green leaf. It is easy to remember; it will help you to think of the hard Greek name—chlorophyll.
The little round, hard kernel is called the nucleus, which means a nut or kernel, and the thin space in the centre is the vacuole or air-cell. It seems to be like a tiny drop of water separated from the rest of the jelly, which contains a good deal of water.
Now that we have described and named each of these different parts, we can go a step or even two steps farther, and tell of what the most of them are made, and of what use they are. The tough wall or sac of the cell is made of the woody matter or cellulose, mixed with a little water and mineral matter. The cellulose or woody part is made of three substances—carbon, hydrogen, and oxygen. If you ask me what these things are, I can only tell you that they belong to what are called the simple chemical elements, because each one is made of just one kind of matter. These three substances, and one other called nitrogen, help to make every thing there is in the world, except a few such things as gold, iron, sulphur, etc., which are also simple elements. The water is made of hydrogen and oxygen. Its use is probably to hold and protect all the inside parts, or contents of the cell. So much for the outside sac of the cell—now for the inside. The cell-jelly, or proto-plasm, is made of water, fat, mineral matters, and protein. The water we already know. The fat is made of carbon, hydrogen, and oxygen. The minerals belong to the simple elements.
The protein we know but little about. We are sure that it contains carbon, hydrogen, oxygen, and nitrogen, with a little sulphur or phosphorus, or both, and we know that it is found in all living matter. There is no life without it, so it has been called the "basis of life." But there is a great deal more to be learned about it. I want you to remember what this word protein stands for, because it is something about this substance that makes one of the greatest differences between vegetables and animals. There is nothing in its appearance that would make you think it of so much importance; it looks to be nothing more than so much light-colored jelly, or white of egg. The word protein means first or chief, and this is the part of the protoplasm-jelly, which is alive. The kernel, or nucleus of the cell, seems to be only a part of the protein-jelly which is harder than the rest, and it has something to do with the making of new cells, as we shall see farther along in our study.
Now, what about the dye-stuff? Is it of any use, or is it just here to make the mould look pretty? It is of great use, as we shall soon see. Each grain is a very clever little chemist that works in the cell, which is his laboratory, or workshop. The sunlight is the fire by which this chemist heats his crucible, or melting-pot. Into this crucible he puts the poisonous gas, carbonic acid, that he gets from the air, and melts it up into carbon and oxygen, the two substances of which it is made. He keeps the carbon to feed upon, and gives back the pure
