The science of life : fully illustrated in tone and line and including many diagrams

BOOK 2

tion of dead things, and that one or two of them have edible, even delicious fructifications. But this scarcely balances the evil they do.

There is, however, a group of organisms, the yeasts, which play an important part in human economy and are highly valuable to us. They differ in structure from other fungi and stand in an order apart. Their chemical reactions are distinctive and extraordinary. They are single-celled moulds ; they thrive on various organic matters and they are cultivated because of their residues, their chemical excretions, of which the most Important ingredients are carbon dioxide and alcohol.

Yeast, as usually purchased, is a moist powder. Every granule of that powder is a microscopic globule of life. An ounce of brewer’s yeast contains about five thousand million of them. It has been known since the dawn of history that if dough is left about for a time before it is baked, the resulting bread is quite different from the unleavened bread produced by baking the dough as soon as it is mixed. And it was known, too, that a little unbaked dough which had already fermented, leaven that is, if it was mixed with newly made dough, leavened the whole lump. But it was reserved for Pasteur to make these facts intelligible. The leavening of bread is due solely to yeast; it nourishes itself on the bread and the changes produced are manifestations of its life—the most important being that the carbon

dioxide it releases gives the bread its light,’

frothy, aerated consistency.

Again, if fruit juices are left undisturbed for a time they undergo changes of flavour and become alcoholic. This also is due to the action of yeasts (of other organisms, too, but chiefly yeasts), which live on the sugar in the juice and produce alcohol and carbon dioxide. If the liquor is bottled while it is still raw the carbon dioxide, unable to escape, dissolves under pressure and a sparkling drink is produced. If not, the carbon dioxide escapes and a still wine or beer or cider results. ‘The various subtle flavours of wines depend partly on the kinds of grapes employed, but much more on the fact that each vineyard has its own peculiar mixture of fermenting organisms, its own stud, so to speak, whose combined excreta have a distinctive bouquet.

From the physiological point of view yeasts are very remarkable because they can live in the absence of oxygen. Nearly all other organisms have to breathe; they can no more live without air than a fire can burn

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THE SCIENCE OF LIFE

CHAPTER 6

without air. But yeasts, and various other humbly organized plants, have different devices. True, if a yeast cell is supplied with oxygen it will breathe, taking in the oxygen and giving out carbon dioxide just as we do. But if it is denied an oxygen supply it falls back on another device ; it turns sugar into alcohol and carbon dioxide, a process which yields energy just as combustion does. It is because of this peculiar method of avoiding suffocation that yeasts are of importance to man. Buried in the dough of his bread they lighten it and make it easily digestible ; drowned in flat fruit juice, they change it to sparkling wine.

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Lichens

And next we must notice a widespread series of curious forms that are really double plants. There are fungi that capture and domesticate other plants, as we domesticate other animals.

Tt was thought until about the middle of the last century that lichens, the familiar encrusting growths that are found on the trunks of trees, on walls, on rocks, and similar places, formed a natural group of plants, equivalent, for example, to the alge, or the fungi, or the mosses and liverworts. But, as a matter of fact, they are not single plants at all. If we examine a shred of lichen through the microscope, we find that it is made of two very different kinds of tissue intimately mixed together. There are little green cells, and surrounding and entangling them there is a closely woven mass of colourless threads. The colourless threads are mould-like fungi, the green cells are single-celled alge that have been captured by the fungi and are held prisoners in the tangled web. A lichen-is no more a single organism than a dairy-farm is a single organism.

Each of the participants in this curious union derives some benefit from the other. The fungus, not being green itself, is able to exploit the synthetic powers of the alge and to use the sugars and other matters that they build up, with the help of light, from air and moisture. In some species of lichen the fungi penetrate into the algal cells and actually consume them, oozing digestive juices on to them and absorbing the resultant broth. On the other hand, the alga is sheltered within the mass of mould-threads and has more chance of getting the moisture and mineral salts that