Feeding and breathing in this way, by taking what it needs from the flour, the cell grows. When it has reached its mature size, it rests quietly for a while as if it were gathering strength for the effort, and then it sends out a little bud, which grows like the parent cell, until another bud and another grow from it. When the plant is grown, it is very unlike our notion of a plant; it is really nothing more than a little chain of sacs growing end to end. As soon as the little plant has exhausted all the sugar and food substance of the flour, it stops growing, the cells separate, and remain quite still.
There is just one time in the growth of the plant when the dough is right for baking. Before it has grown enough, the bubbles through the dough are too few or too small. These bubbles are the carbonic acid gas left behind when the oxygen has been taken out of the sugar, and there must be plenty of them to make the bread light. If the bread is left too long to rise, the cells get more than their share of the wheat sugar, and the bread is sour. Just at the right stage, which every good bread-maker can tell by experience, a thorough baking will destroy the alcohol, and the bread will be both sweet and light.
When the yeast plant is sowed on the top of the flour and water, instead of being buried in it, all this is very different. The plant takes its food from the paste, but it does not need the sugar, so it lets that alone. It can get its oxygen in a much simpler way, right from the air, as we do, and does not need to go through the labor of smelting it out of the sugar. The raising of our bread by yeast is entirely due to the efforts of the tiny cells to get a breath of air when we have smothered them up in the dough.
Fig. 4.—Mould.
a, Rosette Heads; b, Fruit.
There are other plants besides the yeast plant that act in the same way. Have you never heard your mother say, when she opened a jar of preserves, "These are all right, I know, for they are covered with mould"? Mould is a good deal like yeast in some things; if the germ cell, or spore, falls upon the top of the sweetmeats, it can get plenty of oxygen from the air, and so lets the sugar alone. But if it is nearly drowned in the syrup, it will get its oxygen somehow, and so the sugar has to be sacrificed, and the preserves are left to spoil. What else could you expect of such little mischief-makers if you shut them up with the sweetmeats?
The yeast plant is so very, very small that you can not see it except with a very fine magnifying glass. But there are other plants like it which are large enough to be seen with a small and not a costly microscope.[2] These are what we call moulds. If you want to study moulds, nothing is easier than to prepare them. Mix a spoonful of flour with cold water, and spread the paste over the bottom of a plate or saucer. In a few days it will be covered all over. If you put it in a damp and dark place, the mould will sprout sooner. You might put away a piece of bread at the same time, and you will find it covered with a growth too.
Take a bit of this paste on the blade of a knife, and examine it carefully. You will see among the cottony fibres a number of little upright stems with black or white or yellow heads, which give the mould a speckled look. Under the microscope you see a perfect jungle of growth—a tangle of threads, which look like spun glass, running here and there and everywhere. From these, which are the roots of the mould, the stems spring up, bearing, instead of leaves or flowers, tiny glistening toad-stools that look as if they were made out of a pearl; or sometimes the heads are like strings of little pearls (Fig. 3), or at others they are rosettes of such strings (Fig. 4 a). The black and sage green colors come later, and are the fruit or seed-bearing portion of the plant (Fig. 4 b).
Fig. 5.—Mould.
a, Stalk; b, same opened; c, outer skin broken, and spores scattering.