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

BOOK 2

bread broke out into scarlet spots, was due to a bacterium which grows on bread and manufactures a red pigment; the same organism may infect human skins, if not too frequently washed, and thus cause miraculous “‘ sweatings of blood.” There are other bacterial curiosities, too. The phosphorescence that appears on rotting wood, fish, and the like is due to bacteria; and so is the heat of manure heaps and moist haystacks.

But the outstanding thing about bacteria, perhaps the most important of all, is that they are the chief organisms responsible for putrefaction. Usually one regards putrefaction as a nuisance—if anything in the larder goes bad or if a rat dies under the boards of the floor one resents this apparent tendency of dead organic matter to deliquesce and stink. But decay is not a natural property inherent in dead and abandoned things. It is a sign that they are being inhabited and consumed by busy millions of microscopic creatures. And decomposition is a vitally important process. If everything that died were perfectly preserved and lay for ever as a corpse on the surface of the earth we should be hard put to it to find room for our feet. But there is a better reason than that. A living body, a man for example, is a more or less definite weight of organic matter set apart for special use ; when the creature dies its body is finished with, and in the usual course of Nature its substance is melted down, so to speak, and used again, built up again into other bodies. For the amount of available carbon, for example, in the world is limited. Equally important, there is energy even in a dead body ; it is combustible, a potential fuel. A preserved dead body is so much matter, so much energy withdrawn from the general interchanges between living things ; it leaves life as a whole so much the poorer. But normally, as soon as anything dies it is discovered by the floating spores of putrefactive bacteria, Nature’s housebreakers, which proceed to demolish it.

‘There is a great variety of these bacteria. ‘There are, so to speak, a number of rival gangs of housebreakers working without any sign of discipline or co-operation. Some attack proteins and fats and break them up into fluid substances ; others, rivals of the first, fall upon the same material and turn it into stinking gases. Some fall upon the nitrogen-containing molecules and turn them into ammonium carbonate—a_ substance which is wrestled for by two other kinds, the

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THE SCIENCE OF LIFE

CHAPTER 6

first turning it into atmospheric nitrogen and the second into nitrites. Then the nitrites are seized upon by yet other bacteria and converted into nitrates. The confusion is increased by other creatures which butt inmoulds and grubs of various kinds. But the net result of this swarming, struggling activity is good; the useless body is converted into molecules of simple constitution, gases and simple compounds in the soil, which can be used to nourish green plants. ‘The chemical capital, which would otherwise be locked up and unproductive, has been brought back again to play its part in the commerce of living things. .

For the material of living things seldom rests ; it is kept in continual circulation from body to body. Consider, for example, the muscles of a man. Their material may be used up—burnt or worn away—during his life, in which case it will be excreted in his breath or in his urine. Or it may be present at his death, in which case it will be fallen upon by bacteria and demolished, and the chemical bricks of which it is built make their way as before either into the air or into the soil in the form of comparatively sumple molecules. In either case, sooner or later, it will be built again into a living thing. Carbon dioxide in the air is absorbed by plants to be built up into their substance ; nitrogen is absorbed by special bacteria in the soil and turned into nitrogen compounds. And the substances in the soil are sucked up by the roois of plants. The plants are eaten by animals, the animals die or are eaten by other animals —even by men—and thus the material is handed on from one form of life to the other. Life is a continual commerce. There is a rhythm, a cycle, from inorganic substances in the air and soil to plant-tissue, thence to animal-tissue, from either of the last two stages via excretion or death and decay back to the air and soil.

And this everlasting rotation of life is kept in motion by the radiant energy of the sun. For the green plants build up complicated substances out of simple ones by means of solar energy, the vegetarian animals use and burn these compounds and the carnivorous animals, consuming the bodies of the vegetarians, use them too ; true indeed it is that “all flesh is grass.’ And, finally, the last drop of energy is wrung out of these substances by the putrefactive bacteria. So we can compare the life on our planet to a wheel ; matter going round and round, solar energy streaming through it and keeping it in motion.