# The How and Why Library/Life/Plants-Section II
#### II. How the Yeast Plant Grows in a Loaf of Bread

Which would you rather do, get inside a loaf of bread, or put the bread inside of you?
That makes you laugh. But you never tried getting inside a loaf of bread. If you get inside before it is baked, you will find plenty of playmates. Bread dough is just full of little living cells of protoplasm. They eat the bread before you do. They eat and eat and grow, and make millions of other cells like themselves. Let us watch them. They are very active little plants so they can easily be studied.
Your mama buys a yeast cake when she wants to make bread. It may be dry and hard, or it may be a soft paste wrapped in tin foil. It has an odd smell, and does not seem alive at all. Mama isn't sure that it is alive. She takes a half cup of warm water, and a little flour and a tiny bit of sugar and mixes them. Then she breaks her yeast cake into the cup, stirs them all together and stands the cup in a warm place. If you ask her why she does that, she may say: "I want to see if that yeast is alive."
Now you watch it! Soon little bubbles begin to burst. The batter swells and foams, and rises to the top of the cup. Study it with a microscope. The whole mass is in motion. Mama says the yeast ferments, but that is just another name for growing, for yeast plants. There, the secret is out. Mama made a garden. The flour and water and sugar are soil. In the soil she scattered yeast plants-just as a farmer scatters wheat seeds in a field. The yeast is a water plant and will grow in warm water alone, but it will grow faster if given the starch in flour. And it likes sugar, too, just as you do. It uses these things to grow on and, in using them, it changes them. It turns the starch and sugar sour, and gives off the same gas-carbon dioxide—that animals give off in breathing. This gas is what puffs the bread so it rises to the top of the pan. There is very little food in the cup. Soon the yeast plants will stop growing and making gas. Then the paste will fall flat. Mama must hurry and get a big pan full of dough ready, to give the yeast plants more food.
This time she takes a quart of water, and a lot of flour, and the foaming yeast in the cup. She beats the batter with a big spoon to scatter the yeast plants all through the soil and to beat in air, too.All living things need air, you know. Yeast plants want their food warm just as the baby does; but they grow best in the dark, so mama covers the bread pan. The yeast plant is that little round cell filled with the magic jelly, protoplasm. It likes to float around in a warm bath. In some strange way it soaks food through its thin cell wall and grows larger. When it is grown up, it sprouts another little bud of a cell filled with jelly. Sometimes these buds break away from the parent cell and start a new family, but sometimes they hang together in a little knot or string of cells. The yeast plant has neither stem nor roots nor leaves nor blossoms nor fruit. Each little bag of jelly is a whole plant.
There are a great many plants on earth much like yeast, that you can find and study. One of them likes bread after it is baked.
* Yeast, showing single cells and how they grow and multiply by budding.
It is blue mould. Blue mould grows on old bread, and on the top of glasses of jelly. Under a microscope it is very beautiful. It is a feathery mass of delicate blue threads. Black moulds and mildews, rust on wheat, black smut on corn, and puff ball smoke, all belong to the same family of plants. Cells of these kinds of plants are always in the air. You can make a garden of them by leaving a saucer of flour paste, fruit jelly, or a bit of stale bread exposed, for a week or two.
They have a family name. They are called Fungi (fun-ji). The blue mould is often called Fairy Fungi. It looks like a fairy forest. Toad-stools and mushrooms are fungi, too. The fungi all have one very bad habit. They don't earn their own living. They live on other plants, and even on animals. But they like dead or dying things best. Around old trees and fallen logs you will find toad stools and mushrooms. There are two ways in which you may know the fungi. They live on some other living, or dead and decaying, plant or animal, and they are never green. They do not have stems or roots or leaves or flowers or fruit or seeds. Any tiniest cell of a fungus, if put into the right soil will grow and multiply cells, just as the yeast plant does in the batter. It is the very lowest order of plants.
But it became a higher kind of plant when it had to. Fungi like the dark. The first plants were born in the dark of deep sea water. You know earthquakes lifted the floor of the ocean. The plants were lifted, too. As the plants came near the surface of the water they got more light. Do you know what sunlight does to plants? Did you ever find a board lying on the grass? The next time you find one, lift it. You will find that the grass under the board has turned yellow or white. Now, you know that a part of the grass wasn't born green and part of it white. The outside leaves of a head of cabbage or lettuce are green, while the inside leaves, shut away in the dark, are white. Sunshine turns plants green. Nature took as long a step upward as the giant who wore seven league boots, when the first sea plants got enough sunlight to turn green. Green plants were lifted clear out of the fungi class. They began to earn their own living for one thing, and they learned to do a lot of things.
Now, nature might have made one kind of cell and magic jelly for the fungi, and another kind for green plants, but she didn't. She seems to like to see how many different things she can make of a few simple things. All the plants and animals begin to grow in a little cell of protoplasm. So, when we understand the fungi, the simplest of all plants, we have learned the A. B. C.'s of life. Knowing the letters, we can spell the words and read the story of the living world. (See Fungi, Yeast, Mould, Mildew, Rust, Smut, Mushrooms, Toadstools, etc.)