This plant food returns in a slow current, feeding the growing cells under the bark, from leaf tip to root tip, throughout the growing tree. The sap builds solid wood. The maple tree has been built out of muddy water and carbon gas. It stands a miracle before our eyes. In its tough wood fibres is locked up all the heat its leaves absorbed from the sun, since the day the maple seed sprouted and the first pair of leaves lifted their palms above the ground. If my maple tree should die, and fall, and lie undisturbed on the ground, it would slowly decay. The carbon of its solid frame would pass back into the air, as gas, and the heat would escape so gradually that I could not notice it at all, unless I thrust my hand into the warm, crumbling mass. If my tree should be cut down to-day and chopped into stove wood, it would keep a fire in my grate for many months. Burning destroys wood substance a great deal faster than decay in the open air does, but the processes of rotting and burning are alike in this: each process releases the carbon, and gives it back to the air. It gives back also the sun's heat, stored while the tree was growing. There is left on the ground, and in the ashes on the hearth, only the mineral substance taken up in the water the roots gathered underground. If my tree stood in swampy ground and fell over under a high wind, the water that covered it and saturated its substance would prevent decay. The carbon would not be allowed to escape as a gas to the air; the woody substance would become gradually changed into _peat_. In this form it might remain for thousands of years, and finally be changed into coal. Whether it was burned while yet in the condition of peat, or millions of years later, when it was transformed into coal, the heat stored in its substance was liberated by the burning. The carbon and the heat went back to the air. Every green plant we see spreads its leaves to the sun. Every stick of wood we burn, and every lump of coal, is giving back, in the form of light and heat, the energy that came from sunshine and was captured by the green leaves. How long the wood has held this store of heat we may easily compute, for we can read the age of a tree. But the age of coal we cannot accurately state. The years probably should be counted by millions, instead of thousands.