A bit of protoplasm living by itself is called a “cell.”
Many plants and animals have, like the amœba, only one cell. Very often
the little one-celled being has a thick outside wall. The protoplasm
changes part of the food into a hard substance, that is, it builds
itself a wall.
Very often cells live together in colonies instead of living alone. In
such cases, the first cell divides into two cells, but the two stay
together instead of entirely separating. Then each of these two cells
divides again, and the four cells stay together, and so it goes on until
a large body is built up of many cells.
The truth is, plants are only collections of cells which have agreed to
work together. Where there is but one cell, it has to do all sorts of
work; but where there are many, some do one kind of work, some
another,—just as Robinson Crusoe, living all alone on the island of Juan
Fernandez, had to do all sorts of things for himself: make his own shoes
and clothes, get his own food and cook it, build his own house, and
gather his own wood. But in a town one set of men makes shoes, another
chops wood, another raises vegetables and grain, another grinds the
grain, and another bakes the bread; then they all exchange with each
other, and everybody has enough—or ought to have.
So in the plant made of many cells. One set of cells makes hard walls to
protect the plant. Another set draws up water from the earth for all the
cells in the plant, for living things require a great deal of water.
Another set takes gas from the air and changes it into food. Another set
makes tubes for the sap to flow through. Other sets do other things.
Each set of cells does something for the whole plant.
If you look at a leaf or a bit of skin from a stem under a microscope,
you will see they are built up of cells, as a house is built of bricks.
Only the cells are not placed regularly like the bricks in a house, and
they are not solid like bricks. The walls of these cells are sometimes
hard and sometimes soft, sometimes tough and sometimes tender; but the
walls were all built by the protoplasm that lived in them. Sometimes the
protoplasm leaves the little house it has built and goes somewhere else.
Then the empty, wall-surrounded space is left like a cell of honeycomb
before the honey is put in, or an anther cell after the pollen has
fallen out and left nothing in it.
Before microscopes were as perfect as they are now, these empty spaces
with their surrounding walls were discovered. Even where the cells
contained protoplasm the microscope was not strong enough to reveal it,
so only the cell walls were seen.
[Illustration: Some of the cells in one plant.]
It was soon known that plants were built up of these little
compartments, and because they resembled cells in being small and shut
in by walls, they were called “cells.” After awhile it was discovered
that the living part of the plant was the colorless, jelly-like
protoplasm which lived in the cells. Yet later, particles of wall-less
protoplasm were found building up plants and animals. What were these
soft little protoplasmic atoms to be called?
The plant was really built up by them, and only part of them had walls,
so they were called by the name the people had already given to the
walled spaces which they supposed built up the plant, and so got the
name of “cells,” which is not at all an appropriate name.
There is nothing quite so easy as to be mistaken, you see, and the
botanists, having seen that the plant was built of little compartments,
and never suspecting the presence of the living protoplasm lurking in
some of them, had called the compartments “cells”; later, when the
protoplasm was discovered to be the real builder, the old name was kept.
So you see how the amœba came to be called a “cell.”
There are a great many different kinds of cells in one plant.
But every living cell has very much the same powers as the amœba, though
in many of them some one power is developed at the expense of all the
rest. In this way different sets of cells are able to perform different
kinds of work, and do it very well indeed.
The amœba is not the only single-celled creature. There are a great many
different kinds of single-celled plants or animals, and some of them
take very curious and beautiful forms, with streamers floating about
them.
Such are not protean, like the amœba; they do not change their shapes.
Plants are not the only things that have cells. Animals, too, are built
up of them. Animal cells are usually softer than plant cells, because
they very often have no hard walls. Bone cells of course have hard
walls, and there are others, but most of the animal cells are without
walls.
So you see all living things are built of cells, and the living part of
the cells is the protoplasm.
You yourself are built up of millions of cells, and without the help of
protoplasm you would not be living, for protoplasm made your cells, and
protoplasm is the only thing in you that is alive. Your muscles are made
of muscle cells, and the protoplasm in them moves, and when the muscle
cells all move together, that moves your arm or your leg or your head or
some other part of your body.
Since your muscle cells devote themselves to moving, they do not try to
do much else; so other cells digest the food which the blood carries to
the muscle cells. Yet other cells build a good thick skin to protect the
soft muscles, and yet another set of cells _thinks_ for the muscles, and
tells them where and when and how to move. Each set of cells has its own
work.
Your brain is made up of nerve cells, and the protoplasm in them in some
way enables you to think and feel. Your bone cells are hard and
resisting, your sinew cells strong and flexible. So each part of your
body is made up of different kinds of cells.
But what has all this to do with football and parties and picnics you
would like to know?
[Illustration: [Cells]]
Why, a great deal, to be sure. If it were not for cells and protoplasm
there would be no people.
And how could you have football games and picnics without people, _I_
should like to know?