ROOT CELLS.
Roots do their work underground as a rule.
[Roots]
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 themtheywould 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.
Root hairs. Root cap.
SKIN CELLS.
[Skin]
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 theleaf 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.
[Skin cells]
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.dis an opening between two cells, and all is magnified several hundred times.
_a_
Sometimes there are several layers of skin cells where the plant needs a particularly thick skin;ain 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 wastematerials 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.
[Pores]
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._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 upmuch 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—[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.
_a b_
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 doorsto 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 gooutat 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.
[Hairs]
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.
[Hairs]
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.
[Hairs]
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 pointup, 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.
[Hairs]
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 theypass 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.
Thebloomon cabbage leaves and on plums and other fruits is made of tiny scales of wax.
Bayberry.
Bayberry.
Bayberry.
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 toallow water to pass freelyoutof the stomata by evaporation, but not, as a rule, to passintothem. 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 areverysmall, 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.
If you want to know just where the stomata are situated in a leaf, plunge it in water, then shake the drops off and notice what part of the leaf has not been wet. Wherever the leaf is dry, there are the stomata. In many plants, as, for instance, the jewelweed, it is quite impossible to wet the leaf. Soak it in water for an hour, and when you take it out it is dry! The parts that cannot be wet usually have a silvery, glistening appearance. Put the leaf in water and notice where it glistens; there are the stomata,—sometimes all over the under side of the leaf, sometimes in lines or patches, sometimes on both sides of the leaf.
Wax, gum, and resin are not the only things plant glands secrete. There are the glands in the flower cups that secrete nectar. In some plants this breaks through the delicate plant skin and runs into and fills up the little hollows or horns we call nectaries. In others the nectar is provided with stomata by means of which it can escape from the interior of the plant.
You may be surprised to learn that the flower is not the only part of the plant that can secrete nectar!
In some plants the stipules do it, and in some even the stems.
This is not to call visitors to the flowers, but perhaps to keep them away. Where ants trouble the flowers, certain kinds have invented this very clever way of stopping the unwelcome visitors. They do not want the ants to take the honey from the flowers, so they secrete honey on the leaves or stems, and the ants take that instead of traveling on to the flowers.
Of course each living skin cell contains protoplasm. The protoplasm lies in a thin layer against the walls and builds, builds, builds, until the skin is thick enough.
When a good thick wall has been built, the protoplasm passes out through tiny openings in the innerwall into the inside cells, where it goes to work doing something else. The skin cells are then empty of protoplasm; they are only filled with air, and we say they aredeadcells. Their hard walls are a good protection to the plant. In stems there is often a layer of thick cells behind the skin cells which also protects. These are called cork cells.
All very young plants have their stems covered with living skin.
Older plants, particularly woody ones, have their stems covered with the tough, dead skin. And trees have finally a thick layer of dead cork cells. In tree trunks the skin cells have disappeared entirely. The skin protected the young shoot; then its empty cells finally peeled off, as the cork cells formed underneath and made a thick bark. The bark then does the work of the skin. It protects the stem. It becomes very thick sometimes, as layers are constantly added beneath. The outside of the bark keeps peeling and scaling off.
Of course there are no stomata in bark. We find them only in the living skin. Bark does not need stomata, as it does not regulate the water supply. The young green parts of the plant do that by means of their covering of living skin. Living skin is usually transparent like glass.
It is tough and yet transparent. You see, the light must get through it to the cells which lie behind it.
There is usually no green color in skin. Sometimes there are other coloring materials, though not as a rule.
The living skin covers the leaf or stem or other part of the plant like a window of tough glass. Even where the skin is several cells thick, the light can pass through, just as it can through thick glass.
[Flower]
TUBE CELLS.
[Tube Cells]
The top of a tree is a long way from the roots. Yet the leaves must have food from the roots, and the roots must have food from the leaves.
[Tube Cells]
It is not an easy matter to move all this food material up and down, you may be sure.
I wonder howyouwould manage it?
Why, you say, if I had to raise sap from under the ground to the top of the tree, I should certainly build some pipes and have a pump at the top.
That is the way the plant has decided. So pipes there are, plenty of them,—pipes or tubes of many sizes and shapes.
You know how cells grow, lying next each other. Well, tube cells are long and contain protoplasm in the beginning. They lie end to end. But, you see, it would not be very easy for the sap to pass throughmillionsof cell walls on its way up.
So when the protoplasm has built a row of cells with good thick walls, it passes out through thin places or openings it has left in the walls. The end partitions between the tube cells are thin and break away, and lo and behold! we have a long, strong tube with nothing in it but air. Up this tube the sap creeps or down it the sap runs. A great many of these tubes, which are as fine as hairs or much finer in some cases, are needed in a plant. They run all through the stems and out into the leaves. They are collected into bundles, and form part of the veins and the framework of leaves. I do not know what the plant would do without them.
But what makes the sap runupthe tubes?
Now you are asking questions! It took a long time for people to find that out, for there is more than one reason why the sap runs up.
For one thing, the root cells keep drawing in water and other things, and the fluid already in is pushed up by that behind; so there is a sort of pump at the bottom of the plant, you see,—a force pump. The sun shining on the leaves and stems evaporates the water above, and the water below then easily takes its place; so there is a sort of suction pump at the top.
Then the tubes are soveryfine that the fluid in them tends to move up, just as water will soak up into a towel if the fringe happens to get into the water; for you know that if you hang a towel so that the fringe dips into a basin of water, after awhile the whole towel will be wet, as a result of what we call capillary attraction. For all these reasons the sap creeps up the stems through the tubes the cells have made.
Every plant has these tubes, from the tiniest weed in the garden to the tallest forest tree. Although so small, they are often very prettily marked by lines and dots.
STRENGTHENING CELLS.
Plants need something more than cells of working protoplasm and something more than tubes, just as we need more than flesh and blood vessels.
[Strengthening Cells]
We would be in a sad plight if we had no bones to keep us in place, and plants would be in a sad plight if they had no—well, not exactlybones, but something to serve the same purpose.
Think of the weight a tree has to bear. You could notbeginto lift the crown of a large tree, yet the tree trunk has to hold it up in the air. Not only that,—it has to hold on to it when the wind blows, which is a much harder task. Even small bushes and tender garden plants have quite a weight to bear and quite a task to keep their leaves and stems from being blown away. They couldneverhold on to them if it were not for the wood and other tough cells they have,—never in the world.
These wood cells and other tough cells are made by protoplasm, of course.
The protoplasm builds them very much as it does the tube cells, long and slender, as you see in thepicture at the beginning of the chapter, and then when the hard, tough walls are all done, the protoplasm slips out and leaves the strong framework of tough fibres to do its duty. This framework is not only strong, it is elastic, so it can bend easily. If it were not, the first strong wind or the first thing that happened to bend the plant would snap it off short.
You cannot break wood easily, and, if you do succeed, it always bends more or less first. Some wood bends more easily than others, as you know. A willow twig can be tied into a knot, it bends so easily.
Nearly all land plants have these stiffening cells. They run out of the stems down into the leaves and help make their framework of “veins.” The tubes and the strengthening fibres run along in bundles side by side. You see this saves space. If the tubes and strengthening fibres each took a different road, that would not leave much space for the chlorophyll and other working cells. But all the tubes and fibres are closely packed together and run lengthwise, through the stem. All around these long fibres are placed the other cells which are not long and do not form tubes or fibres. Most of those other cells in the leaf contain chlorophyll. Theycontain protoplasm, and do the work of transforming food materials into plant material.
[Strengthening Cells]
WE AND THE PLANT PEOPLE.
[Flower]
We live and the plants live. Probably neither we nor the plants spend much time thinking about what we owe to each other.
The plants are excusable for this, for they are not great thinkers, at least so far as we know.
But we owe so much to them, we ought to stop and think about it once in a while. We are indebted to them not only for the food we eat, but for the air we breathe.
We know about chlorophyll and the starch it makes, and how this starch is stored up in potatoes and wheat and corn and rice and all sorts of food grains and vegetables.
We know, too, how the roots suck up substances from the earth which we need in our bodies, and how they are stored away with the starch or sometimes bythemselves. We know, in short, how all the food we eat is made first or last by the plants. Not only do we owe our food to the plants, but all animals do.
You see, animal cells are not able to take carbon dioxide and water and ammonia and other gases and minerals and work them up into living cells.
The plants have to do this for them; and then the animals eat the plants, for animal cells are able to work starch and sugar and plant protoplasm over into animal protoplasm, which can build all sorts of animal cells. So all the animals in the world get their food from the plant world. If the plants were to stop living, all the animals in the world would soon starve to death. The word “animals,” you know, means every living thing that is not a plant; in this sense flies and bees and oysters and caterpillars are animals as well as dogs and cats and such large creatures. Last of all, we ourselves are animals.
So the animal world would be in a sad predicament if anything should happen to the plants.
But there is more to thank the plants for than food. That is a pretty large item certainly; but what do you think of having to thank them for the air we breathe as well? Yet this we shall have to do if we begin thanking them at all.
You know about oxygen, of course. It is one of the gases that make up the air; and I may as well remind you that air is composed principally of oxygen and nitrogen gases,—about four times as much nitrogen as oxygen, but the oxygen is the most important to us. We do not use the nitrogen in the air at all probably. It serves the purpose of diluting the oxygen, which would be too strong for us if it were not mixed with nitrogen. But what we do use is the oxygen.
That goes into our lungs, and some of it does not come out again. It passes into the lung cells and from them into the blood, and is carried by it all over our bodies to all the millions of cells.
We need a great deal of oxygen, and if the supply should be cut short we would die.
All animals need oxygen; even the worms in the ground and the fishes and oysters in the water must have it. So great quantities are being used up all the time.
Now, you know, when the plants pull carbon dioxide to pieces, they keep the carbon and return the oxygen to the air. In this way we get it to breathe.
But there is more than this to the matter in hand. We are all the time breathing out carbon dioxide asan impurity; so are all the millions upon millions of animals in the world.
The air might in time contain enough carbon dioxide to kill us if there were not some way of getting rid of it. You know what that way is.
The plants use it up. So by giving oxygen into the air and taking out carbon dioxide, the plants keep the air fit for us and all animals to breathe.
[Flower]
But there is more than this we have to thank them for.
They shade the earth and regulate the rainfall and the water supply.
Where forests grow there are always streams of water, and the large water courses are kept full the year round.
The Mississippi River depends upon the far-away forests for its broad stream.
The spreading crowns of the trees shade the earth and prevent the water which falls as rain or dew from evaporating rapidly. It collects into streams and flows through the land, keeping the earth fresh and beautiful.
More than this,—large forests cause the rain to fall and the dew to collect. Their leaves condensethe moisture in the air and cause it to fall as rain or be deposited as dew.
When people recklessly cut down the forests in a country, the water courses dry up, and even the largest rivers are affected.
When the spring rains fall over a country whose trees have been cut away, the water rushes down the little streams all at once and causes a terrific flood in the large rivers. It soon drains away; then the rivers fall lower and lower until they nearly dry up. This state of affairs is a great calamity, because the people can no longer raise crops on the land near where the old forests stood, for it is parched and dry months at a time.
Moreover, boats laden with coal and grain and all sorts of things can no longer pass up and down the rivers, because the water is too low.
People ought to think of these things and not destroy too much forest land. After awhile we shall have to go to work and plant trees instead of cutting them down or burning them; but it takes a long time for trees to grow, and a wiser way would be for us to take care of those we have.
You have heard a great deal about plants eating and the good they do us by eating the carbon dioxide in the air. They take this in through their leaves,and you remember they take in all their other food materials—water, nitrogen compounds, sodium, potassium, magnesium, and many other substances—through their roots.
But they do more than eat; they also breathe.
They breathe everywhere over the surface of their bodies where there are stomata or where the skin is not too thick for the air to penetrate it.
And I must tell you they breathe just as we do,—that is, they take in air, use the oxygen, and give off the carbon dioxide.
It seems rather inconsistent of them to take in carbon dioxide as food and throw it off as a waste at the same time, but that does not troublethem; they do not care whether they are consistent or not. And it is true they take in carbon dioxide and give off oxygen, and take in oxygen (in the air) and give off carbon dioxide, in one breath as it were.
You see, it is different parts of protoplasm at work that does this; one part—that in the chlorophyll bodies—is attracting carbon dioxide, breaking it up, and casting out oxygen. Other protoplasm in the cells outside the chlorophyll bodies attracts and uses the oxygen, while the carbon dioxide comes to the stomata from different parts of the plant as awaste material, just as it comes to the cells of our lungs to be cast out.
So plants, by breathing, make the air a little impure, but they destroy or break up so much more carbon dioxide than they make that on the whole they act as powerful purifiers of the air.
When we think of the great forests of the tropics, all overgrown with luxuriant vegetation, we may remember that those tangles of vines and trees and strange growths are our friends no less than the grass and bushes in our dooryard.
For there is a carrier always at work bringing the pure air to us and carrying away the impure air which we create. This carrier is the air currents. The great winds sweep about the earth, bearing the oxygen from the forests to the crowded cities, and sweeping away the carbon dioxide from the cities to the fields and woods. The winds, too, stir up the water where the water plants and fishes live, and help keep it full of air for the things in it to breathe; the tides and currents help, so as far down in the water as there are living things, you may be sure there is air for them to breathe. There would not be air enough for you, because you need so much; but for them there is plenty.
Swirling around the earth go the winds, carryingthe oxygen to the people and the carbon dioxide to the plants, for the plants are as glad to get the carbon dioxide we breathe out as we are to get the oxygen they give off.
And we are glad, when we come to think about it, that we are able to give them something in return for all they give to us.
You see, we need each other,—plants and people, and the winds are friends to us both.
[Flower]
WHAT ARE THE FLOWERS MADE OF?
[Flower]
I think flowers are “made of sugar and spice and everything nice.” At least, if it is not that, it is something very like it, as I have good reason to believe.
What flowers and all other parts of the plant are made of depends upon protoplasm; and if protoplasm can make sugar and spice and build up flowers that way, we should like to know it.
Wedoknow about sugar and how the little green chlorophyll people run their starch factories in all the green parts of the plant,—under the skin of stems sometimes as well as of leaves, for wherever a stem is green, we may be sure chlorophyll is at work making starch in it. And we know how the protoplasm in the different cells changes the starch into sugar.
We know, too, how wood and other tough substances are made of starch.
But there is something else in plants as important as starch and very different,—the protoplasm. Protoplasm itself is not made entirely of starch; it requires materials not found in starch.
These materials are nitrogen, sulphur, and phosphorus.
Nitrogen is the most important, and this the plant gets chiefly through the roots.
Nitrogen is found in the earth combined with hydrogen and other substances. The protoplasm tears to pieces these nitrogenous substances which the roots suck up, and so enables the plant to take the nitrogen.
The other two substances which the protoplasm needs, sulphur and phosphorus, the plant gets partly from the air and partly from the earth.
Sulphuric acid exists inverysmall quantities in the air and goes in through the stomata, attracted, no doubt, by the protoplasm inside. But other sulphurous and phosphorous compounds are taken up by the roots.
So we see protoplasm is complicated. It contains carbon, hydrogen, oxygen, nitrogen, sulphur, and phosphorus united in a very complicated way.
Although protoplasm itself is made only of carbon, hydrogen, oxygen, nitrogen, sulphur, and phosphorus,it can make use of a great many other things. When the protoplasm of certain cells wants to build hard, tough walls, it uses potash and soda or even silica, which you know glass is made of. Just draw a blade of sedge grass through your fingers if you want to feel the silica in it. You will probably cut your fingers, but that will help make you remember about silica. Then the protoplasm uses iron to color the petals and other parts of the plant. It uses magnesia, too, and salt and lime and a number of other materials for building walls or making dyes or something else.
Every material in our own bodies is found in plants, and sometimes the plants have materials that we do not have.
Of course materials are put together differently in plants from what they are in us. When Mother Nature combines her carbon, hydrogen, oxygen, nitrogen, sulphur, phosphorus, magnesia, iron, and all the other things to make a plant, she does not go to work as she would if she were going to make an animal.
Just what the difference is it would be difficult to tell, but thereisa difference.
Plants contain a good deal of sugar as a rule, and if you remember cloves you will admit that at leastsomeflowers are made of spice, for cloves are the dried flower buds of the clove tree.
Cinnamon is the bark of a plant, and if you are acquainted with orange trees you will be willing to say they are “made of sugar and spice and everything nice,” for the whole tree, wood, bark, stems, leaves, flowers, and fruit, is fragrant and spicy.
Oil is another common substance in plants, and it is made from the materials of starch which, as we know, are carbon, hydrogen, and oxygen; cotton-seed oil, olive oil, and castor oil we are all familiar with.
All nuts contain a great deal of oil, and the skin of a fresh-picked orange is so full of it that it runs down our fingers when we cut the orange.
All the things in a plant—starch, sugar, oils, spices, wood, bark—everything is made by the wonderful protoplasm in the cells.
Starch and the food taken up by the roots pass through all parts of the plant by the sap tubes, and as the sap goes along, each living cell draws into itself the substances from the sap that it needs, and these it combines into the things it wants to make. Some of the cells in an orange skin, for instance, attract out of the sap the materials to make the fragrant, stinging oil that fills the fresh skin, while other cells attract the materials to build the white cottony coveringinside the outer skin, and so the cells in each part of the plant take out what they need to build with.