Part 6

Chlorophyll grains lie just behind the skin cells in all parts of the plant that look green. The cells they lie in are often long with their short ends towards the skin. Leaves contain several layers of chlorophyll cells. The inner ones are not long like the outer ones, and do not contain so many chlorophyll grains. In the illustration, _a_, _a_ represent the upper and lower skin and _b_ the cells containing chlorophyll. The under side of a leaf usually has fewer chlorophyll grains in its cells, for the light is not so bright there, and chlorophyll needs plenty of light.

Sometimes the cells in the middle of a leaf, that is, halfway between the upper and lower surfaces, have no chlorophyll at all.

Now what _do_ you suppose is the work the chlorophyll grains have to do?

You never could guess, so I may as well tell you at once. If it is not making sugar, it is something very like it. To begin at the beginning, which is a long way from sugar, but which will certainly bring us to it, I must tell you that these little round green chlorophyll people have a strong attraction for carbon dioxide, which you know is a gas and is always found in the air. You know, too, we breathe it out as an impurity. Probably you did not know it had anything to do with sugar, but it has a very great deal to do with it.

The chlorophyll grains attract carbon dioxide as strongly as a magnet attracts bits of iron. The carbon dioxide in the air goes through the pores in the leaf skin, right through everything to the cell where the chlorophyll lies. You know carbon dioxide is made of carbon and oxygen. The plant needs a great deal of carbon, for nearly all its hard parts are made of it. Wood for one thing is nearly all carbon.

As soon as carbon dioxide comes where chlorophyll is, the chlorophyll, which of course is chiefly made of protoplasm, tears it to pieces. It pulls the carbon away from the oxygen and the oxygen rushes out through the pores back into the air. But the carbon stays behind.

You see oxygen is a gas and carbon is a solid. When carbon and oxygen unite in a certain way, they make another gas, our carbon dioxide.

It is very queer that carbon should have the form of a gas when united with oxygen, and I cannot explain it here. You must just remember that it is so.

When the oxygen flies away into the air again and leaves the carbon behind, the work of the chlorophyll has but just begun. Raw carbon is of no use whatever,—no more use than carbon dioxide, which we know is good for nothing to the plant or else the chlorophyll would not tear it to pieces.

But if the chlorophyll can only get a little water, something worth while will happen. This it can always do, as the roots take good care to send it plenty.

Water, you know, is made of two gases, hydrogen and oxygen, united together.

Here, you see, gases unite and make a liquid. Well, chlorophyll has a way of its own of uniting the carbon it took away from the carbon dioxide with the hydrogen and oxygen it gets from the water and forming a solid, which the plant cannot live without.

Now what do you suppose this new solid is? Probably you _never_ could guess.

It is _starch_, just _starch_!

Chlorophyll makes starch out of carbon, hydrogen, and oxygen.

Sometimes it makes sugar and oil out of them, but its work is most generally starch-making.

The carbon, you remember, it gets from the carbon dioxide of the air, and the hydrogen and oxygen from the water the roots send it.

The strangest thing about all this is, chlorophyll is the only thing that _can_ make starch.

Perhaps you do not think starch worth making such a fuss about. But wait a moment.

There is more to starch than you ever dreamed of. Really and truly, if it were not for starch you would not be alive to-day, and I would not,—in short nobody would.

All our lives depend upon starch. So when we come right down to the truth, our lives depend upon chlorophyll, because that makes all the starch there is in the world.

You do not think our lives depend upon starch? Wait and see.

Chlorophyll makes starch. Never forget that as long as you live. Forget your own name if you want to, but do not forget that chlorophyll makes starch.

You see starch is the raw material of which plants are made.

After the chlorophyll has made starch, the starch is dissolved, or _melted_ you would likely say, and so is carried all over the plant in the sap. Some parts of the plant change the starch into sugar; for sugar is made of the same things as starch, only in it the carbon, hydrogen, and oxygen are put together a little differently, just as you can make several kinds of cake from flour, butter, sugar, milk, and eggs by stirring them together differently and mixing them in different proportions.

You cannot make cake without flour, sugar, eggs, and milk, and usually butter. But if you have these ingredients you can make a great many kinds of cake.

Starch is the material of which the plant makes a large part of its substance.

Some parts of the plant that need sugar make it from the starch, and we find more or less sugar in all plants. There is, as you know, a great deal in the nectar of flowers, but other parts of the plant need it too, so sugar is a matter of importance to plants as well as to people. But sugar, remember, is made generally from starch, no matter in what part of the plant we find it.

The sweet sap in the sugar maple is made from starch; so is the sweet juice of the sugar beet and of the sugar cane. All the sugar we use, excepting that in homeopathic pills, is made from starch. The sweet juice of fruits, berries, apples, peaches, oranges, contains sugar, which the plant has made from starch. In green fruit the starch has not yet been changed into sugar, so it is not pleasant to the taste.

Some parts of the plant need thick walls, like wood or bark, and these are made by the protoplasm from starch; they are not sugar, however, but a very tough, firm substance so unlike sugar that you wonder how it can be made of the same materials. But it is, for starch is the substance from which both are made.

There are other things in the plant besides starch, and there are things which are not made from starch; for instance, there are acids and minerals of different kinds and there is protoplasm, but the greater part of every green plant is formed from starch.

Some plants make more starch than they need at once, so they store it away for future use, just as people raise extra supplies of wheat and corn, and store them away until they want them.

The potato plant, for instance, stores a large quantity of starch in the potatoes underground. A potato is nearly all starch, and the sweet potato stores up sugar as well as starch in its underground parts.

The potatoes have a reason for this, and, if let alone, would use up the starch and sugar another season; but we do not let them alone, as you know. We too need starch, and so we dig up the potatoes and eat them instead of leaving them for the plant.

A great many plants store up starch in their seeds that the young plant may have food enough to start growing. All our grains do this. Wheat, rye, oats, barley, rice, corn, and all other grains are only the seeds of plants which have been stored full of starch. Peas and beans are also starch-filled seeds. Cabbages store food made from starch in their big thick leaves. Beets store sugar and other starch-food materials in their thick roots; so do carrots and parsnips and turnips. Onions store it in their bulb leaves underground.

You begin to see now how important starch is to our lives. Nearly all the vegetables and grains and fruits we eat are composed almost entirely of starch or the materials of starch. Even meat is made from starch, for what do the animals we kill for meat live on?

Why, plants of course, and chiefly the starch they find in plants.

So now we are just where we started,—we see we really do owe our lives to starch, and we owe starch to chlorophyll, so of course, we owe our lives to chlorophyll. I wonder if we shall think of this next time we look at the green leaves everywhere in the fields and woods.

I wonder if these green leaves will not look more beautiful than ever when we think of the work they are doing.

ROOT CELLS.

Roots do their work underground as a rule.

You might prefer not to be a root, if you had your choice; you might prefer to be a leaf or a flower.

I have never heard that the roots complained of their work, however. For one thing, it is easier. All they have to do is to hold the plant fast, suck up juices from the earth, and in some cases store away food material,—that is, if they are regular, well-behaved, everyday, underground roots.

Sometimes, however, roots come out of the ground and do all sorts of things,—cling to walls and hang in the air and perform in other unroot-like ways; but these are not what we are talking about. We are talking of roots, such as those of the morning-glory and nasturtium and geranium, which stay underground and behave themselves.

Since it is dark where they live, they have no chlorophyll grains, and do not have to make starch. They merely use up the starch that comes to them from above.

Since they are not blown about by the wind, they do not need complicated, stiff, supporting tissues like tree trunks. On the whole, they are rather a simple people. They are made of cells, of course. But there are not so many kinds of cells in them as in the stems and leaves.

They have skin cells, but no pores. Out of their skin cells grow their most interesting and important parts. These are called root hairs. They are made of cells lying next each other, like other hairs, but they do all the sucking up of food materials for the whole root. These root hairs draw the water and other food out of the soil for the use of the plant, and the rest of the root only stores it up and conducts it to the stem and leaves above and anchors the plant to the ground.

The root’s work as an anchor is important, as you can imagine.

Just suppose that plants had no strong roots twisting around stones and bits of earth underground and holding them fast! What a time there would be whenever the wind blew.

Even a light breeze would be worse than a cyclone at present, for it would send the wheat in the wheatfields flying before it.

All the plants would go hurry-skurry wherever the wind blew—excepting the morning-glories and others that were twined about trellises or fences or rocks; and even they would be blown all out of shape.

And when a strong wind came, if the trees had no roots to anchor them _they_ would go hurry-skurry in the direction in which the wind blew, even if they were balanced so that they could not fall over; and we should see the forests sliding about the country and probably right on our houses, knocking them down, so we would not be able to have any houses, but would have to live in caves. It is a very good thing for us that the plants are held fast by their roots.

Well, the root hairs do the most important work of the plant after all. It is they who go poking their noses through the soil, and with their cells draw up water and potash and nitrogen and sulphur and iron and many other things which have become dissolved in the water. They are even able to dissolve rocks and such delicacies for themselves.

Now a growing root tip is a very delicate thing. You could not expect it to go pushing its tender tip through the hard earth without some kind of protection. And it does not: it wears a cap. This cap fits over the tip of the root and is hard. The cap is not alive, that is, the outside of it is not. The growing part of the root tip is just behind the cap.

The root tip grows by adding on new cells and so pushes the root cap ahead of it. The hard root cap finds its way between the particles of earth and so opens a channel for the growing root tip behind it.

The cap wears off on the outside as the bark does on a tree, and, like that, is continually renewed from the inside where the cells are alive.

SKIN CELLS.

Skin covers over and protects what is underneath. It is thin compared with what it covers, but it is important, as we discover when we lose a piece of our own skin. A fluid substance or even blood oozes out, and the spot where the skin is off is very painful.

Plants have a skin too, and it does for them what our skin does for us. It is tough and protects the soft inner parts and keeps the sap from oozing out.

Skin, of course, is built up of cells. These cells generally lie close together, touching each other, except at certain spots, where there is an opening.

Skin cells are usually long and wide, and their outer walls, as you would expect, are thicker than the inside walls. The protoplasm builds up hard material on the outside to protect the rest of the leaf or stem. Leaves and young stems and roots and flower parts all have skin.

The skin is alike in all in a general way, just as all houses are alike in a general way. They all have a roof, walls, partitions, doors, and windows, though these are of different sizes and arranged differently in different houses to suit the needs of the people who live in them. So with plants. The skin cells are different in size and shape and thickness in different plants to suit the needs of the plants, though in all there is a general resemblance.

Here is a row of skin cells (_a_) with other cells (_b_) back of them. See how thick the skin cells are on the outside (_c_). They are very tough there too. _d_ is an opening between two cells, and all is magnified several hundred times.

Sometimes there are several layers of skin cells where the plant needs a particularly thick skin; _a_ in the illustration is an example of such a skin.

But it would not do to have an air-tight skin, even for a plant.

Our own skins are full of holes, or pores, as you know, to let out the extra water and other waste materials in what we call perspiration. The plants need such an arrangement as much as we do. So in their skin we find pores. You see the plant needs a great deal of water. The water is used in making the substance of the plant. It is also used in the sap to carry food about from place to place. Sap contains a great deal of water in order that it may flow easily. This water cannot all be used by the plant, and when it comes up from the roots in the sap a large part of it has to be got rid of by the leaves.

If the skin were solid, the water could not escape. But you know what protoplasm can do.

If the skin needs pores, it will make them. And this is how it does it.

If you peel off a bit of skin from the under side of a leaf and put it under the microscope, you will see something like this.

The round forms are the pores. The crooked lines between are the edges of the cell walls, and you are looking at them right through the outer wall of the skin, which is transparent like glass, otherwise you could not see the edges of the partitions.

Let us look at these pores, or stomata as we must call them, if we want to talk like botanists.

One of the stomata is called a “stoma”; stoma comes from the Greek and means a “mouth,” or “opening.” These little mouths, or stomata, are made of two cells lying close together. These cells reach through the skin into an open space back of it.

There are open spaces between many of the inner plant cells, and there is always one behind a stoma. There are very few spaces between skin cells, excepting, of course, the openings between the two cells of a stoma. The two cells which make a stoma are called “guard cells,” because they guard the opening into the plant.

They are shaped, you see, something like half-moons. When the plant is full of water these half-moons swell up and their edges are drawn apart—so. [Illustration: _x_]

This, you see, makes an opening (_x_) into the plant. This little mouth through the skin opens into the space back of the skin, and this space connects with other spaces all through the plant. Through these stomata all parts of the plant can communicate with the outer air. The extra water and other waste materials pass out through the open stomata and air and other gases pass in and out.

Now, if the air outside is very dry and the earth is dry so that the roots are not able to send up much water, these wise little guard cells do not swell up and separate.

They are too good gatekeepers for that. They straighten out, their edges meet—so—[Illustration: [Cell]] and the opening is closed.

Now the water cannot so readily escape and the plant will not wither so soon. In dry climates the stomata are often surrounded by hairs which prevent too rapid evaporation; these hairs are often thick enough to make the plant look woolly. In fact, many plants have hairs upon those parts of the leaves where the stomata are found; they not only prevent too rapid evaporation, but also keep the rain or dew from getting into the stomata and closing them up. They hold off the water so that it cannot wet that part of the leaf.

There are a great many stomata on one leaf,—on some kinds as many as thousands to a square inch.

Usually, among land plants, there are more on the under side of the leaf, and in very dry places all are on the under side. The sun shining on the upper side would often cause too great evaporation, so the stomata are found underneath. In very hot, dry air there will be a little evaporation, even when the stomata are closed.

But when we come to look at leaves that lie on the surface of the water, like water lily leaves, of course the stomata are all on top, as that is the only part of the leaf the air can reach.

Many water plants have their stomata above, for you see there is no danger of their water supply running short.

It is very important for a plant to keep its pores open and it is quite ingenious in contriving ways to do this. Perhaps hairs are most frequently used.

They often cover the under side of the leaf where the stomata are thickest, or are found in lines along the leaf, when the stomata are distributed in this way.

But, you say, rain cannot get to the under side of the leaf. No, but dew can. Dew wets the under side of the leaf quite as much as the upper side, for dew does not fall, as some people think, but is deposited all over the surface of a cool object like a leaf, for dew is nothing but the vapor in the air which is deposited in the form of water at night.

To see better how the stomata work, here is a side view of one closed (_a_) and one open (_b_).

Stomata, you see, are the doors to the plant through which things pass in and out. Not only water goes out through them, but also other waste substances, such as oxygen and carbon dioxide.

You must not suppose because so many things go _out_ at the doors that nothing goes in; for air passes in and also carbon dioxide.

Carbon dioxide passes out from the plant and in from the air! That seems curious, but you must remember the plant has to use its stomata for both lungs and mouths,—lungs to breathe out impure air, which contains carbon dioxide, and mouths to take in carbon dioxide, which is one of its principal foods.

Besides stomata, plant skin has other kinds of special cells. These other cells form hairs or prickles or scales or glands. The hairs, prickles, and scales form on the outside of the skin, as you can see by the illustration.

On the side of a regular skin cell the protoplasm builds a small cell; this grows long and divides and makes two; these may again divide, and so on until the plant has as long a hair as it needs. Sometimes the hair is made of but one long cell.

Hairs, as we know, protect the plant from too great evaporation and from changes of temperature; they also keep the dew and rain from settling in the stomata and filling them up so they cannot do their work.

Here is a picture of four stomata, growing about a hollow filled with hairs. These hairs prevent the outside water from running in and wetting the stomata.

Prickles and some kinds of hairs and scales protect the outside of the plant from animals. When the animals bite the plant, these things stick into their mouths and they are glad to let it alone.

If you want to be sure that prickles and hairs protect the outside of a plant, go take hold of a nettle!

Madam Nettle does not wish to be taken hold of nor eaten nor touched by cows or sheep or anything else.

So her skin has hairs on it that sting. The hairs are very sharp and they are hollow. There is a poisonous juice inside, something the protoplasm has made; and when the sharp end of a hair sticks into your finger, the little turned-up end breaks off, and the poisonous juice gets into the wound and irritates and causes the finger to swell a little.

There is a way to take hold of a nettle so that it cannot sting. The little poison-filled hairs all point _up_, as you see in the picture. So if you stroke the nettle or draw your hand over it from root to tip, it cannot hurt you. Your hand presses the hairs flat against the stem and they cannot stick into you.

Sometimes hairs branch and make a thick network, like felt, over the leaf. They do this in the mullein, and here is a picture of mullein hairs very highly magnified.

Prickles and scales are made of cells as hairs are.

All parts of the plant above ground and sometimes the roots are covered with skin, but only the parts above ground are covered with hairs or prickles. Some plants are abundantly supplied with these protections; others manage to get along without them.

Plants very often have glands in their skins. These glands are merely cells which take certain things from the sap and pour them out on the outside of the plant.

Glands secrete their fluids inside the skin cells, and these fluids finally break through the outer wall of the skin cell and so get to the surface, or else they pass through stomata specially provided for them. They sometimes cover the surface of the plant with a sticky substance, as is the case with young birch twigs.

Glands also secrete the gum or resin which covers up the winter buds and keeps out the rain, and which makes the young leaves of the cherry shine so.

Some plants secrete wax which covers leaves or stems or fruits. Bayberry berries are covered with white wax, of which fragrant candles can be made.

Bayberry grows abundantly all along the New England coast, and friends of Thoreau used to make these fragrant candles as Christmas presents. Whenever Thoreau went to visit them, he insisted upon having a bayberry candle to go to bed by.

The _bloom_ on cabbage leaves and on plums and other fruits is made of tiny scales of wax.

Wax is a very good substance to keep the plant dry. You may be sure the plant knows this and often uses it about the stomata. You see, the object is to allow water to pass freely _out_ of the stomata by evaporation, but not, as a rule, to pass _into_ them. So the clever plants often have wax instead of hairs as a protection to the stomata. It would not do at all to let the stomata get closed up, so they are always protected in some way. Sometimes little projections grow out of the skin, close to the stomata. The raindrops fall upon these little knobs and stay there, instead of settling down into the stomata. You see, the pegs are _very_ small, and when the rain falls on them there is a layer of air below them which the water cannot displace, and which prevents it from going any farther.