first comes out of the ground it grows up straight like any plant.
Pretty soon, however, being but a slender vine, it begins to bend
over. Thereupon, it begins to sweep its tip slowly round in a
circle—hop and honeysuckle toward the left, bean and morning-glory
toward the right. As the stem grows longer, the circle gets bigger,
the tip reaches out farther and farther after a support. When at
length it does swing round against pole or trellis, it still keeps
on winding, and so continuing to grow, winds itself up toward the
top. If one pole is not high enough, when it reaches the top, it
again sweeps its long growing end round till it catches something
else and winds up that. Thus the vine finds its support in the first
place, having reached the top of that jumps across to another,
almost as if it could see where it was going.
The coiled tendrils of grape vines manage in the same way. They,
too, sweep round in a circle till they catch a support. Sometimes,
too, the tendril, instead of merely growing round an object,
actually closes down and grips it like a shutting hand.
[Illustration: A root much enlarged, shows cells and hairs.]
Or perhaps you have wondered how the roots of a plant manage to find
their way thru the soil, always picking out the cracks and openings
and never butting up against a pebble and having to stop. This is
the way it manages. Instead of growing straight forward, steadily,
the tip of the root grows out by perhaps the thickness of a sheet of
tissue paper. Then it pulls back again not quite so far. Then,
perhaps half a minute later, it grows out a little farther, and
again draws back. Meantime, the root tip is writhing and twisting
like an earth worm, only much more slowly. Whenever the moving tip
touches a pebble or a grain of sand, the growing region, which is
just back of the tip, grows a little faster on the side where the
touch came, and so throws the tip of the root away from the
obstacle. In this way, sooner or later, the root hits the open space
and grows thru. You see, it is almost exactly like the way in which
the infusorian gets by obstacles with its touch, back, turn, and go
ahead; except that where the plant grows the animal swims.
Not only the root-tips but all the soft, green, growing parts of a
plant are continually pushing out and drawing back, twisting,
turning and bending; only the movement is generally so very slow
that one can hardly make it out at all. Yet there are certain
“sensitive plants” which when touched, pricked, heated or cooled,
roughly handled, jarred, or in almost any other way made to sit up
and take notice, fold up their leaves or drop them.
All plants, however, give some sort of slow jump or twitch or bend
when anything is done to them. They are made sluggish with cold, put
to sleep with ether and chloroform, revived by water when they are
thirsty, even made uncertain of movement when beer is poured over
their roots; all of course, just about like an animal under the same
circumstances. Both alike move more feebly when they are tired; both
alike stop moving when they are dead.
The plant, in short, is a very sluggish animal, shut up tight in a
wooden box, so that only the ends of its roots and shoots stick out
where we can see them move. We know, however, that all the living
jelly of the plant does move, tho we cannot see it inside the wood.
To be sure, the plant moves only very slowly. A leaf will turn when
a lighted match is held near it; but if somebody held the same
lighted match equally near your nose, your jump would be something
like four thousand times quicker. Nevertheless, some of the slowest
animals are not a bit more rapid of movement than the quickest
plants. The fig tree and the fresh water clam, for example, are
equally slow to move when they are touched, but move they both do.
But we must not forget the turning of the plants’ leaves toward the
light, for that is, after all, the one movement of plants which we
have all seen for ourselves.
The curious thing about it is that the leaf turns, not because the
light falls on the leaf itself, but because the light falls on the
stem. If we cover the blade of the leaf, but let the light fall on
the stem, then the leaf will turn; but if we shade the stem and
leave the blade uncovered, then the leaf will not turn. Or in case
there is a joint in the stem where the turning takes place, as in
the clover leaf, then there is where the plant does its seeing.
Allow the light to strike the leaf, cover that spot only, and the
leaf is blind, but cover everything else, and the leaf turns as
before.
Nor is it only sunlight toward which the plant turns its leaves. The
great Darwin, who was one of the first to study this matter
carefully, had a plant that after being kept a long while in the
dark, screwed round its leaves to face a small lamp twelve feet
away. Some of the so-called “sensitive plants,” will start turning
toward a candle ten seconds after they first catch sight of it.
Oddly enough, however, the leaf will move in exactly the same way,
if instead of letting light strike the stem, one rubs salt on it, or
brings a hot wire near by. In fact, leaves, tendrils, and other soft
green parts will turn toward a red hot wire till they touch it and
are burned to death. So the plant is after all much like the
infusorian. It can do one thing, which is generally right; but it
does that one thing just the same, even when that is the worst thing
it possibly could do.
XXIV
Some Plant-Like Doings Of Animals
The plants, then, know enough to do two things—to grow up or down
with stem or root, and to turn toward the light or away from it.
This really, if you can call this knowing, is about all they do
know. Now I am going to tell you about some common animals which are
not much better off than the plants; which know up and down, and
know the direction of the sunlight, and know mighty little else.
Happy is the community which does not know the brown-tail moth.
Wherever it appears, it spreads like a pestilence, eating every
green leaf off a tree, and leaving it in mid-summer as forlorn and
bare as at Christmas time. A great tree that has taken a hundred
years to grow, the progeny of one moth will kill in three.
The brown-tail moths, their cousins the golden-tail moths, and
several other sorts of moths, lay their eggs in the late summer and
early fall. The little caterpillars hatch out that same season, grow
to be something like a quarter inch long, and spend the winter in a
cocoon-like nest which they spin for themselves much as does a
silk-worm or a spider. In the spring, having eaten nothing all
winter, they leave the nest, crawl to the ends of the branches, and
proceed to devour the new leaves.
But how does a little worm, no bigger round than a slender pin,
finding itself in the midst of a great tree, with nothing near it
but tough bark which it cannot eat, know in the first place that
there are fresh green buds anywhere, and how in the second place,
does it find its way to the tips of the twigs where the buds are?
The answer is, that it doesn’t. The little caterpillar knows no more
about buds and food and the way to them than the tree itself does.
It is simply built like a tree, so that when it first leaves its
nest, it always turns its head up, and when it has a choice between
light and darkness, turns toward the light.
So the caterpillar simply turns up and toward the light, just as a
plant would, and with no more intelligence than a plant has, and no
more idea what it is about. But of course, crawling up and toward
the light, sooner or later brings it to the outer ends of the
branches where its food is.
You can easily prove this by putting the young caterpillars in a
bottle, or a wide-mouthed jar. If you lay the jar down on the table
with the closed end toward the window, every caterpillar will crawl
to the closed end, and never a one will crawl back away from the
light to the open end and escape. You don’t need any cover; the
light holds them fast prisoners. But turn the jar round, open end
toward the window, and soon there will not be a caterpillar left in
it.
Suppose now, when your caterpillars are at the closed end of the jar
toward the window, you take some fresh leaves, from the tree on
which you found the insects (since these are presumably the sort
they eat) and put them in the open end of the jar away from the
window. The little caterpillars will stay where they are till they
all starve to death, before one of them will turn round and crawl
away from the light toward its dinner. They are even more helpless
than a plant, which can at least send its roots toward water, no
matter how the light comes.
Also, as I have explained, the caterpillars must crawl up. So they
cannot escape from an open jar placed mouth down. Neither can they
escape from an open jar placed mouth up; because when they come to
the lip of the jar, in order to go farther, they must turn head down
to crawl down the outside. But they cannot crawl head down so there
they must stay. Moreover, if you put food in the bottom of the jar
while they are at the top, they can never crawl down to get it. They
cannot turn head down, and they do not have sense enough to crawl
backwards.
There is, nevertheless, this difference between caterpillars and
plants. If the plant grows up and turns toward the sun at all, it
does so always; but the caterpillar changes its nature, and after it
has reached the buds and once fed, then the impulse to move upwards
and lightward, is no longer useful, and so in a large measure
disappears.
Still many sorts of caterpillars keep these willy-nilly turnings, until
they are full grown. Our common—our much too common—tent-caterpillar,
is accustomed to leave its tent during the warm part of the day, crawl
to the tips of the branches where its food is, eat until the cool of
the evening begins, and then return to its tent. In no sense, however,
does it go after its food, knowing what it wants. During the warm part
of the day, it simply becomes like a plant stem, head up, and crawls.
It has to head up, and it has to crawl. So in the end it reaches its
food, but it doesn’t know anything about how it gets there.
I have often on an early summer afternoon, when the caterpillars are
getting restless and just ready to start out, taken the tent,
inhabitants and all, and put it on top of a post or a smooth rock.
The caterpillars being disturbed, at once start to crawl away. They
start in all directions. In a moment, of course, they find
themselves crawling head down. That being against the rules, they
turn and crawl up again. In no possible way can a single caterpillar
get off the top of that rock or post, until the regular time for
them to knock off eating and go back to the tent. Then they have to
crawl down; and cannot crawl up if they try. So the chief difference
between tent-caterpillars and plants, is that while the plant always
turns its root down and its stem up, the animal turns its whole body
down at certain times of day, and turns its whole body up at certain
others. One can hardly say that either has any more sense, or
intelligence, or knowledge, than the other.
All caterpillars, while they remain caterpillars, have to crawl
toward the light. All caterpillars, also, after they have changed
into butterflies and moths, when they fly, have to fly toward the
light also. That is why the swarms of moths collect around the arc
lights on the streets, fly into lighted rooms thru unscreened
windows in the evening, circle about the reading lamp, or if the
light chances to be a candle with an uncovered flame, fly into it
and are burned.
People will tell you that the moth is curious, wants to see what the
light is. But he isn’t; any more than the leaf is curious to look
out of the window to see what is going on in the street. Both alike
simply turn toward the brightest light. The moth, having turned
toward the light, when he flies, flies toward it. If the leaf could
fly, it also would fly into the flame and be burned.
The reason why moths only fly into the lamp is that they are about
the only insects that fly at all while the lamps are lighted; most
other winged insects, also, head toward the brightest light. So do
vast numbers of other small animals, snails and crabs and various
water bugs, earth worms, leeches, infusoria, and even minute fishes
just hatched out of their eggs. But older fishes and all the larger
animals with fur and feathers, have more sense. They go where they
please and turn any way they like just as we do.
Many small animals, on the other hand, are like plant roots. They
have to head according to the light, but they head away from it, and
so move toward the darker places. In fact, it is rather the rule for
the young insect, before it gets its wings, to burrow like a root
and turn away from the light, but to turn toward it later after it
gets its wings.
Perhaps the strangest fact of all is that some water animals which
ordinarily head away from the light, turn round and head toward it,
as soon as a little acid is added to the water. Alcohol, even common
soda water or ordinary salt, has the same effect. But some salt
water animals which normally head lightward, if put into slightly
fresher water, promptly turn tail to the light. All of which shows
that the creature himself hasn’t much choice in the matter, and
probably doesn’t know much about it anyway, any more than if it were
a plant.
These turnings of plant or animal, toward the light or away from it,
up or down, the heading up-stream of many fishes, and the necessity
for crowding into cracks and corners of many insects and other small
creatures, all these are called “tropisms.” Tropism is merely the
Greek word for turning. I tell you the name, because we human beings
who have speech, if we want to think about a matter, have to have a
name for it to act as a handle for our minds to take hold of.
We see, then, that the various sorts of living creatures which we
have met thus far in this book, tho they are all made of much the
same sort of living jelly, have really quite different sorts of
minds. We ourselves, as you know, have reason, speech, intelligence,
feeling, and instinct. The animals most like ourselves, dogs and
cats and horses and the like, have also intelligence, feeling and
instinct. Animals very different from ourselves such as fishes,
insects, and the various strange sea creatures, have some
intelligence, some feeling, a few strong instincts; and besides
these, certain tropisms. But the simplest animals of all, and the
plants, have neither intelligence nor instinct, but only feelings
and tropisms.
All living things, then, plants and animals alike, have feeling. I
have already explained something about instincts and tropisms; and
told you, if not much about intelligence or reason, at least
something about speech. Now I shall tell you something about the one
thing which all living things have in common, and that is feeling.
XXV
The Five Senses and The Other Five
Traditionally, of course, we have five senses—sight, hearing, touch,
taste and smell. Yet we sometimes say that we are “frightened out of
our seven senses,” as if there were seven and not five. Really, the
number of our different ways of feeling is neither five nor seven,
as we shall now see by counting them up for ourselves.
Five at least we are sure of, the traditional five. There is no need
for anyone to tell us what our eyes, ears, and noses are for; nor
that we taste with our mouths as well as talk and eat, nor that we
feel touches anywhere over our skins. As to this last, however, I
don’t think we always realize how completely the sense of touch is
confined to the skin. Headaches, for example, we feel on the outside
of the head; the brain itself can be pinched, cut, burned, and
generally maltreated, and we not feel it so much as we feel a
pinprick on a finger tip.
Do you remember how, a few years ago, when you had not so many
things to do as you have now, you used to amuse yourself by whirling
rapidly round in one direction, till you were dizzy and could hardly
stand up. For a few moments, or till you whirled around in the other
direction, you lost your sense of right-side-up-ness; you didn’t know
sky from ground; and if you had been whirling especially fast or
long, you probably fell down flat, and couldn’t get up again. When
you dance, too, without reversing, by and by you get giddy, the
floor begins to curl up at the edges, and you become uncertain as to
which way is down.
The trouble is not with the eyes, because we get dizzy with our eyes
shut. We really have a sense of up and down, which is neither sight,
hearing, touch, taste nor smell; but a sixth sense. Its organ is a
portion of the inner ear, the so-called labyrinth; and it tells us,
not how things look or sound or taste or smell or feel, but whether
we ourselves are right side up. Too much whirling upsets this sense
of equilibrium, just as too bright a light dazzles the eyes, or too
loud a sound stuns the ears. A shark will swim with his head cut
off; but his ears being gone, he is as likely as not to swim upside
down.
There, then, are six senses. Let us see if we can find number seven.
While you are sound asleep at night, you do more or less turning
over in bed, so that in the morning, you are lying in quite a
different position from that in which you went to sleep. Yet altho
you remember nothing of what you did in the night, when you awake in
the morning, and before you open your eyes, you can tell exactly the
position of every arm, leg, or finger. You know just how much each
joint is bent. You have, in short, a sense of where the several
parts of your body are, which is none of the six senses which we
have recognized heretofore.
So, too, if you shut your eyes, and let some other person move any
member into another position, you feel it move, and know all the
while just where it is. Or suppose you are playing short-stop, and
fielding a hot grounder. Your eye is on the ball, and your mind on
the game; but your hands drop, and your back bends, and your knees
sink, all to precisely the right degree to bring your hands where
the ball is. You have a most accurate sense of where these hands
are, tho you neither see nor hear nor touch nor taste nor smell
them. A piano player relies on this same muscular sense, when
without looking at the keyboard he skips unerringly from one note
to another, never going too far nor stopping too short. In fact,
this muscular sense is really one of the most accurate of all our
senses. We can see a speck of dust much too small to feel; but a
person used to using a microscope or doing other nice work, can make
a movement with his fingers as small as the twentieth part of the
width of a hair—and that is a good deal smaller than any unaided
human eye can possibly see.
As you might guess from its name, this muscular sense is located
partly in the muscles. Still more, however, do we depend on the
joints. We do not feel one bone slip over the other, that would be
plain touch; but we know with extraordinary accuracy just how far
the joint has moved and just where it is at any moment.
Here then are seven senses; now for number eight. Take something not
too sharp, a tooth-pick will do, or the point of a lead pencil, and
touch the skin lightly on the back of the hand or on the forehead or
near the elbow. You feel, of course, the touch. Now move the point
ever so little, by the thickness say of a small pin, and touch
again. You probably feel the same touch as before. Continue this
touching, and before you have tried twenty times, you ought to
discover a spot where the point feels suddenly cold. In short, some
points on the skin feel pressure, which is the ordinary sense of
touch. But other points feel cold, which is a different sense, an
eighth sense, and not touch at all.
There is also a ninth sense, the sense of heat. If you are very
careful indeed, and notice very closely, you can find heat spots,
just as you found the cold spots. Only these are much harder to
locate, so that you will probably not be able to make them out.
One more sense is located in the skin besides these three. That is
the sense of pain. Tho to be sure other parts of the body feel pain
also—the muscles and joints when we have rheumatism, and the teeth
when we have the tooth ache. Pain is not touch. In the first place
it feels very different, and in the second place, it is quite
possible to lose one sense and keep the other. In certain diseases
of the nerves, the ordinary sense of touch remains as before, but
the sense of pain completely disappears. One can be pinched, cut,
burned, in the most violent manner, feel the touch of fingers or
knife or hot wire, and yet not feel the slightest pain. And the same
thing is true, after one has taken the right amount of ether or
chloroform; one isn’t asleep, and one can feel touches but one does
not feel pain—and a wonderful blessing it is sometimes.
I wonder whether many of you have ever heard of cocain, or had it
used on you when you got something in your eye, or had to have
something done to the inside of nose or mouth. (Only you musn’t call
it “co-cane,” in two syllables, as many people do and as it looks in
print, but “co-ca-in,” in three syllables; for it is extracted from
coca leaves, and the name is “coca,” with the “in” added on, like
strychnin, atropin, protein, and the names of so many drugs and
medicines). Cocain, then, looks a good deal like common salt, but
fit grain or two in a drop of water, placed anywhere where the skin
is thin, soaks thru to the ends of the nerves, and for ten minutes
or so, puts an end to all feeling there. If now we apply cocain to
the tongue in just the right strength, it takes away for the moment
all sense of pain, while it leaves the sense of taste as before, and
the sense of touch. One can bite his tongue and feel the bite, yet
it does not hurt. One can drop hot syrup on it, taste the sweet, and
yet not feel the burn. But if we make the cocain a little stronger,
then when the pain goes, the taste goes with it. One can feel
something in his mouth, but cannot tell sugar from salt; while a
drop of strong acid, tho it burned a hole in the tongue, would
neither taste sour nor hurt. A still stronger dose of the drug
suspends all feeling—pain, taste, and touch alike.
So pain is a different sense from touch, just as much as taste is.
Besides, if you take a sharp pin and make a line of pricks close
together, much as you did when hunting for cold spots, taking pains
to prick equally hard each time, you will be pretty sure to discover
certain points where the prick hurts much more than it hurts the
thickness of the pin away. This, of course, is a pain spot. Between
these you hardly feel the pain at all.
There are, then, in the skin, cold spots, hot spots, touch spots,
and pain spots, from two to three times as many of each as there are
fine hairs on the skin. Under each of these spots, is the end of a
nerve, either branching like a little bush or ending in a sort of
oval knob, much the shape of a foot ball. And just as the eye sees,
but doesn’t hear, and the ear hears, but doesn’t see, so each of
these nerve endings feels either pain, or cold, or heat, or
pressure, but only one of them.
How many senses have we then? At least ten, which is twice the
traditional five. Besides these there are thirst and hunger, which
are certainly feelings, tho neither sight, hearing, touch nor any of
the rest. Then there is that peculiarly unpleasant feeling which
comes to us after we have dined less wisely than well, or have been
rocked too fondly in the cradle of the deep, the sensation, I mean,
which we call “sickness” or nausea. This makes thirteen senses.
There are several more in addition to these, more or less vague
affairs, which for the most part tell us only what is going on
inside our own bodies.
If therefore, you ask how many senses we really have, I shall have
to say that we had better call it ten. At least we have ten well
defined sorts of feelings, which tell us what is going on outside
our bodies—and after all, that is what senses are for. These, then,
will be sight, hearing, touch, taste, smell, heat, cold, pain,
equilibrium, and the muscular sense. Each of these has its own
special place, in eye, ear, joint between two bones, or little spot
in the skin. If we lacked any one of these (as indeed many creatures
do) there would be something which it is important for us to know,
but which would be forever impossible for us to find out.
XXVI
Eyes
I am not going to tell you about the wonderful structure of the eye,
nor about how it works. That, if you have not learned something
about it already in your school physiology, you will get sooner or
later, certainly before you get thru the high school. This book is
mostly about things that you do not learn in school.