cavity. Cklo-
rella is a genus whose cells are symbiotic with Radiolaria (yellow cells). Other
forms live in a similar relation with certain Coelenterates and Platyhelminths.
Several help to form lichens. Since the various genera of Pleurococcacece differ
thus very widely in the form and structure of their cells, and indeed are only united
by the negative character of the absence of zoospores, it is almost certain that they
cannot be considered as forming a natural group. The various genera are very
probably allied to different neighbouring groups from which they have been derived
by the suppression of the habit of forming zoospores.

The JEndosphceracece are a small and very natural group of unicellular Algae,
characterized by their habit of living in the intercellular spaces of various higher
plants. They possess motile zoospores, or gametes, or both, but the motionless cells
produced from these do not undergo vegetative divisions. Very possibly they
represent a separate line of descent from the Chlamydomonadese, a line of descent
in which the motionless cell has become the dominant phase in the life-cycle, and
has been specially adapted to the new conditions of life, but differs from the motion-
less cells of the " Tetrasporacese " in directly forming zoospores without undergoing
purely vegetative divisions.

Two forms of Endosphaeraceas may be taken as illustrations of this type of life-
history.

Chlorochytrium Lemnce inhabits the intercellular spaces immediately under the
epidermis of the leaves of Lemna trisulca (the Ivy-leaved Duckweed). Each plant
consists of a single, thick-walled, oval cell with a parietal chromatophore containing
numerous pyrenoids and a large central vacuole. Very numerous pear-shaped
isogametes are formed by successive divisions of the protoplasm of the cell. Then
a layer of substance outside the mass of gametes (probably the ectoplasm of the
cell) begins to swell strongly, and bursts not only the cell- wall but also the super-
incumbent tissue of the Duckweed leaf, forming a sphere of mucilage in which the
gametes begin to swarm and to conjugate in pairs. Spherical zygozoospores are
thus produced; these escape from the mucilage, and after some free swarming in
the surrounding water, settle on the boundary between two epidermal cells of a
Duckweed leaf, draw in their flagella, put on a cell-membrane, and form a definite
parietal chlorophyll-body with a single pyrenoid. After two or three days a delicate,
colourless tube is put out, which forces its way between the two epidermal cells of
the leaf, and reaches an intercellular space. The contents of the zygote slowly pass
over into the apex of this tube, which gradually increases in size and assumes the



638 THE SUBDIVISIONS OF THE VEGETABLE KINGDOM.

characters of a young vegetative cell, the original zygote-wall remaining on the
surface of the leaf as a mere cellulose knob.

The generations rapidly succeed one another during the summer months, the
last-formed cells of the season becoming packed with starch grains and passing the
winter in this state. These resting cells can withstand desiccation, in case the pond
in which the duckweed lives becomes dried up.

Phyllobium dimorphum forms large iminotile cells between the tracheids of
the vascular bundles in the leaves of the creeping Moneywort (Lysimachia
nummularia). This plant lives in damp woods and other shady places. The Rhine
plain in the neighbourhood of Strasburg, where Phyllobium was first found in the
leaves of the Moneywort, is usually flooded during the month of June, partly by
the rising of the river, and partly by the thunderstorms which usually occur about
that time of the year. The Phyllobium-cells take this opportunity to form their
gametes, which are of two distinct sizes, each cell producing gametes of one size
only. After the escape of the gametes into the surrounding water conjugation
occurs. The zygozoospores produced have only two flagella, the body and flagella
of each microgamete being completely lost in the megagamete, just as the body of
a spermatozoid is completely lost in the substance of the egg. After coming to rest
on the surface of a Lysimachia leaf, and acquiring cell-membranes, the zygotes put
out delicate tubes which enter the stomata of the leaf. If a leaf is infected by a
few zygotes only, the tubes formed reach the vascular bundles, and forcing their
way between the elements of the wood, grow forward in the bundles, branching
when they branch, and attaining to a considerable length. Eventually, towards the
end of the summer, the protoplasmic contents of each tube becoming concentrated
in one spot, this part of the tube swells and is cut off from the remainder by the
formation of transverse partitions. The swollen part of the tube thus forms a large
cell which rests during the winter, and in the next summer will produce gametes.
If, on the other hand, the leaf is infected by a large number of zygotes, most of the
tubes never get any further than the intercellular spaces immediately under the
stomata. In this position they form small resting cells in large numbers. These
eventually form zoospores, which apparently behave, on germination, just like the
zygozoospores. The dimorphism of the resting cells of Phyllobium thus depends
directly on the amount of space at the disposal of the germ tubes. This conclusion
can be confirmed by cultivating the germ tubes apart from the leaves of the host.

The purpose of the germ tubes of Chlorochytrium, Phyllobium, and their allies
in penetrating the leaves of their hosts, seems to be simply that they may gain the
advantage of a quiet protected place for their development. Just in the same way
Diatoms and other unicellular forms often live comfortably in the empty cells of
Algae, the intercellular spaces of the Bog-moss (Sphagnum), and similar situations.
Only in the case of these Endosphserese the association of the Alga with its habitat
is invariable and adaptive, not merely casual and unrelated. But the Endosphserese
are not parasites in any sense. They take no food from their " hosts " nor do they
exercise any appreciable influence on the latter. This is sufficiently proved by the



THALLOPHYTA. (539

fact that Lemna trisulca lives quite happily and can flower when infested with
Chlorochytrium, and that the germ tubes of Phyllobium dimorphum usually enter
dead leaves of the Moneywort. Another form which always enters the living leaves
of a river-weed, continues its course of development whether the leaves die or re-
main alive. It is not, however, difficult to imagine how a form like Phyllobium,
living as it does in the vascular bundles of its host, might acquire a parasitic habit
by tapping the food supplies. As a matter of fact certain confervoid Algse are
known whose presence results in the death of the leaves they inhabit, though pro-
bably not by direct appropriation of the food of the host.

Resembling the Endosphcerece in possessing motionless cells which form zoospores
but do not undergo vegetative divisions, are certain common fresh-water forms of
which Characium and Sciadium may be mentioned. A plant of Sciadium ori-
ginally consists of a single cylindrical cell whose contents breaks up into zoospores.
These zoospores have acquired the peculiar habit of settling on the rim of the
mother-cell, instead of seeking out fresh spots for their development. Each zoospore
produces a single cell like the mother, so that a whorl of cells of the new genera-
tion is formed on the top of the original cell. This process may be repeated for
two or three generations, after which the zoospores will settle on some other object
.and start fresh " plants ".

The Hydrodictyacece are a group of Algae which form immotile colonies. The
cells of these colonies resemble the single cells of the forms we have just been con-
sidering in producing zoospores or gametes, but undergoing no vegetative divisions.
The colony is formed by the joining together in a definite way of the group of
-zoospores formed in a single cell of the mother-colony. Each of these zoospores
then develops into an adult vegetative cell

The recently discovered genus Euastropsis (so called from its likeness to the
Desmid Euastrum) is the simplest type of the family. It consists of two mitre-
shaped cells joined to one another by their bases. Each cell contains a parietal
chromatophore with a single pyrenoid, and a single nucleus. The contents break
up by successive divisions into 2-32 zoospores, which escape from the cell surrounded
by a general membrane. After oscillating for about a quarter of an hour, the zoo-
spores become attached in pairs by their anterior ends. Each pair then takes on
the characters of the two-celled colony.

Pediastrum (fig. 370 6 ) consists of a disc of cells, of which the marginal ones are
often drawn out into lobes or processes. The chromatophore is parietal with a
single pyrenoid; there are numerous nuclei. The formation of zoospores is like that
of Euastropsis, but their movement is more lively, and eventually all the zoospores
formed in a single cell join together to form a new Pediastrum-colony (figs. 370 7
and 370 8 ). Gametes are formed in the same way as the zoospores, but are smaller
and more numerous. They escape from the investing membrane, swim freely in
the water, and fuse in pairs to form zygotes. From these zygotes new Pediastrum-
colonies are produced indirectly, probably by a method like that obtaining in
Hydrodictyon.



6 40 THE SUBDIVISIONS OF THE VEGETABLE KINGDOM.

Hydrodictyon, the Water-net (figs. 370' and 370') is a beautiful organism
forming net-like colonies of cylindrical cells, which are joined end to end, forming
the sides of the polygonal meshes. Each cell may be as much as 1 centimetre m
length A thin layer of protoplasm containing numerous small nuclei lines
wafl and incloses a large central vacuole. The chromatophore or chlorophyll-layer
of the protoplasm, contains many pyrenoids, each surrounded by a sheath of starch
grains Fine-grained starch is also scattered through the substance of the




Fig. 370. Hydrodictyacese.

i The Water-net (Hydrodictyon utriculatum), nat. size. 2 A portion of the same magnified 50 diameters. , * and s Formation
of zoospores in a cell of Hydrodictyon, showing their union together, and escape as a young net ; x 300. 6 Pediastrum,
granulatum; development and escape of zoospores, the lightly-dotted chambers already vacated. 1 and Zoospores after
their escape arranged as a new Pediastrum plant ; x 240.

matophore. This stroma-starch appears in great quantity when growth is checked
and assimilation remains active, disappearing again if assimilation is stopped. The
pyrenoid-starch, on the other hand, seems to be withdrawn from the ordinary meta-
bolism of the cell, since it is formed round each pyrenoid early in the life of the
cell, and remains there under all circumstances, unless the cell is on the point of
actual starvation in the dark, till the onset of reproduction. When zoospores are
about to be formed the pyrenoids together with their starch disappear, and abund-
ant stroma-starch appears. At the same time the nuclei multiply a good deal by
division, and eventually the whole of the protoplasm divides to form a great num-



THALLOPHYTA. 641

ber of zoospores. These zoospores, however, never become free like those of Pedi-
astrum, but remain joined together by strands of protoplasm, and after a certain
amount of shifting backwards and forwards, come to rest with their ends in contact.
Each then gradually assumes the characters of a Hydrodictyon-cell, the young
colony eventually escaping from the mother-cell (figs. 370 3 > 4 ' 5 ). Gametes are formed
in the same way as zoospores, but are smaller and more numerous. The spherical
zygote gradually increases in size, and its contents breaks up into 2-5 large zoo-
spores, which develop into large cells with pointed processes, the so-called poly-
hedra. In the interior of each polyhedron an embryonic Hydrodictyon-iiet is
developed from swarm-spores, and in the cells of this ordinary Hydrodictyon
colonies are found.

It has been shown experimentally that any Hydrodictyon-net above a certain
size and age is capable of producing either zoospores or gametes, and that the
stimulus to the formation of one or the other is given by external conditions.
Thus bright light, fresh water rich in inorganic nutritive salts, and fairly high
temperatures, are favourable to the production of zoospores, while the reverse of
these conditions, and especially the presence of organic substances, such as sugar,
tend to make the cells of a net produce gametes. The conditions favourable to
zoospore- formation are also of course, favourable to active vegetative growth,
and no doubt the abundant formation of new protoplasm is a necessary preliminary
to the production of zoospores. A slight check to the processes of assimilation and
growth is apparently necessary in order to give play to the zoospore-f orming forces.
Thus, experimentally, a change from a strong solution of nutritive salts to fresh
water will induce the formation of zoospores in nets which would simply have gone
on growing if left in the nutritive solution. A similar check is probably given by
the waning light in many Algae in which zoospores are produced at night. For the
production of gametes, on the other hand, an actual reversal of the conditions
favourable to growth is necessary. In nature this probably happens when by very
active growth the whole of the water of a pool is filled with nets, the inorganic
food and oxygen are exhausted, and the normal chemical processes of the cell
receive a check. The formation of gametes and zygotes under these conditions is
obviously adaptive, since the zygote can, although it need not, rest during several
months till the conditions are quite altered. We may therefore conclude that,
whereas zoospores are especially designed to multiply and distribute the species,
zygotes are intended to preserve it under unfavourable conditions. It is probable
that the production of large zoospores and polyhedra is a necessary part of the
life-cycle following the germination of the zygote, and cannot be altered by the
incidence of different conditions.

Alliance VII. Siphoneae.

Thallus consisting of a tube, often much branched, and containing many nuclei.
This tube is the production of a single cell, but in the more complicated forms is
often shut off into compartments by transverse septa. Reproduction by zoospores

VOL. II. 91



(542 THE SUBDIVISIONS OF THE VEGETABLE KINGDOM.

and planogametes, or spermatozoids and eggs: in very many forms no reproductive
cells are known. The higher forms of Siphonese often produce plant-bodies of very
definite and characteristic external form, and of considerable size. In some cases
these simulate the external form of various higher plants.

Families: Botrydiacece, Phyllosiphonacece, Vaucheriacecs, Bryopsidacece,
CaulerpacecB, Codiacece, Valoniacece, Verticellatce.

Botrydiacece. Botrydium granulatum is a little plant found growing espe-
cially on loam at the damp edges of ponds and ditches. It consists of a club-shaped
or balloon-shaped green shoot-portion, about 1-4 millimetres in diameter, continuous
with a simple or branched tubular colourless root-portion which is embedded in the
substratum. The entire plant consists of a single cell, that is to say, its cavity is
continuous throughout. The wall is lined with a thin layer of protoplasm, which
contains many nuclei, and, in the shoot, a net-like chlorophyll-layer.

Botrydium can reproduce itself in very various ways, according to the incidence
of external conditions. The simplest form of propagation is by budding, which
takes place under conditions favourable to the ordinary vegetation of the plant.
The shoot-portion of a small vegetative plant sends out a process which swells to
the size of the mother shoot, puts out a colourless root, and is then constricted off
to form a separate plant. But if the plants are covered with water they cannot
go on growing comfortably, and accordingly the protoplasm breaks up to form ,-i
number of zoospores, each with a single flagellum and two lateral chromatophores.
The mass of zoospores is subjected to considerable pressure by the swelling up of a
ring-like area of the wall, and the tension becomes so great as to rupture the wall
in the centre of the ring and expel the mass of zoospores into the water. On damp
soil the zoospores come to rest, and germinate to form new plants. If a zoospore
cannot escape from the water it enters on a resting stage, which gives rise to a new
plant directly it finds itself on damp soil. Further, if a young plant is exposed to
bright sunlight, its protoplasm breaks up into a number of spherical cells, each of
which puts on a cell-wall. If now these spherical cells (gametangia) are placed in
water, the contents of each breaks up into spindle-shaped, biflagellate gametes,
which conjugate in pairs to form zygotes. These zygotes can rest for a longer or
shorter time, but if placed on damp earth they at once germinate to form new
plants. If, on the other hand, the gametangia are placed in water after being kept
for two years they give rise to biflagellate cells which rather resemble the gametes,
but which on damp soil germinate directly to form vegetative plants. Finally, if



Online LibraryAnton Kerner von MarilaunThe natural history of plants, their forms, growth, reproduction, and distribution: from the German of Anton Kerner von Marilaun (Volume 2) → online text (page 78 of 128)