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How trees feed


I have spoken of the breathing of a tree, - the means by which the free oxygen of the air is brought into contact with the living cells of the cambium. It is not easy to explain what business the oxygen has there. Nor is it easy to understand how the chemical disturbance caused by the presence of the oxygen contributes to the well-being of the tree, - is even a necessity to its life.

The power to work comes only to the cell attacked by oxygen. Carbon exists in combination with other elements, notably hydrogen, oxygen and nitrogen, in the cell substance. Free oxygen induces some of the carbon to leave the other elements. The carbon and oxygen chemically unite, forming carbon dioxid. The molecules which have lost some atoms of carbon are disorganized. But the energy liberated by the oxygen's disturbance enables the cell to take in food, and so to get carbon in such amount and proportion that the disturbed chemical equilibrium is restored, the broken down molecules replaced or rebuilt, the cell made whole.

All the tree accomplishes in the way of growth depends upon the energy and the food supply of its individual cells. The activity of the oxygen supplies the energy, and the elaborated sap furnishes the food which repairs the waste caused by the oxygen, and enables the cells to grow and multiply, thereby increasing the tree's size.

The food supply comes to the cambium in a continuous descending current of rich sap that flows through the cells, furnishing them with food at all times. But it has to be made ready before it can thus be used. The raw material out of which this food is made comes to the tree from two sources, - the soil and the air. The roots absorb water and with it many substances held in solution, that may or may not be useful to the tree as food. During the growing season there is a continuous flow of this crude sap from the roots to the leaves, where it is converted into nutritious plant food.

The course of this current is through the sap wood. The water which the hoots take up in such quantities has, furthermore, a mechanical use in the feeding of the tree. It forms a great complex waterway which bears raw materials for food up from the roots, and carries the prepared food from the leaves down through the cambium, supplying nourishment to every cell that needs it, from leaf to root, and storing the surplus as starch along the way in older cells. The carbon supply comes largely from the air in the form of carbon dioxid, a form unfit for cell food. The leaves receive and prepare the food for the use of the growing parts of the tree.

In the leaf cells crude elements are elaborated. Carbon dioxid comes in through the open doorways of the leaf when there is less of this gas inside than outside. Where there are no doorways the gas may pass by osmosis through the cell walls. Here are granules of "leaf green" arranged around the walls or in the clear substance of the cell. Many chemical elements are present, some in simple combinations, as hydrogen and oxygen united in the form of water; others in more complex combinations. The sunlight beats upon the leaf. Its upper wall is transparent. The granules of leaf green absorb the heat and light. Through the energy thus obtained the living leaf cells are able to dissociate the two gases that compose certain molecules of water.

Some carbon and oxygen that came in from the air as carbon dioxid are also divorced. By a selective and constructive power that is past human understanding these elements are made to reunite in new proportions into new molecules. The substance formed is known as Starch. It is of very high complexity, containing carbon, hydrogen and oxygen. The new product changed to soluble form makes its way down through the cells of the newest bark, forming a current of nutritious sap. In this the cambium finds food to rebuild its broken molecules, to nourish and multiply its cells. Here is excess of food materials to be stored away.

In the making of starch, oxygen is left over. The amount required in a molecule of starch does not use the amount supplied by the breaking up of the molecules of water and of carbon dioxid. Hence oxygen accumulates in the cell, and under osmotic pressure passes out into the air. Leaves prepare food only in the daytime, and in the presence of sunlight. The more warmth, the more work accomplished. Moist, sunny regions produce the most luxuriant vegetation.

Symbiosis is the name applied by the physiological botanists to the mutually helpful relation that exists between the roots of certain trees and certain parasitic fungi. The delicate filamentous threads of these were formerly mistaken for root hairs. They closely enmesh the tips of the tree roots, which are destitute of root hairs.

The tree being unable to absorb food from the soil, the fungi do this work, and transmit the soil water to the tree. The leaves prepare it for use. The returning sap current feeds the cambium cells from leaves to roots, and the fungi get their share. Having no leaf green, they are utterly dependent upon the tree for nutrition.