As fire is no part of the mineral kingdom, we find it surprisingly apt to discuss it as though it were one among the kingdoms of living things. Copious fire arose only after life crossed the margins of its watery world and came to thrive on dry land, under air of near-modern oxygen content, a state reached after 90 percent of earth history had passed. We can divide fire, like the grasses, into wild and domesticated forms, both of them very widespread. Wildfire is not always truly wild. Like the cereal grasses, it may have been molded by human intervention and yet may still bloom under the open sky.
Green plants synthesize themselves anew from the sun's brilliance.
The very cell walls of most plants and the woody tissues, too, are largely fibrous bundles of cellulose, a defined polymer of glucose, the simplest of sugars, linked by the 100,000 into rings and chains of the atoms H, C and O. Plant-stored energy is the chief metabolic support of the life of fire, as it is of our own lives. But we do not see fire grow regularly from the fields as we see green blades grow. Some special act of ignition, a tiny spark struck from a rock or a bolt of lightning, always precedes. The fresh, greenish marble of the newly restored Parthenon was quarried from the same layers whose rock had given the old temple its rosy glow. The difference was those centuries in open air.