Scientific American Supplement, Vol. XXI., No. 531, March 6, 1886 by Various
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Various >> Scientific American Supplement, Vol. XXI., No. 531, March 6, 1886
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A bulbil is a bud which becomes an independent plant before it commences
to elongate; it is generally fleshy, somewhat after the manner of a
bulb, hence its name. Examples occur in the axillary buds of _Lilium
bulbiferum_, in some _Alliums_, etc.
The gemma is found most frequently in the liverworts and mosses, and is
highly characteristic of these plants, in which indeed vegetative
reproduction maybe said to reach its fullest and most varied extent.
Gemmae are here formed in a sort of flat cup, by division of superficial
cells of the thallus or of the stem, and they consist when mature of
flattened masses of cells, which lie loose in the cup, so that wind or
wet will carry them away on to soil or rock, when, either by direct
growth from apical cells, as with those of the liverworts, or with
previous emission of thread-like cells forming a "protonema," in the
case of the mosses, the young plant is produced from them.
The lichens have a very peculiar method of gemmation. The lichen-thallus
is composed of chains or groups of round chlorophyl-containing cells,
called "gonidia," and masses of interwoven rows of elongated cells which
constitute the hyphae. Under certain conditions single cells of the
gonidia become surrounded with a dense felt of hyphae, these accumulate
in numbers below the surface of the thallus, until at last they break
out, are blown or washed away, and start germination by ordinary cell
division, and thus at once reproduce a fresh lichen-thallus. These
masses of cells are called soredia.
Artificial budding and grafting do not enter into the scope of this
paper.
As in the general growth and the vegetative reproduction of plants
cell-division is the chief method of cell formation, so in the
reproduction of plants by special cells the great feature is the part
played by cells which are produced not by the ordinary method of cell
division, but by one or the other processes of cell formation, namely,
free-cell formation or rejuvenescence.
If we broaden somewhat the definition of rejuvenescence and free-cell
formation, and do not call the mother-cells of spores of mosses, higher
cryptogams, and also the mother-cells of pollen-grains, reproductive
cells, which strictly speaking they are not, but only producers of the
spores or pollen-grains, then we may say that _cell-division is confined
to vegetative processes, rejuvenescence and free-cell formation are
confined to reproductive processes_.
Rejuvenescence may be defined as the rearrangement of the whole of the
protoplasm of a cell into a new cell, which becomes free from the
mother-cell, and may or may not secrete a cell-wall around it.
If instead of the whole protoplasm of the cell arranging itself into one
mass, it divides into several, or if portions only of the protoplasm
become marked out into new cells, in each case accompanied by rounding
off and contraction, the new cells remaining free from one another, and
usually each secreting a cell wall, then this process, whose relation to
rejuvenescence is apparent, is called free-cell formation.
The only case of purely vegetative cell-formation which takes place by
either of these processes is that of the formation of endosperm in
Selaginella and phanerogams, which is a process of free-cell formation.
On the other hand, the universal contraction and rounding off of the
protoplasm, and the formation by either rejuvenescence or free-cell
formation, distinctly mark out the special or true reproductive cell.
Examples of reproductive cells formed by rejuvenescence are:
1. The swarm spores of many algae, as Stigeoclonium (figured in Sachs'
"Botany"). Here the contents of the cell contract, rearrange themselves,
and burst the side of the containing wall, becoming free as a
reproductive cell.
2. The zygoblasts of conjugating algae, as in Spirogyra. Here the
contents of a cell contract and rearrange themselves only after contact
of the cell with one of another filament of the plant. This zygoblast
only becomes free after the process of conjugation, as described below.
3. The oosphere of characeae, mosses and liverworts, and vascular
cryptogams, where in special structures produced by cell-divisions there
arise single primordial cells, which divide into two portions, of which
the upper portion dissolves or becomes mucilaginous, while the lower
contracts and rearranges itself to form the oosphere.
4. Spores of mosses and liverworts, of vascular cryptogams, and pollen
cells of phanerogams, which are the analogue of the spores.
The type in all these cases is this: A mother-cell produces by
cell-division four daughter-cells. This is so far vegetative. Each
daughter-cell contracts and becomes more or less rounded, secretes a
wall of its own, and by the bursting or absorption of the wall of its
mother-cell becomes free. This is evidently a rejuvenescence.
Examples of reproductive cells formed by free-cell formation are:
1. The ascospores of fungi and algae.
2. The zoospores or mobile spores of many algae and fungi.
3. The germinal vesicles of phanerogams.
The next portion of my subject is the study of the methods by which
these special cells reproduce the plant.
1st. Asexual methods.
1. Rejuvenescence gives rise to a swarm-spore or zoospore. The whole of
the protoplasm of a cell contracts, becomes rounded and rearranged, and
escapes into the water, in which the plant floats as a mass of
protoplasm, clear at one end and provided with cilia by which it is
enabled to move, until after a time it comes to rest, and after
secreting a wall forms a new plant by ordinary cell-division. Example:
Oedogonium.
2. Free-cell formation forms swarm-spores which behave as above.
Example: Achlya.
3. Free-cell formation forms the typical motionless spore of algae and
fungi. For instance, in the asci of lichens there are formed from a
portion of the protoplasm four or more small ascospores, which secrete a
cell-wall and lie loose in the ascus. Occasionally these spores may
consist of two or more cells. They are set free by the rupture of the
ascus, and germinate by putting out through their walls one or more
filaments which branch and form the thallus of a new individual. Various
other spores formed in the same way are known as _tetraspores_, etc.
4. Cell-division with rejuvenescence forms the spores of mosses and
higher cryptogams.
To take the example of moss spores:
Certain cells in the sporogonium of a moss are called mother-cells. The
protoplasm of each one of these becomes divided into four parts. Each of
these parts then secretes a cell-wall and becomes free as a spore by the
rupture or absorption of the wall of the mother-cell. The germination of
the spores I shall describe later.
5. A process of budding which in the yeast plant and in mosses is merely
vegetatively reproductive, in fungi becomes truly reproductive, namely,
the buds are special cells arising from other special cells of the
hyphae.
For example, the so-called "gills" of the common mushroom have their
surface composed of the ends of the threads of cells constituting the
hyphae. Some of these terminal cells push out a little finger of
protoplasm, which swells, thickens its wall, and becomes detached from
the mother-cell as a spore, here called specially a _basidiospore_.
Also in the common gray mould of infusions and preserves, Penicillium,
by a process which is perhaps intermediate between budding and
cell-division, a cell at the end of a hypha constricts itself in several
places, and the constricted portions become separate as _conidiospores_.
_Teleutospores, uredospores_, etc., are other names for spores similarly
formed.
These conidiospores sometimes at once develop hyphae, and sometimes, as
in the case of the potato fungus, they turn out their contents as a
swarm-spore, which actively moves about and penetrates the potato leaves
through the stomata before they come to rest and elongate into the
hyphal form.
So far for asexual methods of reproduction.
I shall now consider the sexual methods.
The distinctive character of these methods is that the cell from which
the new individual is derived is incapable of producing by division or
otherwise that new individual without the aid of the protoplasm of
another cell.
Why this should be we do not know; all that we can do is to guess that
there is some physical or chemical want which is only supplied through
the union of the two protoplasmic masses. The process is of benefit to
the species to which the individuals belong, since it gives it a greater
vigor and adaptability to varying conditions, for the separate
peculiarities of two individuals due to climatic or other conditions are
in the new generation combined in one individual.
The simplest of the sexual processes is conjugation. Here the two
combining cells are apparently of precisely similar nature and
structure. I say apparently, because if they are really alike it is
difficult to see what is gained by the union.
Conjugation occurs in algae and fungi. A typical case is that of
Spirogyra. This is an alga with its cells in long filaments. Two
contiguous cells of two parallel filaments push each a little projection
from its cell-wall toward the other. When these meet, the protoplasm of
each of the two cells contracts, and assumes an elliptical form--it
undergoes rejuvenescence. Next an opening forms where the two cells are
in contact, and the contents of one cell pass over into the other, where
the two protoplasmic bodies coalesce, contract, and develop a cell-wall.
The zygospore thus formed germinates after a long period and forms a new
filament of cells.
Another example of conjugation is that of Pandorina, an alga allied to
the well-known volvox. Here the conjugating cells swim free in water;
they have no cell-wall, and move actively by cilia. Two out of a number
approach, coalesce, contract, and secrete a cell-wall. After a long
period of rest, this zygospore allows the whole of its contents to
escape as a swarm-spore, which after a time secretes a gelatinous wall,
and by division reproduces the sixteen-celled family.
We now come to fertilization, where the uniting cells are of two kinds.
The simplest case is that of Vaucheria, an alga. Here the vegetative
filament puts out two protuberances, which become shut off from the body
of the filament by partitions. The protoplasm in one of these
protuberances arranges itself into a round mass--the oosphere or female
cell. The protoplasm of the other protuberance divides into many small
masses, furnished with cilia, the spermatozoids or male cells. Each
protuberance bursts, and some of the spermatozoids come in contact with
and are absorbed by the oosphere, which then secretes a cell-wall, and
after a time germinates.
The most advanced type of fertilization is that of angiosperms.
In them there are these differences from the above process: the contents
of the male cell, represented by the pollen, are not differentiated into
spermatozoids, and there is no actual contact between the contents of
the pollen tube and the germinal vesicle, but according to Strashurger,
there is a transference of the substance of the nucleus of the pollen
cell to that of the germinal vesicle by osmose. The coalescence of the
two nuclei within the substance of the germinal vesicle causes the
latter to secrete a wall, and to form a new plant by division, being
nourished the while by the mother plant, from whose tissues the young
embryo plant contained in the seed only becomes free when it is in an
advanced stage of differentiation.
Perhaps the most remarkable cases of fertilization occur in the Florideae
or red seaweeds, to which class the well-known Irish moss belongs.
Here, instead of the cell which is fertilized by the rounded
spermatozoid producing a new plant through the medium of spores, some
other cell which is quite distinct from the primarily fertilized cell
carries on the reproductive process.
If the allied group of the Coleochaeteae is considered together with the
Florideae, we find a transition between the ordinary case of Coleochaete
and that of Dudresnaya. In Coleochaete, the male cell is a round
spermatozoid, and the female cell an oosphere contained in the base of a
cell which is elongated into an open and hair-like tube called the
trichogyne. The spermatozoid coalesces with the oosphere, which secretes
a wall, becomes surrounded with a covering of cells called a cystocarp,
which springs from cells below the trichogyne, and after the whole
structure falls from the parent plant, spores are developed from the
oospore, and from them arises a new generation.
In Dudresnaya, on the other hand, the spermatozoid coalesces indeed with
the trichogyne, but this does not develop further. From below the
trichogyne, however, spring several branches, which run to the ends of
adjacent branches, with the apical cells of which they conjugate, and
the result of this conjugation is the development of a cystocarp similar
to that of Coleochaete. The remarkable point here is the way in which the
effect of the fertilizing process is carried from one cell to another
entirely distinct from it.
Thus I have endeavored to sum up the processes of asexual and of sexual
reproduction. But it is a peculiar characteristic of most classes of
plants that the cycle of their existence is not complete until both
methods of reproduction have been called into play, and that the
structure produced by one method is entirely different from that
produced by the other method.
Indeed, it is only in some algae and fungi that the reproductive cells of
one generation produce a generation similar to the parent; in all other
plants a generation A produces are unlike generation B, which may either
go on to produce another generation, C, and then back to A, or it may go
on producing B's until one of these reproduces A, or again it may
directly reproduce; A. Thus we have the three types:
1. A-B-C.--A-B-C.--A..................... etc.
2. A-B-B.--B-B...................B--A ... etc.
3. A B A B A............................. etc.
The first case is not common, the usual number of generations being two
only; but a typical example of the occurrence of three generations is in
such fungi as _Puccinia Graminis_. Here the first generation grows on
barberry leaves, and produces a kind of spore called an _aecidium spore_.
These aecidium spores germinate only on a grass stem or leaf, and a
distinct generation is produced, having a particular kind of spore
called an _uredospore_. The uredospore forms fresh generations of the
same kind until the close of the summer, when the third generation with
another kind of spore, called a _teleutospore_, is produced.
The teleutospores only germinate on barberry leaves, and there reproduce
the original aecidium generation.
Thus we have the series A.B.B.B ... BCA
In this instance all the generations are asexual, but the most common
case is for the sexual and the asexual generations to alternate. I will
describe as examples the reproduction of a moss, a fern, and a
dicotyledon.
In such a typical moss as Funaria, we have the following cycle of
developments: The sexual generation is a dioecious leafy structure,
having a central elongated axis, with leaves arranged regularly around
and along it. At the top of the axis in the male plant rise the
antheridia, surrounded by an envelope of modified leaves called the
perigonium. The antheridia are stalked sacs, with a single wall of
cells, and the spiral antherozoids arise by free-cell formation from the
cells of the interior. They are discharged by the bursting of the
antheridium, together with a mucilage formed of the degraded walls of
their mother cells.
In the female plant there arise at the apex of the stem, surrounded by
an envelope of ordinary leaves, several archegonia. These are of the
ordinary type of those organs, namely, a broad lower portion, containing
a naked oosphere and a long narrow neck with a central canal leading to
the oosphere. Down this canal pass one or more antherozoids, which
become absorbed into the oosphere, and this then secretes a wall, and
from it grows the second or asexual generation. The peculiarity of this
asexual or spore-bearing plant is that it is parasitic on the sexual
plant; the two generations, although not organically connected, yet
remain in close contact, and the spore-bearing generation is at all
events for a time nourished by the leafy sexual generation.
The spore-bearing generation consists of a long stalk, closely held
below by the cells of the base of the archegonium; this supports a
broadened portion which contains the spores, and the top is covered with
the remains of the neck of the archegonium forming the calyptra.
The spores arise from special or mother-cells by a process of division,
or it may be even termed free-cell formation, the protoplasm of each
mother-cell dividing into four parts, each of which contracts, secretes
a wall, and thus by rejuvenescence becomes a spore, and by the
absorption of the mother-cells the spores lie loose in the spore sac.
The spores are set free by the bursting of their chamber, and each
germinates, putting out a branched thread of cells called a protonema,
which may perhaps properly be termed a third generation in the cycle of
the plant; for it is only from buds developed on this protonema that the
leafy sexual plant arises.
The characteristics, then, of the mosses are, that the sexual generation
is leafy, the one or two asexual generations are thalloid, and that the
spore-bearing generation is in parasitic connection with the sexual
generation.
In the case of the fern, these conditions are very different.
The sexual generation is a small green thalloid structure called a
prothallium, which bears antheridia and archegonia, each archegonium
having a neck-canal and oosphere, which is fertilized just as in the
moss.
But the asexual generation derived from the oospore only for a short
while remains in connection with the prothallium, which, of course,
answers to the leafy portion of the moss. What is generally known as the
fern is this asexual generation, a great contrast to the small leafless
moss fruit or sporogonium as it is called, to which it is
morphologically equivalent. On the leaves of this generation arise the
sporangia which contain the spores. The spores are formed in a manner
very similar to those of the mosses, and are set free by rupture of the
sporangium.
The spore produces the small green prothallium by cell-division in the
usual way, and this completes the cycle of fern life.
The alternation of generations, which is perhaps most clear and typical
in the case of the fern, becomes less distinctly marked in the plants of
higher organization and type.
Thus in the Rhizocarpae there are two kinds of spores, _microspores_ and
_macrospores_, producing prothallia which bear respectively antheridia
and archegonia; in the Lycopodiaceae, the two kinds of spores produce
very rudimentary prothallia; in the cycads and conifers, the microspore
or pollen grain only divides once or twice, just indicating a
prothallium, and no antheridia or antherozoids are formed. The
macrospore or embryo-sac produces a prothallium called the endosperm, in
which archegonia or corpuscula are formed; and lastly, in typical
dicotyledons it is only lately that any trace of a prothallium from the
microspore or pollen cell has been discovered, while the macrospore or
embryo-sac produces only two or three prothallium cells, known as
antipodal cells, and two or three oospheres, known as germinal vesicles.
This description of the analogies of the pollen and embryo-sac of
dicotyledons assumes that the general vegetative structure of this class
of plants is equivalent to the asexual generation of the higher
cryptogams. In describing their cycle of reproduction I will endeavor to
show grounds for this assumption.
We start with the embryo as contained in the seed. This embryo is the
product of fertilization of a germinal vesicle by a pollen tube. Hence,
by analogy with the product of fertilization of rhizocarp's, ferns, and
mosses, it should develop into a spore bearing plant. It does develop
into a plant in which on certain modified leaves are produced masses of
tissue in which two kinds of special reproductive cells are formed. This
is precisely analogous to the case of gymnosperms, lycopods, etc., where
on leaf structures are formed macro and micro sporangia.
To deal first with the microsporangium or pollen-sac. The pollen cells
are formed from mother cells by a process of cell division and
subsequent setting free of the daughter cells or pollen cells by
rejuvenescence, which is distinctly comparable with that of the
formation of the microspores of Lycopodiaceae, etc. The subsequent
behavior of the pollen cell, its division and its fertilization of the
germinal vesicle or oosphere, leave no doubt as to its analogy with the
microspore of vascular cryptogams.
Secondly, the nucleus of the ovule corresponds with the macrosporangium
of Selaginella, through the connecting link of the conifers, where the
ovule is of similar origin and position to the macrosporangium of the
Lycopodiaceae. But the formation of the macrospore or embryo-sac is
simpler than the corresponding process in cryptogams. It arises by a
simple enlargement of one cell of the nucleus instead of by the division
of one cell into four, each thus becoming a macrospore. At the top of
this macrospore or embryo-sac two or three germinal vesicles are formed
by free cell formation, and also two or three cells called antipodal
cells, since they travel to the other end of the embryo-sac; these
latter represent a rudimentary prothallium. This formation of germinal
vesicles and prothallium seems very different from the formation of
archegonia and prothallium in Selaginella, for instance; but the link
which connects the two is in the gymnosperms, where distinct archegonia
in a prothallium are formed.
Thus we see that the flowering plant is essentially the equivalent of
the asexual fern, and of the sporogonium of the moss, and the pollen
cell and the embryo-sac represent the two spores of the higher
cryptogams, and the pollen tube and the germinal vesicles and antipodal
cells are all that remain of the sexual generation, seen in the moss as
a leafy plant, and in the fern as a prothallium. Indeed, when a plant
has monoecious or dioecious flowers, the distinction between the asexual
and the sexual generation has practically been lost, and the
spore-bearing generation has become identified with the sexual
generation.
Having now described the formation of the pollen and the germinal
vesicles, it only remains to show how they form the embryo. The pollen
cell forms two or three divisions, which are either permanent or soon
absorbed; this, as before stated, is the rudimentary male prothallium.
Then when it lies on the stigma it develops a long tube, which passes
down the style and through the micropyle of the ovule to the germinal
vesicles, one of which is fertilized by what is probably an osmotic
transference of nuclear matter. The germinal vesicle now secretes a
wall, divides into two parts, and while the rest of the embyro-sac fills
with endosperm cells, it produces by cell division from the upper half a
short row of cells termed a suspensor, and from the lower half a mass of
cells constituting the embryo. Thus while in the moss the asexual
generation or sporogonium is nourished by the sexual generation or leafy
plant, and while in the fern each generation is an independent
structure, here in the dicotyledon, on the other hand, the asexual
generation or embryo is again for a time nourished in the interior of
the embryo-sac representing the sexual generation, and this again
derives its nourishment from the previous asexual generation, so that as
in the moss, there is again a partial parasitism of one generation on
the other.
To sum up the methods of plant reproduction: They resolve themselves
into two classes.
1st. Purely vegetative.
2d. Truly reproductive by special cells.
In the second class, if we count conjugation as a simple form of
fertilization, there are only two types of reproductive methods.
1st. Reproduction from an asexual spore.
2d. Reproduction from an oospore formed by the combination of two sexual
cells.
In the vast majority of plant species these two types are used by the
individuals alternately.
The extraordinary similarity of the reproductive process, as shown in
the examples I have given, Achlya, Spirogyra, and Vaucheria among algae,
the moss, the fern, and the flowering plant, a similarity which becomes
the more marked the more the details of each case and of the cases of
plants which form links between these great classes are studied, points
to a community of origin of all plants in some few or one primeval
ancestor. And to this inference the study of plant structure and
morphology, together with the evidence of palaeobotany among other
circumstances, lends confirmatory evidence, and all modern discoveries,
as for instance that of the rudimentary prothallium formed by the pollen
of angiosperms, tend to the smoothing of the path by which the descent
of the higher plants from simpler types will, as I think, be eventually
shown.
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