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.
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—
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.
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 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.
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 inner wall 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 are dead cells. 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.
