Newly formed cells have the wall composed of cellulose, a substance similar to starch in composition. The contents of the cell are made up of a number of substances, the chief of which are albuminoids, starchy matters, oils, and water with dissolved sugars, gums, and acids.

In the heart-wood the contents have disappeared, air taking their place, and the cell-wall has become very much thickened by a deposit within the cellulose of a dense substance called lignin, which gives to wood its elasticity and hardness.

In the living tree, air and water are present in varying quantities, depending on the season and kind of wood. The amount of water is frequently as much as fifty per cent. During the seasoning of pine, about twenty per cent of water is removed from the wood. This may be called free water, because it exists in the plant with all the ordinary properties of water. But there is also in pine-wood about the same amount of water, which is chemically combined with carbon to form cellulose and lignin. The presence of this modified water may be demonstrated by placing the wood in a partially closed iron vessel, and heating it red hot; the wood is reduced to charcoal, while water is given off, together with a small quantity of gases, oils, and other matters.

The elementary composition of wood varies according to the kind, the soluble matters in the soil, and the amount of moisture absorbed by the tree. Generally wood contains large quantities in proportion of carbon, hydrogen, and oxygen; less of nitrogen, sulphur, and potassium; and small quantities of iron, phosphorus, calcium, sodium, and silicon, with traces of many other elements.

Fig. 10. - Diagram showing growth of the cell-wall.

1. Cambium-cell: a, protoplasm or living contents of the cell; b, nucleus in the protoplasm; c, thin mem-, brane through which the sap passes. In the heart-wood this membrane has broken down, as at d.

2. Protoplasm forming a wall of cellulose.

3. Protoplasm has disappeared. Cellulose changing into lignin.

4. Cell-wall composed of lignin and thin membrane.

If wood is burned in the open air, the carbon, hydrogen, nitrogen, sulphur, and part of the oxygen are driven off in gaseous form; the other elements remain, and constitute the ash, of which the principal ingredient is potassium.

The amount of ash is greater in the palms and least in the pines. The percentage of a few are as follows: