Part 6

It is the same way with the plants. They too are built of these living bricks. Each leaf and blade of tree or grass is covered with a sheet of colorless cells one layer deep, which one can often peel off from the green pulp underneath. The green pulp, in turn, is a rather loose pile a half dozen thick, of roundish brick-shaped cells, each containing scattered grains of green coloring matter. The solid wood of a tree is only the thick walls of long slender cells, overlapping at the ends and packed tightly together. These cells lie lengthwise of the tree; that is why wood splits with the grain so much easier than it cuts across it.

I have already said that at the time of year when the tree is growing rapidly, these woody cells are large; but when the tree is growing slowly, they are small. So each year there is a change from large cells formed in the spring to smaller ones grown in the fall. The next year, the living substance of the cell moves off to the growing region next the bark, and leaves the old wood cells empty. These, therefore, never change; and because the large cells and the small ones do not look quite alike, we see the annual rings of wood in the tree trunk, as thick as card board, which give us the light and dark lines in our furniture and our hard wood floors. From these one can tell, not only how old the tree is, but also what were its good years when it grew rapidly, and what its poor seasons when it hardly grew at all. If a drought came along any summer, or if insects one year ate off all the leaves, that too shows in the wood. But trees which grow in the tropics, where they keep growing the whole year thru, do not have annual rings.

While some cells of the tree form wood and some green pigment, others in the bark produce cork, as one can see nicely in the thin layers of cork in the bark of an elm. The cells of juicy fruits swell up with water, and form sugar and various flavoring matters and pleasant acids. Where the animal cells swell up with oil and become fat, the plant cells swell up with starch grains and become a potato or the thick seed-leaves of a bean. But other cells form gum, rosin, turpentine, pitch, and the various oils and the like, pleasant or bitter, which we use for food and medicines.

So the plant, like the animal, is just a great mass of different sorts of these living bricks, and of the various substances which they form within and around them.

Naturally it takes millions upon millions of these living bricks to build up the body of a man or an apple tree, still more of a whale or one of the giant redwood trees of California. Many humbler creatures, on the other hand, both animals and plants, contain comparatively few. Our common green pond scums, for example, which tho they are plants, have neither leaves nor stems nor roots, are like single long lines of tiny green barrels set end to end. Our common sea-lettuce is a sheet of cells only one layer thick; while other sea-weeds and water plants are but bundles of a score or more. Often the fewer such bricks there are, the larger they are; even at times, to a half-inch in thickness and an inch or more in length.

A vast number of plants and animals, moreover, are single cells. Such among plants are the yeasts with which most of us make our bread, and a few of us brew our beer. Such also are the hundreds of different sorts of bacteria, which tho some of them are the germs of various catching diseases, are for the most part useful enough. But of these we shall learn more by and by. The green spots and patches on the bark of old trees and fences, and sometimes even on damp earth, are due to enormous numbers of minute plants, green with the same green pigment as the leaves of the largest tree; while the green tint of the gray lichens on rocks and tree trunks is caused by similar single-celled plants which grow among the white fibers of the lichen proper. Besides these, there are many like plants which float about in fresh water, each a single cell.

The diatoms which one finds in the mud at the bottom of ditches and mud-puddles, tho they have shells and move about, are usually counted among plants; but the water of most ditches and puddles swarms with amoebas, infusoria, animalcules of various sorts, most of them large enough to be made out with the unaided eye when seen in a tumbler against the light, and each a single cell.

Many animals, then, and many plants are just one single cell and no more. Many others, like pond scums and sea-lettuces,—which are plants,—and sponges and jelly-fishes,—which are animals,—are composed of many cells, but all pretty much alike. But the animals and plants which we know best, kittens and oak trees and horses and grass, and the creatures we know best of all which are ourselves, are made up of many cells, and many different sorts—skin and bark and wood, flesh and fat and leaves and hair and all the rest, so many that it would take half an hour merely to write them all down.

These “cells” then, are the living part of every plant and animal. Each of them became by the splitting in halves of an older cell; each of these in turn by the splitting in halves of a still older cell, until we get back to the egg which is the great-great-great-grandfather of them all. But the egg itself arose by the splitting of still another cell, which, of course, was part of the parent’s body. This came from yet another, and so on back to the beginning of life on this earth, tho nobody knows how long ago that was.

So the living flesh of us has always been alive. Most of it will die; but some of it will live on in our children and our children’s children, until the and of the world.

VII

How Much of Us Is Alive

How much of a tree is alive? Certainly not the outer bark. That falls off in dry scales, or can be scraped off down to the white layers within, and the tree be none the worse. Certainly not the wood. One often comes across old trees that have lost limbs or been carelessly pruned, which are entirely decayed out on the inside, so that nothing is left but a thin shell next the bark. Yet these trees grow as vigorously as ever, and bear leaves and fruit like a solid tree. The bark is dead; and the wood is dead. Between the two is a thin layer, perhaps a quarter inch thru, which is alive. On one side, it is changing into dead wood. On the other side, it is changing into dead bark. The new wood is alive, and the new bark. Between them is something neither wood nor bark, but just living tree-stuff. The green leaves also are alive, and the green twigs, and the blossoms, and the growing buds. But at least half of every living tree is already dead; while the larger and longer lived a tree is, the smaller proportion of it is alive at one time.

How much of a hen’s egg is alive? Not the shell, for that is mostly just chalk. Not the white, for that is merely the little chicken’s pantry shelf where it keeps the food on which it is to grow. The living part of the egg is the yolk—unless somebody boils the egg and so kills it. Sometimes, too, the egg dies, as any living thing may; then we usually find it out.

Even we ourselves are not all alive. I have already pointed out that our hair and nails are not alive at all, and that our outer skin, the thin skin, that is, which we tear off when we bark our shins, is fully alive only on the inside. Our “bark” in fact, is very like a tree’s. Each has a soft, thin, living layer on the inside, which grows, hardens, dies, forms a water-tight layer over the rest of the body, cracks into scales, and drops off. Where one forms cork, the other forms horn. Indeed the cork stoppers of our bottles are made from nothing more than an especially thick corky bark of a certain kind of oak, like the especially thick and homy soles of all bare-footed savages and some bare-footed little boys.

“The blood,” we say, “is the life.” And yet the blood itself is dead. The watery part is just soup; water and salt and fat and jelly. The minute, coin-like, red blood corpuscles carry the oxygen of the air from the lungs all over the body. But there are similar oxygen-carriers, likewise dead, in bottles in the drug-stores. The corpuscles are dead cells alive once, and like the hard skin cells, a great deal more useful dead than alive.

As for our teeth, the hard white enamel on the outside is just about as much alive as a clam shell. The baby tooth, as I have already explained, is formed in a little pocket in the gum. The inner part of the tooth grows up from the cells at the bottom, very much as a hair grows out from the bottom of the still smaller pocket where it starts. In fact, the tusks of pigs and the long front gnawing teeth of squirrels and rats are still more like hairs, for they keep growing from the root, and wear off at the outer end. The tooth pushes thru the gum; and as it goes by, the cells at the sides of the pocket and on top plaster it with a coating of enamel. Therefore, as most of us find out to our cost, this enamel once destroyed, can never grow again. Once clear of the pocket where it was formed, it has to last us the rest of our lives; and little boys and girls who don’t keep their teeth clean when they are young, have to put up with something not nearly so good when they are grown up.

The inside of the tooth is not quite so dead as the outside—one sometimes gets the impression during a visit to the dentist that it isn’t dead at all. The tooth, inside the enamel, is mostly bone; and bone is mostly lime, like clam-shells, mortar and chalk, plaster, and the great boulders and ledges of rock in a limestone country. The rest of the bone is living substance, scattered cells far apart from one another with long roots, that look as if they had grown out into the bone like tree roots into the soil. Really, however, it is the other way. Before there were bones, there were bone cells. These build themselves round with the hard bone substance, pushing their neighbors away and leaving only the long root-like strands of living substance. It is thru these root-like living strands that we feel the dentist’s auger bore into the solid tooth. But cutting the outer enamel does not hurt at all simply because no part of that is alive.

We are, then, built of living bricks, but of living bricks set in dead mortar. We saw that the great trees, complex and long lived, have more wood and bark and other dead substances in them than the shrubs, herbs, and grass. These in turn are less alive than the lowly water plants and yeasts and molds which have no wood or bark at all. The same is true of animals. The jelly-fishes and infusoria have neither skin, hair, bones, nails, nor blood, and are pretty much all alive. So the more a creature’s life is worth, the less of it is alive.

Even the living cells themselves are not wholly alive. The thin living jelly always contains water and salt, which are—just water and salt. Fat cells contain drops of oil, which are simply stored up food material, no more alive than the oil in an oil can. Plants, on their side, store up their food largely as starch, no more alive than that in a package from the grocers. Besides oil and starch, some cells contain gum or rosin or saliva or milk or sweat, which they pour out from time to time. These substances, too, can hardly be considered more alive while they are in the body than when they are outside.

So the living substance is the cell jelly. Everything outside the cell is dead; many cells even are dead, while not a few, even while alive, contain so much dead stuff within them, that there is more oil in a young hen’s egg than there is chick, and more starch than corn-plant in a grain of corn.

VIII

How We Grow

By “we” I mean all living things, trees and grass and dogs and cats and boys and girls. For as you, my reader, have I hope already discovered, we who have the breath of life in us are a good deal alike, whether we are oaks or men. We don’t look much alike, to be sure. But when we consider the things that are not alive—the stones and stoves and bats and balls and such—and see how very different we are from these, then we get some idea of what being alive is, and understand how being alive makes us blood-brothers with everything else that is alive also.

Now things that are alive usually do more or less growing. We have already learned something of this growing of little creatures in the egg—how the eye buds out as a ball and afterwards folds into a cup, how the limbs sprout out from the body as shapeless lumps which only gradually turn into hands or feet or wings. We have learned something of the way the bones grow, and the skin, and the hair, and the nails. Now we have to learn something more about the way a little child or a little tree grows up to be a big one.

The tree, we already know, grows larger round only between bark and wood. It grows taller only at the tips of its branches. The solid wood, once formed, does not change. If then, you drive two nails into the trunk of a little tree, say a foot apart and one above the other, even if that tree should grow to be a hundred feet high, those two nails would remain just where they were, a foot apart and just the same height from the ground as before. The little tree looks so much like the big one, that one cannot help thinking that it has simply grown thruout, so that the same branch which was once at the height of one’s head has now been lifted to the height of the house eaves. But this is not the fact. The lower branches of the little tree have died and dropped off; what are now the lower branches of the large tree, were once the top twigs of the little one, which have always been at the same level where they now are. The top branches of the large tree, as the tree grows still larger, will in their turn become, first the middle branches, then the lower ones, then will drop off entirely.

Now this growing at one end which looks like growing thruout is pretty common in our own bodies. We have seen how the hair grows at the inner end only, and the nails likewise, and the skin. Ignorant people will tell you that cutting off the ends of your hair, or singeing the ends; and that smearing various messes on the outside of your skins will change the quality of either. Don’t you believe them. After wood and skin and hair and teeth are once grown, all we can do is to protect them. Really to affect their growth for good or ill, we have to do something to the growing end.

The bones also grow in spots. The child’s leg bones and arm bones and finger bones do not simply swell up to become the man’s. The head of each bone, the rounding end, that is, where it touches the next, grows on the outside. But the shaft does its growing chiefly at two spots, one at each end where the shaft joins the head.