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The leaves themselves take in from the atmosphere carbon dioxide gas, and this gas dissolves in the water contained in the leaves . In the leaves, as well as in some other parts of the tree, there is a green-coloured matter called chlorophyll, which has the remarkable property of using the energy contained in sunlight to cause the elements provided by water and carbon dioxide to become united into complex carbon compounds, which form the greater portion of the actual food of the tree. Chlorophyll is itself only found in the presence of light, and an essential element in its composition is iron. The composition of the raw material used in the chemical laboratory of the plant is complex, but, in order that the plant may continue to live, the following elements must all be absorbed in the form of some soluble compound. The essential elements are:—
Hydrogen.
Oxygen.
Carbon.
Nitrogen.
Potassium.
Calcium.
Phosphorus.
Iron.
Carbon and oxygen make up carbonic acid gas which is absorbed through the leaves, but all other elements must enter as soluble compounds through the minute hairs on the tiny rootlets. Even the largest tree must depend for its food on these fine hairs on its smallest rootlets. The large roots only seem to hold the tree upright, and to convey .the food materials in solution from the root hairs to the portion of the tree above the ground.
The inorganic chemical compounds which are absorbed by the plant are transformed in the leaves into organic compounds, chief among which are carbohydrates. This process is known as "photosynthesis," a word which means building up through the action of light. The first stable carbohydrate formed is usually some form of sugar. Photosynthesis only goes on in the presence of light, and often in strong light the production of sugar proceeds at a faster rate than the product can be removed from the leaf to other parts of the tree. The stoppage of the life processes in this way is prevented by the fact that, when the concentration of sugar reaches a certain point, new activities of the protoplasm of the living tree-cells are awakened and the formation of insoluble starch at the expense of the sugar begins. As the starch is insoluble, the action of photosynthesis can continue unchecked seeing that the concentration of the sugar will not rise beyond a critical point. Sugars as they arise are also continuously passing away from the leaf to other parts of the tree. They have to be used up along with other materials in the formation of new living substance, wood and other materials, as well as to supply the chemical energy needed for vital processes. When the photosynthetic activity of the leaf ceases at night time, the starch formerly produced in the leaves is re-converted by special ferments into sugar, which thus continues to replace that which is being still withdrawn from the leaves till all the starch may be finally removed from them. This explains why it is that, towards the end of a summer's day, the leaves may be full of starch, but next morning they contain little or none. In other words, the process of absorption of food by trees goes on continuously day and night.
Let us now turn to the mechanics of tree-food. The water from the roots, as has been said, passes upwards into the leaves, in the outer layer of the wood. The sugary sap already referred to travels from the leaves to the parts where it is required in the inner layers of the bark. Hence, if a tree be "ringbarked" all round the stem through the bark and the outer layers of the wood, it will be killed, as no water can then pass upwards, and it follows, of course, that no life-giving
