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.