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;[32] and the chlorophyll[33] 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.
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 _Oscillatoriæ_, and 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[34] 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.
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 _Heteromitæ_ 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 _Heteromitæ_, 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. These, in about
four hours from their being set free, 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
_Peronosporæ_ 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
_hyphæ_, 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 hyphæ into separate rounded bodies, the spores;
so, in the _Peronospora_, certain of the hyphæ grow out into the air
through the interstices of the superficial cells of the potato plant,
and develop spores. Each of these hyphæ 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 hyphæ, 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 _Peronospora_ 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.
There are some small plants known to botanists as members of the genus
_Coleochæte_, which, without being truly parasitic, grow upon certain
water-weeds, as lichens grow upon trees. The little plant has the form
of an elegant green star, the branching arms of which are divided into
cells. Its greenness is due to its chlorophyll, and it undoubtedly
has the manufacturing power in full degree, decomposing carbonic acid
and setting oxygen free, under the influence of sunlight. But the
protoplasmic contents of some of the cells of which the plant is made
up occasionally divide, by a method similar to that which effects the
division of the contents of the _Peronospora_ spore; and the severed
portions are then set free as active monad-like zoospores. Each is oval
and is provided at one extremity with two long active cilia. Propelled
by these, it swims about for a longer or shorter time, but at length
comes to a state of rest and gradually grows into a _Coleochæte_.
Moreover, as in the _Peronospora_, conjugation may take place and
result in an oospore; the contents of which divide and are set free as
monadiform germs.
If the whole history of the zoospores of _Peronospora_, and of
_Coleochæte_ were unknown, they would undoubtedly be classed among
“Monads” with the same right as _Heteromita_; why then may not
_Heteromita_ be a plant, even though the cycle of forms through
which it passes shows no terms quite so complex as those which occur
in _Peronospora_ and _Coleochæte_? And, in fact, there are some
green organisms, in every respect characteristically plants, such
as _Chlamydomonas_, and the common _Volvox_, or so-called “Globe
animalcule,” which run through a cycle of forms of just the same simple
character as those of _Heteromita_.
The name of _Chlamydomonas_ is applied to certain microscopic green
bodies, each of which consists of a protoplasmic central substance
invested by a structureless sac. The latter contains cellulose, as
in ordinary plants; and the chlorophyll which gives the green colour
enables the _Chlamydomonas_ to decompose carbonic acid and fix carbon
as they do. Two long cilia protrude through the cell-wall, and effect
the rapid locomotion of this “monad,” which, in all respects except its
mobility, is characteristically a plant. Under ordinary circumstances,
the _Chlamydomonas_ multiplies by simple fission, each splitting
into two or into four parts, which separate and become independent
organisms. Sometimes, however, the _Chlamydomonas_ divides into eight
parts, each of which is provided with four instead of two cilia. These
“zoospores” conjugate in pairs, and give rise to quiescent bodies,
which multiply by division, and eventually pass into the active state.
Thus, so far as outward form and the general character of the cycle
of modifications, through which the organism passes in the course of
its life, are concerned, the resemblance between _Chlamydomonas_ and
_Heteromita_ is of the closest description. And on the face of the
matter there is no ground for refusing to admit that _Heteromita_ may
be related to _Chlamydomonas_, as the colourless fungus is to the green
alga. _Volvox_ may be compared to a hollow sphere, the wall of which
is made up of coherent Chlamydomonads; and which progresses with a
rotating motion effected by the paddling of the multitudinous pairs of
cilia which project from its surface. Each _Volvox_monad, moreover,
possesses a red pigment spot, like the simplest form of eye known among
animals. The methods of fissive multiplication and of conjugation
observed in the monads of this locomotive globe are essentially similar
to those observed in _Chlamydomonas_; and, though a hard battle has
been fought over it, _Volvox_ is now finally surrendered to the
Botanists.
Thus there is really no reason why _Heteromita_ may not be a plant; and
this conclusion would be very satisfactory, if it were not equally easy
to show that there is really no reason why it should not be an animal.
For there are numerous organisms presenting the closest resemblance to
_Heteromita_, and, like it, grouped under the general name of “Monads,”
which, nevertheless, can be observed to take in solid nutriment, and
which, therefore, have a virtual, if not an actual, mouth and digestive
cavity, and thus come under Cuvier’s definition of an animal. Numerous
forms of such animals have been described by Ehrenberg, Dujardin, H.
James Clark, and other writers on the _Infusoria_. Indeed, in another
infusion of hay in which my _Heteromita lens_ occurred, there were
innumerable infusorial animalcules belonging to the well-known species
_Colpoda cucullus_.[35]
Full-sized specimens of this animalcule attain a length of between
1/300 or 1/400 of an inch, so that it may have ten times the length
and a thousand times the mass of a _Heteromita_. In shape, it is
not altogether unlike _Heteromita_. The small end, however, is not
produced into one long cilium, but the general surface of the body is
covered with small actively vibrating ciliary organs, which are only
longest at the small end. At the point which answers to that from which
the two cilia arise in _Heteromita_, there is a conical depression, the
mouth; and, in young specimens, a tapering filament, which reminds one
of the posterior cilium of _Heteromita_, projects from this region.