When the seed or seedlings are ready, the garden plot is prepared. The cook heaps up in her bread bowl quarts of snowy flour. Into this heap, after making a hole, she pours her prepared yeast. Working the bread is only another name for the careful scattering of the seed through all the dough, that it may spring up and grow, and fill the whole mass with the tiny plants.
The yeast plant is not a common kind of plant, but belongs to the same class as mushrooms and toad-stools, and the fuzzy, cottony growth that we call mould. There are two kinds of plants that we may find almost anywhere in the fields and woods, and even in the city yards—the fungi and the green plants. The yeast plant is one of the fungi. These are very different in most respects from the green plants: they can live and grow and thrive in darkness; they do not have either leaves or flowers, and they usually spring up and die very quickly. The greatest real difference between the two kinds is, however, that the fungi live on food that has been alive before—on plants, or animals, or decaying matter, while the green plants live on what they get out of the earth and the air and the water.
Fig. 2.—Yeast Plant.
a, Single cells; b, growing plants.
The simplest of all the fungi is the yeast plant. It begins its life as a tiny egg-shaped bag, or sac (Fig. 2 a). This cell, as it is called, is filled with a very curious jelly, perhaps the most wonderful thing in all the world. It is found in everything that lives and grows. By its help the little yeast plant can take the flour and water, and can change it so that while the paste is used up and disappears, the cells grow larger, and sprout out buds. You have particles of this jelly, or protoplasm, lining your mouth and stomach, and the food you eat is changed into flesh and blood and bones by this wonder-working magician. In the figures, the grainy substance is the protoplasm.
This jelly all seems to be pretty much alike, but there is some marvellous difference somewhere—a difference that science has never reached. The yeast cell takes in certain food, and grows, but it never makes anything but other or larger yeast cells. The food you eat and digest makes just you; more of you, perhaps, but still you, yourself, and nobody else.
Fig. 3.—Mould.
Like all living things, the tiny yeast cell must eat and breathe, or it will die. It feeds, not by opening its mouth and taking in its food, but by lying bathed in it, and soaking it up through its skin. When the cook dissolves her yeast cake, and puts it into the spongy dough, she is putting the little plant into its food bath. The cells which have been so long in prison, shut up in the darkness and cold of the dried yeast, begin to look alive, and stretch themselves, and enjoy their liberty. They take kindly to their food right away, and begin helping themselves to what they find about them. They do not merely soak up the flour and water in which they are plunged, but they manage to extract from the compound just what they need.
The cells must not only feed in order to live, but they must breathe, they must somehow get oxygen, which is the gas that our breathing takes out of the air. And this they extract, as a miner does gold, by separating it from its ore. There is a certain amount of sugar in wheat, which gives to good bread and to cracked wheat their delicate sweetness of flavor. Sugar is made up of a number of different substances, which the yeast cell has the power of separating. It takes the oxygen for its own use, and leaves behind the other things that make up the sugar.