: Trees Worth Knowing by Rogers Julia Ellen - Trees

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send up escapes by evaporation through the curtained doorways of the leaves.

Starch contains carbon, hydrogen, and oxygen, the last two in the exact proportion that they bear to each other in water, H^O. The carbon comes in as carbon dioxide, CO^. There is no lack of this familiar gas in the air. It is exhaled constantly from the lungs of every animal, from chimneys, and from all decaying substances. It is diffused through the air, and, entering the leaves by the stomates, comes in contact with other food elements in the palisade cells.

The power that runs this starch factory is the sun. The chlorophyll, or leaf green, which colors the clear protoplasm of the cells, is able to absorb in daylight some of the energy of sunlight, and to enable the protoplasm to use the energy thus captured to the chemical breaking down of water and carbon dioxide, and the reuniting of their free atoms into new and more complex molecules. These are molecules of starch, C^H^O^.

The new product in soluble form makes its way into the current of nutritious sap that sets back into the tree. This is the one product of the factory--the source of all the tree's growth--for it is the elaborated sap, the food which nourishes every living cell from leaf to root tip. It builds new wood layers, extends both twigs and roots, and perfects the buds for the coming year.

Sunset puts a stop to starch making. The power is turned off till another day. The distribution of starch goes on. The surplus is unloaded, and the way is cleared for work next day. On a sunless day less starch is made than on a bright one.

Excess of water and of free oxygen is noticeable in this making of starch. Both escape in invisible gaseous form through the stomates. No carbon escapes, for it is all used up, and a continual supply of CO^2 sets in from outside. We find it at last in the form of solid wood fibres. So it is the leaf's high calling to take the crude elements brought to it, and convert them into food ready for assimilation.

There are little elastic curtains on the doors of leaves, and in dry weather they are closely drawn. This is to prevent the free escape of water, which might debilitate the starch-making cells. In a moist atmosphere the doors stand wide open. Evaporation does not draw water so hard in such weather, and there is no danger of excessive loss. "The average oak tree in its five active months evaporates about 28,000 gallons of water"--an average of about 187 gallons a day.

In the making of starch there is oxygen left over--just the amount there is left of the carbon dioxide when the carbon is seized for starch making. This accumulating gas passes into the air as free oxygen, "purifying" it for the use of all animal life, even as the absorption of carbon dioxide does.

When daylight is gone, the exchange of these two gases ceases. There is no excess of oxygen nor demand for carbon dioxide until business begins in the morning. But now a process is detected that the day's activities had obscured.

The living tree breathes--inhales oxygen and exhales carbonic-acid gas. Because the leaves exercise the function of respiration, they may properly be called the lungs of trees, for the respiration of animals differs in no essential from that of plants.

The bulk of the work of the leaves is accomplished before midsummer. They are damaged by whipping in the wind, by the ravages of fungi and insects of many kinds. Soot and dust clog the stomates. Mineral deposits cumber the working cells. Finally they become sere and russet or "die like the dolphin," passing in all the splendor of sunset skies to oblivion on the leaf mould under the trees.

The great chestnut tree on the hillside has cast its burden of ripe nuts, flung down the empty burs, and given its yellow leaves to the autumn winds. Now the owner has cut down its twin, which was too near a neighbor for the well-being of either, and is converting it into lumber. The lopped limbs have gone to the woodpile, and the boards will be dressed and polished and used for the woodwork of the new house. Here is our opportunity to see what the bark of the living tree conceals--to study the anatomy of the tree--to learn something of grain and wood rings and knots.

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