of animals converge towards the conclusion that the nerve fibres, which
we have hitherto regarded as ultimate elements of nervous tissue, are not
such, but are simply the visible aggregations of vastly more attenuated
filaments, the diameter of which dwindles down to the limits of our
present microscopic vision, greatly as these have been extended by modern
improvements of the microscope; and that a nerve is, in its essence,
nothing but a linear tract of specially modified protoplasm between two
points of an organism--one of which is able to affect the other by means
of the communication so established. Hence, it is conceivable that even
the simplest living being may possess a nervous system. And the question
whether plants are provided with a nervous system or not, thus acquires a
new aspect, and presents the histologist and physiologist with a problem
of extreme difficulty, which must be attacked from a new point of view
and by the aid of methods which have yet to be invented.
Thus it must be admitted that plants may be contractile and locomotive;
that, while locomotive, their movements may have as much appearance of
spontaneity as those of the lowest animals; and that many exhibit
actions, comparable to those which are brought about by the agency of a
nervous system in animals. And it must be allowed to be possible that
further research may reveal the existence of something comparable to a
nervous system in plants. So that I know not where we can hope to find
any absolute distinction between animals and plants, unless we return to
their mode of nutrition, and inquire whether certain differences of a
more occult character than those imagined to exist by Cuvier, and which
certainly hold good for the vast majority of animals and plants, are of
universal application.
A bean may be supplied with water in which salts of ammonia and certain
other mineral salts are dissolved in due proportion; with atmospheric air
containing its ordinary minute dose of carbonic acid; and with nothing
else but sunlight and heat. Under these circumstances, unnatural as they
are, with proper management, the bean will thrust forth its radicle and
its plumule; the former will grow down into roots, the latter grow up
into the stem and leaves of a vigorous bean-plant; and this plant will,
in due time, flower and produce its crop of beans, just as if it were
grown in the garden or in the field.
The weight of the nitrogenous protein compounds, of the oily, starchy,
saccharine and woody substances contained in the full-grown plant and its
seeds, will be vastly greater than the weight of the same substances
contained in the bean from which it sprang. But nothing has been supplied
to the bean save water, carbonic acid, ammonia, potash, lime, iron, and
the like, in combination with phosphoric, sulphuric, and other acids.
Neither protein, nor fat, nor starch, nor sugar, nor any substance in the
slightest degree resembling them, has formed part of the food of the
bean. But the weights of the carbon, hydrogen, oxygen, nitrogen,
phosphorus, sulphur, and other elementary bodies contained in the bean-
plant, and in the seeds which it produces, are exactly equivalent to the
weights of the same elements which have disappeared from the materials
supplied to the bean during its growth. Whence it follows that the bean
has taken in only the raw materials of its fabric, and has manufactured
them into bean-stuffs.
The bean has been able to perform this great chemical feat by the help of
its green colouring matter, or chlorophyll; for it is only the green
parts of the plant which, under the influence of sunlight, have the
marvellous power of decomposing carbonic acid, setting free the oxygen
and laying hold of the carbon which it contains. In fact, the bean
obtains two of the absolutely indispensable elements of its substance
from two distinct sources; the watery solution, in which its roots are
plunged, contains nitrogen but no carbon; the air, to which the leaves
are exposed, contains carbon, but its nitrogen is in the state of a free
gas, in which condition the bean can make no use of it;[3] and the
chlorophyll[4] is the apparatus by which the carbon is extracted from the
atmospheric carbonic acid--the leaves being the chief laboratories in
which this operation is effected.
[Footnote 3: I purposely assume that the air with which the bean is
supplied in the case stated contains no ammoniacal salts.]
[Footnote 4: The recent researches of Pringsheim have raised a host of
questions as to the exact share taken by chlorophyll in the chemical
operations which are effected by the green parts of plants. It may be
that the chlorophyll is only a constant concomitant of the actual
deoxidising apparatus.]
The great majority of conspicuous plants are, as everybody knows, green;
and this arises from the abundance of their chlorophyll. The few which
contain no chlorophyll and are colourless, are unable to extract the
carbon which they require from atmospheric carbonic acid, and lead a
parasitic existence upon other plants; but it by no means follows, often
as the statement has been repeated, that the manufacturing power of
plants depends on their chlorophyll, and its interaction with the rays of
the sun. On the contrary, it is easily demonstrated, as Pasteur first
proved, that the lowest fungi, devoid of chlorophyll, or of any
substitute for it, as they are, nevertheless possess the characteristic
manufacturing powers of plants in a very high degree. Only it is
necessary that they should be supplied with a different kind of raw
material; as they cannot extract carbon from carbonic acid, they must be
furnished with something else that contains carbon. Tartaric acid is such
a substance; and if a single spore of the commonest and most troublesome
of moulds--_Penicillium_--be sown in a saucerful of water, in which
tartrate of ammonia, with a small percentage of phosphates and sulphates
is contained, and kept warm, whether in the dark or exposed to light, it
will, in a short time, give rise to a thick crust of mould, which
contains many million times the weight of the original spore, in protein
compounds and cellulose. Thus we have a very wide basis of fact for the
generalisation that plants are essentially characterised by their
manufacturing capacity--by their power of working up mere mineral matters
into complex organic compounds.
Contrariwise, there is a no less wide foundation for the generalisation
that animals, as Cuvier puts it, depend directly or indirectly upon
plants for the materials of their bodies; that is, either they are
herbivorous, or they eat other animals which are herbivorous.
But for what constituents of their bodies are animals thus dependent upon
plants? Certainly not for their horny matter; nor for chondrin, the
proximate chemical element of cartilage; nor for gelatine; nor for
syntonin, the constituent of muscle; nor for their nervous or biliary
substances; nor for their amyloid matters; nor, necessarily, for their
fats.
It can be experimentally demonstrated that animals can make these for
themselves. But that which they cannot make, but must, in all known
cases, obtain directly or indirectly from plants, is the peculiar
nitrogenous matter, protein. Thus the plant is the ideal _prolétaire_ of
the living world, the worker who produces; the animal, the ideal
aristocrat, who mostly occupies himself in consuming, after the manner of
that noble representative of the line of Zähdarm, whose epitaph is
written in "Sartor Resartus."
Here is our last hope of finding a sharp line of demarcation between
plants and animals; for, as I have already hinted, there is a border
territory between the two kingdoms, a sort of no-man's-land, the
inhabitants of which certainly cannot be discriminated and brought to
their proper allegiance in any other way.
Some months ago, Professor Tyndall asked me to examine a drop of infusion
of hay, placed under an excellent and powerful microscope, and to tell
him what I thought some organisms visible in it were. I looked and
observed, in the first place, multitudes of _Bacteria_ moving about with
their ordinary intermittent spasmodic wriggles. As to the vegetable
nature of these there is now no doubt. Not only does the close
resemblance of the _Bacteria_ to unquestionable plants, such as the
_Oscillatorioe_ and the lower forms of _Fungi_, justify this conclusion,
but the manufacturing test settles the question at once. It is only
needful to add a minute drop of fluid containing _Bacteria_, to water in
which tartrate, phosphate, and sulphate of ammonia are dissolved; and, in
a very short space of time, the clear fluid becomes milky by reason of
their prodigious multiplication, which, of course, implies the
manufacture of living Bacterium-stuff out of these merely saline matters.
But other active organisms, very much larger than the _Bacteria_,
attaining in fact the comparatively gigantic dimensions of 1/3000 of an
inch or more, incessantly crossed the field of view. Each of these had a
body shaped like a pear, the small end being slightly incurved and
produced into a long curved filament, or _cilium_, of extreme tenuity.
Behind this, from the concave side of the incurvation, proceeded another
long cilium, so delicate as to be discernible only by the use of the
highest powers and careful management of the light. In the centre of the
pear-shaped body a clear round space could occasionally be discerned, but
not always; and careful watching showed that this clear vacuity appeared
gradually, and then shut up and disappeared suddenly, at regular
intervals. Such a structure is of common occurrence among the lowest
plants and animals, and is known as a _contractile vacuole_.
The little creature thus described sometimes propelled itself with great
activity, with a curious rolling motion, by the lashing of the front
cilium, while the second cilium trailed behind; sometimes it anchored
itself by the hinder cilium and was spun round by the working of the
other, its motions resembling those of an anchor buoy in a heavy sea.
Sometimes, when two were in full career towards one another, each would
appear dexterously to get out of the other's way; sometimes a crowd would
assemble and jostle one another, with as much semblance of individual
effort as a spectator on the Grands Mulets might observe with a telescope
among the specks representing men in the valley of Chamounix.
The spectacle, though always surprising, was not new to me. So my reply
to the question put to me was, that these organisms were what biologists
call _Monads_, and though they might be animals, it was also possible
that they might, like the _Bacteria_, be plants. My friend received my
verdict with an expression which showed a sad want of respect for
authority. He would as soon believe that a sheep was a plant. Naturally
piqued by this want of faith, I have thought a good deal over the matter;
and, as I still rest in the lame conclusion I originally expressed, and
must even now confess that I cannot certainly say whether this creature
is an animal or a plant, I think it may be well to state the grounds of
my hesitation at length. But, in the first place, in order that I may
conveniently distinguish this "Monad" from the multitude of other things
which go by the same designation, I must give it a name of its own. I
think (though, for reasons which need not be stated at present, I am not
quite sure) that it is identical with the species _Monas lens_ as defined
by the eminent French microscopist Dujardin, though his magnifying power
was probably insufficient to enable him to see that it is curiously like
a much larger form of monad which he has named _Heteromita_. I shall,
therefore, call it not _Monas_, but _Heteromita lens_.
I have been unable to devote to my _Heteromita_ the prolonged study
needful to work out its whole history, which would involve weeks, or it
may be months, of unremitting attention. But I the less regret this
circumstance, as some remarkable observations recently published by
Messrs. Dallinger and Drysdale[5] on certain Monads, relate, in part, to
a form so similar to my _Heteromita lens_, that the history of the one
may be used to illustrate that of the other. These most patient and
painstaking observers, who employed the highest attainable powers of the
microscope and, relieving one another, kept watch day and night over the
same individual monads, have been enabled to trace out the whole history
of their _Heteromita_; which they found in infusions of the heads of
fishes of the Cod tribe.
[Footnote 5: "Researches in the Life-history of a Cercomonad: a Lesson in
Biogenesis"; and "Further Researches in the Life-history of the Monads,"
--_Monthly Microscopical Journal_, 1873.]
Of the four monads described and figured by these investigators, one, as
I have said, very closely resembles _Heteromita lens_ in every
particular, except that it has a separately distinguishable central
particle or "nucleus," which is not certainly to be made out in
_Heteromita lens_; and that nothing is said by Messrs. Dallinger and
Drysdale of the existence of a contractile vacuole in this monad, though
they describe it in another.
Their _Heteromita_, however, multiplied rapidly by fission. Sometimes a
transverse constriction appeared; the hinder half developed a new cilium,
and the hinder cilium gradually split from its base to its free end,
until it was divided into two; a process which, considering the fact that
this fine filament cannot be much more than 1/100000 of an inch in
diameter, is wonderful enough. The constriction of the body extended
inwards until the two portions were united by a narrow isthmus; finally,
they separated and each swam away by itself, a complete _Heteromita_,
provided with its two cilia. Sometimes the constriction took a
longitudinal direction, with the same ultimate result. In each case the
process occupied not more than six or seven minutes. At this rate, a
single _Heteromita_ would give rise to a thousand like itself in the
course of an hour, to about a million in two hours, and to a number
greater than the generally assumed number of human beings now living in
the world in three hours; or, if we give each _Heteromita_ an hour's
enjoyment of individual existence, the same result will be obtained in
about a day. The apparent suddenness of the appearance of multitudes of
such organisms as these in any nutritive fluid to which one obtains
access is thus easily explained.
During these processes of multiplication by fission, the _Heteromita_
remains active; but sometimes another mode of fission occurs. The body
becomes rounded and quiescent, or nearly so; and, while in this resting
state, divides into two portions, each of which is rapidly converted into
an active _Heteromita_.
A still more remarkable phenomenon is that kind of multiplication which
is preceded by the union of two monads, by a process which is termed
_conjugation_. Two active _Heteromitoe_ become applied to one another,
and then slowly and gradually coalesce into one body. The two nuclei run
into one; and the mass resulting from the conjugation of the two
_Heteromitoe_, thus fused together, has a triangular form. The two pairs
of cilia are to be seen, for some time, at two of the angles, which
answer to the small ends of the conjoined monads; but they ultimately
vanish, and the twin organism, in which all visible traces of
organisation have disappeared, falls into a state of rest. Sudden wave-
like movements of its substance next occur; and, in a short time, the
apices of the triangular mass burst, and give exit to a dense yellowish,
glairy fluid, filled with minute granules. This process, which, it will
be observed, involves the actual confluence and mixture of the substance
of two distinct organisms, is effected in the space of about two hours.
The authors whom I quote say that they "cannot express" the excessive
minuteness of the granules in question, and they estimate their diameter
at less than 1/200000 of an inch. Under the highest powers of the
microscope, at present applicable, such specks are hardly discernible.
Nevertheless, particles of this size are massive when compared to
physical molecules; whence there is no reason to doubt that each, small
as it is, may have a molecular structure sufficiently complex to give
rise to the phenomena of life. And, as a matter of fact, by patient
watching of the place at which these infinitesimal living particles were
discharged, our observers assured themselves of their growth and
development into new monads. In about four hours from their being set
free, they had attained a sixth of the length of the parent, with the
characteristic cilia, though at first they were quite motionless; and, in
four hours more, they had attained the dimensions and exhibited all the
activity of the adult. These inconceivably minute particles are therefore
the germs of the _Heteromita_; and from the dimensions of these germs it
is easily shown that the body formed by conjugation may, at a low
estimate, have given exit to thirty thousand of them; a result of a
matrimonial process whereby the contracting parties, without a metaphor,
"become one flesh," enough to make a Malthusian despair of the future of
the Universe.
I am not aware that the investigators from whom I have borrowed this
history have endeavoured to ascertain whether their monads take solid
nutriment or not; so that though they help us very much to fill up the
blanks in the history of my _Heteromita_, their observations throw no
light on the problem we are trying to solve--Is it an animal or is it a
plant?
Undoubtedly it is possible to bring forward very strong arguments in
favour of regarding _Heteromita_ as a plant.
For example, there is a Fungus, an obscure and almost microscopic mould,
termed _Peronospora infestans_. Like many other Fungi, the _Peronosporoe_
are parasitic upon other plants; and this particular _Peronospora_
happens to have attained much notoriety and political importance, in a
way not without a parallel in the career of notorious politicians,
namely, by reason of the frightful mischief it has done to mankind. For
it is this _Fungus_ which is the cause of the potato disease; and,
therefore, _Peronospora infestans_ (doubtless of exclusively Saxon
origin, though not accurately known to be so) brought about the Irish
famine. The plants afflicted with the malady are found to be infested by
a mould, consisting of fine tubular filaments, termed _hyphoe_, which
burrow through the substance of the potato plant, and appropriate to
themselves the substance of their host; while, at the same time, directly
or indirectly, they set up chemical changes by which even its woody
framework becomes blackened, sodden, and withered.
In structure, however, the _Peronospora_ is as much a mould as the common
_Penicillium_; and just as the _Penicillium_ multiplies by the breaking
up of its hyphoe into separate rounded bodies, the spores; so, in the
_Peronospora_, certain of the hyphoe grow out into the air through the
interstices of the superficial cells of the potato plant, and develop
spores. Each of these hyphoe usually gives off several branches. The ends
of the branches dilate and become closed sacs, which eventually drop off
as spores. The spores falling on some part of the same potato plant, or
carried by the wind to another, may at once germinate, throwing out
tubular prolongations which become hyphoe, and burrow into the substance
of the plant attacked. But, more commonly, the contents of the spore
divide into six or eight separate portions. The coat of the spore gives
way, and each portion then emerges as an independent organism, which has
the shape of a bean, rather narrower at one end than the other, convex on
one side, and depressed or concave on the opposite. From the depression,
two long and delicate cilia proceed, one shorter than the other, and
directed forwards. Close to the origin of these cilia, in the substance
of the body, is a regularly pulsating, contractile vacuole. The shorter
cilium vibrates actively, and effects the locomotion of the organism,
while the other trails behind; the whole body rolling on its axis with
its pointed end forwards.
The eminent botanist, De Bary, who was not thinking of our problem, tells
us, in describing the movements of these "Zoospores," that, as they swim
about, "Foreign bodies are carefully avoided, and the whole movement has
a deceptive likeness to the voluntary changes of place which are observed
in microscopic animals."
After swarming about in this way in the moisture on the surface of a leaf
or stem (which, film though it may be, is an ocean to such a fish) for
half an hour, more or less, the movement of the zoospore becomes slower,
and is limited to a slow turning upon its axis, without change of place.
It then becomes quite quiet, the cilia disappear, it assumes a spherical
form, and surrounds itself with a distinct, though delicate, membranous
coat. A protuberance then grows out from one side of the sphere, and
rapidly increasing in length, assumes the character of a hypha. The
latter penetrates into the substance of the potato plant, either by
entering a stomate, or by boring through the wall of an epidermic cell,
and ramifies, as a mycelium, in the substance of the plant, destroying
the tissues with which it comes in contact. As these processes of
multiplication take place very rapidly, millions of spores are soon set
free from a single infested plant; and, from their minuteness, they are
readily transported by the gentlest breeze. Since, again, the zoospores
set free from each spore, in virtue of their powers of locomotion,
swiftly disperse themselves over the surface, it is no wonder that the
infection, once started, soon spreads from field to field, and extends
its ravages over a whole country.
However, it does not enter into my present plan to treat of the potato
disease, instructively as its history bears upon that of other epidemics;
and I have selected the case of the _Peroganspora_ simply because it
affords an example of an organism, which, in one stage of its existence,
is truly a "Monad," indistinguishable by any important character from our
_Heteromita_, and extraordinarily like it in some respects. And yet this
"Monad" can be traced, step by step, through the series of metamorphoses
which I have described, until it assumes the features of an organism,
which is as much a plant as is an oak or an elm.
Moreover, it would be possible to pursue the analogy farther. Under
certain circumstances, a process of conjugation takes place in the
_Peronospora_. Two separate portions of its protoplasm become fused
together, surround themselves with a thick coat and give rise to a sort
of vegetable egg called an _oospore_. After a period of rest, the
contents of the oospore break up into a number of zoospores like those
already described, each of which, after a period of activity, germinates
in the ordinary way. This process obviously corresponds with the
conjugation and subsequent setting free of germs in the _Heteromita_.
But it may be said that the _Peronospora_ is, after all, a questionable
sort of plant; that it seems to be wanting in the manufacturing power,
selected as the main distinctive character of vegetable life; or, at any
rate, that there is no proof that it does not get its protein matter
ready made from the potato plant.
Let us, therefore, take a case which is not open to these objections.