[Illustration: [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 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.
[Illustration: [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. _d_ is an opening between two cells, and all is
magnified several hundred times.
[Illustration: _a_]
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.
[Illustration: [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. [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.
[Illustration: _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 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.
[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.
[Illustration: [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.
[Illustration: [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 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.
[Illustration: [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 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.
[Illustration: 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.
[Illustration: [Flower]]