Fill a glass with water, and let a piece of common tape or a strip of
muslin hang so that its lower end shall dip into the water, and then
notice it: the liquid creeps slowly but surely up the strip. If the end
which you have in your hand is dropped on the table beside the glass,
the goblet may be entirely emptied, the water running up over the edge
of the glass before it runs down again. This behavior of water would
seem very queer if we had not noticed something of the kind all our
lives. It is caused by what is called capillary attraction. Whenever one
part of a material full of fine openings which lead through it is dipped
into a liquid, the fluid runs through the whole stuff, even if it has to
run upward. Try a lump of sugar: put one corner into your cup of tea or
hot milk, and watch it soak the lump through. The burning of a lamp is
upon the same principle. The wick serves to carry the oil from the globe
of the lamp to feed the flame. As soon as the oil gives out, the light
fades and dies away.
[Illustration: FIG. 1.--CELLS. A, Leaf of Geranium Flower; B, Leaf of
Sorrel.]
Every part of a plant needs water: it must be close around every little
cell. These cells are the tiny queer-shaped bags full of liquid that are
packed close together, and make up the leaves, stems, and flowers of
plants. In Fig. 1 you see the cells of a leaf of geranium flower, and
one of sorrel or sour grass, which, if you are like the children I know,
you have many a time eaten to get the pleasant sour taste. Well, every
one of these tiny cells must be kept wet all the time, or the plant will
die. The only way we can think of that water could get up into the
leaves and flowers from the earth is by capillary attraction, as it runs
up the slip of muslin. And if it were not for this singular behavior of
water, the only plants in the world would be those that grow in the seas
and rivers and lakes. The land would be as barren as the desert of
Sahara.
Now try to think of some plant with all the earth away--a tree, for
instance--and you will see that it is a sort of double growth; that
there is an upside-down tree in the ground, with its trunk and branches
and twigs, as well as one above the ground. The under-ground twigs do
not bear leaves, but each one of them wears on its head a little cap or
helmet to protect the tender growing part from being injured as it
pushes its way through the hard earth. The most important parts of a
tree are those that seem of least consequence, the rootlets and the
leaves. These are to the tree what our mouths and stomachs and our lungs
are to us: the roots are the feeders and the leaves the breathing
apparatus of plants.
As the under-ground tree grows, the tender little roots push their way
down into the darkness and cold of the deep soil; they find their way
around stones and through great clods of earth, anywhere and everywhere,
until they get their little noses into water or damp earth, and then
they begin to suck. Sometimes it is only pure water that they take up
from the earth, but generally it is a sort of broth--water with plant
food dissolved in it.
[Illustration: FIG. 2.--CORN STALK CUT ACROSS.]
The roots and stems and leaves are all full of little passageways
running upward and branching and dividing until they reach the leaves.
Fig. 2 shows a corn stalk cut across. You see some roundish holes,
marked _a_; these are the ends of tubes that run through the stalk. When
the corn begins to grow, take a stalk about two feet high, and cut it
across; you will see little white spots all over the cut place. This
figure is one of those white dots magnified.
[Illustration: FIG. 3.--PLANT MOUTHS.