is that solid part of a plant that makes a bright fire. Coal is nearly all carbon, and coal you know was made of plants pressed to a kind of stone. Nitrogen is a plant food. The roots get some of it out of the earth, and the leaves get some out of the air. Nitrogen is what is left when the oxygen is burned out of the air. (See Air.)
A leaf is very thin. The sun can shine right through it, for the cell walls or skin is as transparent as glass. In some way sunlight mixes with the water and minerals from the roots and the oxygen, from the air, and makes green plant cells. The clear, unused part of the water is drawn away in vapor, and most of the oxygen is given back to the air for animals to breathe. The carbon is laid away in the plant cells. The nitrogen is sent clear back to the roots to make nitrates, before the plant can use it. Clover draws a great deal of nitrogen from the air, to make this plant food.
The new plant food made in the leaves is sent back to all the growing parts of the plant where it is needed. Some of it stays in the leaves to build them larger. Some goes into flowers, fruit and seeds. In grasses and straight-veined plants without bark, the new plant food goes to every part. But in plants that grow by adding a new ring every year, the green cells form a layer between the old wood and the bark.
You can find this soft, green layer under the bark of a rose bush, or the twig of an orchard or nut tree, in the spring. It loosens the bark so it can easily be peeled off. That is why you can make a willow whistle in the spring, or peel a little switch. The new green layer makes the heart wood just that much larger, so the bark has to stretch to fit it. You know your skin stretches as you grow larger or become fatter, doesn't it? Maybe this stretching, year after year, is what makes the bark on hard wood trees crack in long deep ridges.
All summer a plant is busy feeding itself, growing and bringing up seed babies. But in the fall the leaves close their mouths and stop pulling up water. They know hard times are coming, when there will be little water to draw upon. So they must stop giving water to the air. They seem to rob themselves of the food they no longer need, and to send it all into ripening seeds, into the roots, and into next year's leaf-buds that form at the base of the leaf stalks. Food for the seeds, when they begin to grow, is stored in fruits and nuts, into thickened stems like lily bulbs, into tubers like potatoes, and into grains of corn and wheat. Everything is done to keep the plant alive over the winter, and to give it a new start in the spring.
