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 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. 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 decay