By ANDREW WILSON.
One of the plainest points connected with the study of living things
is the power we apparently possess of separating animals from plants.
So self-evident appears this power that the popular notion scoffs at
the idea of science modestly disclaiming its ability to separate the
one group of living beings from the other. Is there any danger of
confusing a bird with the tree amid the foliage of which it builds its
nest, or of mistaking a cow for the grass it eats? These queries are,
of course, answerable in one way only. Unfortunately (for the
querists), however, they do not include or comprehend the whole
difficulty. They merely assert, what is perfectly true, that we are
able, without trouble, to mark off the higher animals from the higher
plants. What science inquires is, whether we are able to separate
_all_ animals from _all_ plants, and to fix a definite boundary line,
so as to say that all the organisms on the one side of the line are
assuredly animals, while all the others on the opposite side of the
line may be declared to be truly plants. It is exactly this task which
science declares to be among the impossibilities of knowledge. Away
down in the depths of existence and among the groundlings of life the
identity of living things becomes of a nature which is worse than
confusing, and which renders it a futile task to attempt to separate
the two worlds of life. The hopelessness of the task, indeed, has
struck some observers so forcibly that they have proposed to
constitute a third kingdom--the _Regnum Protisticum_--between the
animal and the plant worlds, for the reception of the host of doubtful
organisms. This third kingdom would resemble the casual ward of a
workhouse, in that it would receive the waifs and strays of life which
could not find a refuge anywhere else.
A very slight incursion into biological fields may serve to show forth
the difficulties of naturalists when the task of separating animals
from plants is mooted for discussion. To begin with, if we suppose our
popular disbeliever to assert that animals and plants are always to be
distinguished by shape and form, it is easy enough to show him that
here, as elsewhere, appearances are deceptive. What are we to say of a
sponge, or a sea anemone, of corals, of zoophytes growing rooted from
oyster shells, of sea squirts, and of sea mats? These, each and all of
them, are true animals, but they are so plant-like that, as a matter
of fact, they are often mistaken by seaside visitors for plants. This
last remark holds especially true of the zoophytes and the sea mats.
Then, on the other hand, we can point to hundreds of lower plants,
from the yeast plant onward, which show none of the ordinary features
of plant life at all. They possess neither roots, stems, branches,
leaves, nor flowers, so that on this first count of the indictment the
naturalist gains the day.
Power of movement, to which the popular doubter is certain to appeal,
is an equally baseless ground of separation. For all the animals I
have above named are rooted and fixed, while many true plants of lower
grade are never rooted at all. The yeast plant, the _Algæ_ that swarm
in our ponds, and the diatoms that crowd the waters, exemplify plants
that are as free as animals; and many of them, besides, in their young
state especially (e.g., the seaweeds), swim about freely in the water
as if they were roving animalcules. On the second count, also, science
gains the day; power of motion is no legitimate ground at all for
distinguishing one living being as an animal, while absence of
movement is similarly no reason for assuming that the fixed organism
must of necessity be a plant. Then comes the microscopic evidence.
What can this wonder glass do in the way of drawing boundary lines
betwixt the living worlds? The reply again is disappointing to the
doubter; for the microscope teaches us that the tissues of animals and
plants are built upon kindred lines. We meet with cells and fibers in
both. The cell is in each case the primitive expression of the whole
organism. Beyond cells and fibers we see the wonderful living
substance, _protoplasm_, which is alike to our senses in the two
kingdoms, although, indeed, differing much here and there in the
results of its work. On purely microscopic grounds, we cannot separate
animals from plants. There is no justification for rigidly assuming
that this is a plant or that an animal on account of anything the
microscope can disclose. A still more important point in connection
with this protoplasm question consists in the fact that as we go
backward to the beginnings of life, both in animals and plants, we
seem to approach nearer and nearer to an identity of substance which
baffles the microscope with all its powers of discernment. Every
animal and every plant begins existence as a mere speck of this living
jelly. The germ of each is a protoplasm particle, which, whatever
traces of structure it may exhibit, is practically unrecognizable as
being definitely animal or plant in respect of its nature. Later on,
as we know, the egg or germ shows traces of structure in the case of
the higher animals and plants; while even lowly forms of life exhibit
more or less characteristic phases when they reach their adult stage.
But, of life's beginnings, the microscope is as futile as a kind
scientific touchstone for distinguishing animals from plants as is
power of movement, or shape, or form.
A fourth point of appeal in the matter is found within the domain of
the chemist. Chemistry, with its subtile powers of analysis, with its
many-sided possibilities of discovering the composition of things, and
with its ability to analyze for us even the light of the far distant
stars, only complicates the difficulties of the biologist. For, while
of old it was assumed that a particular element, nitrogen, was
peculiar to animals, and that carbon was an element peculiar to
plants, we now know that both elements are found in animals, just as
both occur in plants. The chemistry of living things, moreover, when
it did grow to become a staple part of science, revealed other and
greater anomalies than these. It showed that certain substances which
were supposed to be peculiar to plants, and to be made and
manufactured by them alone, were also found in animals. Chlorophyl is
the green coloring matter of plants, and is, of course, a typical
product of the vegetable world; yet it is made by such animals as the
hydra of the brooks and ponds, and by many animalcules and some worms.
Starch is surely a typical plant product, yet it is undoubtedly
manufactured, or at least stored up, by animals--a work illustrated by
the liver of man himself, which occasionally produces sugar out of its
starch.
Again, there is a substance called _cellulose_, found well nigh
universally in plants. Of this substance, which is akin to starch, the
walls or envelopes of the cells of plant tissues are composed. Yet we
find those curious animals, the sea squirts, found on rocks and stones
at low-water mark, manufacturing cellulose to form part and parcel of
the outer covering of their sac-like bodies. Here it is as if the
animal, like a dishonest manufacturer, had infringed the patent rights
of the plant. On the fourth count, then--that of chemical
composition--the verdict is that nothing that chemistry can teach us
may serve definitely, clearly, and exactly to set a boundary line or
to erect a partition wall between the two worlds of life. There yet
remains for us to consider a fifth head--that of the food.
In the matter of the feeding of the two great living worlds we might
perchance light upon some adequate grounds for making up the one
kingdom from the other. What the consideration of form, movement,
chemical composition, and microscopic structure could not effect for
us in this way, it might be supposed the investigation of the diet of
animals and plants would render clear. Our hopes of distinguishing the
one group from the other by reference to the food on which animals and
plants subsist are, however, dashed to the ground; and the diet
question leaves us, therefore, when it has been discussed, in the same
quandary as before.
Nevertheless, it is an interesting story, this of the nutrition of
animals and plants. A large amount of scientific information is to be
gleaned from such a study, which may very well be commenced by our
having regard to the matters on which a _green_ plant feeds. I
emphasize the word "green," because it so happens that when a plant
has no chlorophyl (as green color is named in the plant world) its
feeding is of diverse kind to that which a green plant exhibits. The
mushroom or other fungus may be taken as an illustration of a plant
which represents the non-green race, while every common plant, from a
bit of grass to an oak tree, exemplifies the green-bearing order of
the vegetable tribes.
Suppose we were to invite a green plant to dinner, the _menu_ would
have to be very differently arranged from that which would satisfy a
human or other animal guest. The soup would be represented for the
plant's delectation by water, the fish by minerals, the joint by
carbonic acid gas, and the dessert by ammonia. On these four items a
green plant feeds, out of them it builds up its living frame. Note
that its diet is of inorganic or non-living matter. It derives its
sustenance from soil and air, yet out of these lifeless matters the
green plant elaborates and manufactures its living matter, or
protoplasm. It is a more wonderful organism than the animal, for while
the latter can only make new protoplasm when living matter is included
in its food supply, the green plant, by the exercise of its vital
chemistry, can transform that which is not living into that which is
life-possessing.
The green plant in other words, raises non-living into living matter,
while the animal can only transform living matters into its like. This
is why the plant is called a constructive organism, while the animal
is, contrariwise, named a destructive one. The result of the plant's
existence is to build up, that of the animal's life is to break down
its substance, as the result of its work, into non-living matter. The
animal's body is, in fact, breaking down into the very things on which
the green plant feeds. We ourselves are perpetually dissipating our
substance in our acts of life and work into the carbonic acid, water,
ammonia, and minerals on which plants feed. We "die daily" in as true
a sense as that in which the apostle used the term. And out of the
debris of the animal frame the green plant builds up leaf and flower,
stein and branch, and all the other tokens of its beauty and its life.
If, then, an animal can only live upon living matter--that is to say
on the bodies of other animals or of plants--with water, minerals and
oxygen gas from the air thrown in to boot, we might be tempted to hold
that in such distinctive ways and works we had at last found a means
of separating animals from plants. Unfortunately, this view may be
legitimately disputed and rendered null and void, on two grounds.
First of all, the mushrooms and their friends and neighbors, all true
plants, do not feed as do the green tribes. And secondly, many of the
green plants themselves can be shown to have taken very kindly to an
animal mode of diet.
A mushroom, thus, because it has no green color, lives upon water,
oxygen, minerals, and organic matter. You can only grow mushrooms
where there is plenty of animal matter in a state of decay, and as for
the oxygen, they habitually inhale that gas as if they were animals.
Non-green plants thus want a most characteristic action of their green
neighbors. For the latter in daylight take in the carbonic acid gas,
which is composed of carbon and oxygen. Under the combined influence
of the green color and the light, they split up the gas into its two
elements, retaining the carbon for food and allowing the oxygen to
escape to the atmosphere. Alas! however, in the dark our green plant
becomes essentially like an animal as regards its gas food, for then
it is an absorber of oxygen, while it gives off carbonic acid. If to
take in carbonic acid and to give out oxygen be held to be a feature
characteristic of a plant, it is one, as has been well said, which
disappears with the daylight in green plants, and which is not
witnessed at all in plants that have no green color.
So far, we have seen that not even the food of plants and animals can
separate the one kingdom of life from the other. The mushroom bars the
way and the green plant's curious behavior by night and by day
respectively, in the matter of its gas food, once more assimilates
animal life and plant life in a remarkable manner. Still more
interesting is the fact, already noticed, that even among the green
tribes there are to be found many and various lapses from the stated
rules of their feeding. Thus what are we to say of the parasitic
mistletoe, which, while it has grown leaves of its own, and can,
therefore, obtain so much carbon food from the air on its own account,
nevertheless drinks up the sap of the oak or apple which forms its
host, and thus illustrates the spectacle of a green plant feeding like
an animal, on living matter? Or, what may we think of such plants as
the sundew, the Venus' fly trap, the pitcher plants, the side saddle
plants, the butterworts and bladderworts, and others of their kind,
which not only capture insects, often by ingenious and complex lures,
but also digest the animal food thus captured? A sundew thus spreads
out its lure in the shape of its leaf studded with sensitive
tentacles, each capped by a glistening drop of gummy secretion.
Entangled in this secretion, the fly is further fixed to the leaf by
the tentacles which bend over it and inclose it in their fold. Then is
poured out upon the insect's body a digestive acid fluid, and the
substance of the dissolved and digested animal is duly absorbed by the
plant. So also the Venus' fly trap captures insects by means of its
leaf, which closes upon the prey when certain sensitive hairs have
given the signal that the animal has been trapped. Within the leaf the
insect is duly digested as before, and its substance applied to the
nutrition of the plant. Such plants, moreover, cannot flourish
perfectly unless duly supplied with their animal food. Such
illustrations of exceptions to the rule of green plant feeding simply
have the effect of abolishing the distinctions which the diet question
might be supposed to raise between animals and plants. We may return
to the sundews and other insect catchers; meanwhile, I have said
enough to show that to the question, "Can we separate animals from
plants?" a very decided negative reply must be given. Life everywhere
exhibits too many points of contact to admit of any boundary line
being drawn between the two great groups which make up the sum total
of organic existence.--_Illustrated London News._
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