Part 14

For instance, one sees little pine trees growing in a pasture. Each year they send up a “leading shoot” a foot or more long, straight up into the air, to become by and by the main trunk. Each year, also from the point where the trunk ended the year before, there starts out a whorl of little branches, a half dozen or so of them, some of which will by and by become the main limbs. It is the same way with the firs and spruces. If you haven’t seen these growing out of doors, you have at least had them for Christmas trees in the house.

When these little trees grow in the pasture, pretty soon a cow comes along and eats off the top so that there is no longer any leading shoot to grow up into a trunk. What does the tree do then, but pick out one of the side branches from the uppermost whorl, and turn that up into a new leading shoot. So the tree gets a trunk in spite of the cow. Somehow or other, nobody understands how, one of those green tufts which was meant to grow into a horizontal branch, changes its mind, turns up, and becomes the vertical trunk.

This may happen five or six times to a single tree—for when it isn’t cows, then it’s wind, or insects, or something else, that kills the bud that ought to grow up into the main trunk. So one finds often in the woods, trunks of evergreen trees that grow up straight for a few feet; then take a sharp turn to one side, and another straight run of trunk; then another turn. Each of these turns means that something has happened to the leading shoot, and that the tree, like a very wise vegetable, has at once made another leading shoot out of a side branch. But still the curve remains to show where the branch had to change its mind.

It certainly is most clever of the tree. I for one, should like much to know how that side branch finds out that something has happened to the leader, and that it must step into the gap; and how the tree decides which side branch it shall be that is to make the change. Sometimes, indeed, the tree doesn’t seem to be able to decide, so that two branches turn up instead of one; and after that, the tree has a double trunk. Sometimes, too, when the leading shoot is weak but still alive, a side branch turns up; and if the leader recovers itself and grows up strongly again, there will be here also a double trunk; but in such a case, one trunk will grow up straight, while the other starts out with a turn. So the tree sometimes makes a mistake, just as we all do. But it almost never makes so bad a mistake as to have three branches turn up into trunks, tho as far as numbers go, it might have six.

Another matter that all plants seem to understand is the difference between up and down. At least they never make the mistake of sending their roots into the air and their stems down. You recall the bean plant that is inside the bean, with its little root and its tiny stem and leaves tucked snugly away between the two big seed-leaves which are most of the bean. You may plant the bean any way you like, right side up or wrong side up, point the stem up and the root down, or the stem down and the root up, put the bean flat on its side, even plant it in one position for a while and then dig it up and turn it over to another. It makes no difference what you do; that little stem will twist round and grow up, and the little root will twist round the other way and grow down, tho each has to travel half way round the bean to do it. Somehow every seedling does tell which from t’other.

Now the question is, does the plant grow up because up is up, or because up is toward the light? The matter is easily settled. If we plant a seed in a pot, pack the earth in solidly, put a screen over the surface to keep the earth from falling out, and then tip the pot upside down then if the stem wants to grow up it will have to grow away from the light, and if it wants to grow toward the light it will have to grow down. Thus we shall find out whether the plant goes by light-and-darkness or by up-and-downness.

As a matter of fact, while nearly all plants are influenced by both, for young seedlings just getting their start in the world, the question of up and down is much the more important. So a seed planted in an upside down pot, will grow its stem up into the dirt, and its roots downward into the air.

So a plant knows in a way, up from down, as indeed it must, else a tree would not be able to send a tall trunk straight up into the air. Besides, when a tree happens to get uprooted, and yet lives, the new part of the trunk which grows after the accident, does not continue the direction of the old fallen portion, but turns and grows straight up. You can see this sort of thing almost anywhere in the woods.

The plant also, in a way, knows which way the light is falling on it. Commonly, as everybody is supposed to know, the plant grows toward the light. Yet the curious thing about it is that some parts of some plants always grow away from the light. The leafless runners of the strawberry geranium grow away from the light; but when they begin to form leaves on their ends, then they change and grow toward the light like other plants. The tendrils of many vines also, always grow away from the light, while the leaves and, stems are growing toward it. The reason is that by turning away from the light, they turn back toward the rock or tree trunk or wall or trellis which gives them support.

Thus the plant, that seems to know two things, is twice as well off as the infusorian that knows only one.

XXIII

What Plants Can Do

As trees and vines and shrubs and bushes are wiser than they look, so they can do more than we commonly suppose. We think of all plants as merely sitting still and growing; but they really do much more. Most ponds and ditches, the water squeezed out of bog moss, even damp spots on rocks or the ground, often swarm with minute green plants, that swim about quite as freely as if they were animals. Some of these, single-celled, pear-shaped affairs, have two long tails at the smaller end, with which they lash the water and so get about as freely as do the equally small animals which live with them. In fact, some of these little plants are so much like some of the infusoria, which I have already told you about, that about the only way to tell them apart is by the green color of the vegetable—tho to be sure the plant is apt to have two tails, while the animal has only one.

Then there are the so-called “diatoms” which live, absolutely millions upon millions, in the slippery coating which covers the sand and stones at the bottom of streams and ponds. These are commonly counted among plants; but they have two shells like an oyster and swim about freely—as an oyster does not, for all it is an animal. Then too, there are the “slime moulds,” which at some times of the year look like common puff-balls, and at other times change into a soft jelly, and crawl away to find a new place to change back into a puff-ball again.

In short, there is simply no end to the animal-like actions of the simpler plants, for after all, plants and animals are a good deal alike. To be sure, you don’t have any difficulty in telling a cow from an apple tree, but that is because a cow is a very complex sort of animal, and an apple tree is a very complex sort of plant. But the simpler plants, which have neither stem nor twigs nor leaves nor roots nor branches, and the simpler animals, which have neither heads nor legs nor bones nor muscles nor skins, are naturally not nearly so different from one another as apple trees and cows. And when you come to the very smallest and simplest creatures, the distinction between the two seems hardly worth counting. Some animals grow on stalks, and some plants swim about or crawl. Many plants are not even green; a few animals are. Once in a while, you find the very same creature described as an animal in one book, and as a plant in another.

However, I began to tell you about the animal-like actions of the plants which we see more commonly, the ordinary trees and shrubs and bushes, grass and house plants and the like.

We say that plants grow toward the light. They really do much more than that. When a houseplant has stood for some time at a window, in the same position, every leaf, as you know, is set to face the light, so that as much sunshine as possible falls on the upper surface of each. But if you turn the pot round, so that the leaves face away from the light, within a day or two, every several leaf will have skewed itself round toward the window again. So the plant can move its leaves about as much as an animal can move its head; only it moves very much more slowly. But the sunflower, grown out of doors, can wag its head fast enough to keep up with the sun. Indeed, it is called the sunflower, not so much because its blossom looks like the sun, as because, in the morning at sunrise, it bends its tip over toward the east so that the rising sun shall strike the upper sides of its leaves, follows the sun around thru the sky all day, and in the evening finds itself with all its upper leaves facing west. Then in the night it nods back again ready for the next sunrise.

Many leaves, if you notice them closely, have a soft bunch or cushion, either where the blade of the leaf joins the stem or where the stem of the leaf joins the branch or sometimes at both places. This is the joint on which the leaf does its turning. The clover, which is an especially active little plant has one of these joints for each of its three leaflets.

Not only the leaves of a plant, but the tendrils also, and the soft green parts of the stem, and the slender tips of the roots, turn and twist slowly, moving like the limbs of a very sluggish animal. Did you ever wonder how the climbing vines, the beans, peas, morning glories, woodbines, and the like, manage to find the poles and trellises on which they grow? The seedling comes out of the ground six inches or a foot away from the nearest support. Next thing you know, it has grown straight for the support and begun to climb.

This is the way it manages to find its way. When the young shoot 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.

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.