A tree in three directions: trunk and branches grow upward, roots grow downward, and all grow laterally, that is in diameter. As with all living things, trees are made up of cells, and growth occurs by means of cell division. Vertical growth is of little interest, because the most part of the wood in the tree trunk is formed by lateral growth. Growth in diameter, also called secondary growth, takes place in a very narrow zone between the wood of a tree trunk and the bark. This area, called cambium, is only a few cells thick, but it produces all the different types of cells in both the wood and the bark. The cambium itself consists of a layer only one cell thick, but as the cells divide and mature, there is a region on each side of the cambium which contains living cells in various stages of development.
When a wood cell is mature, it is technically dead, for it contains no nucleus or protoplasm. Thus , even the wood of a living tree is made up mainly of dead cells, although certain kinds of cells in the sapwood remain alive longer than others.
During a normal growing season, the cambium produces millions of cells, and a layer of new wood is formed. Since the cambium is a cover surrounding the tree trunk, the layer of wood produced each year is in the same form, and when the tree is only a year or two old, the layer of wood is a cone as high as the tree. During each successive growing season, another cone – shaped layer of wood is added around underneath. Thus , in order to find the age of a tree by the time- honored method of counting growth rings, one must cut the tree very near the ground or the first year or two is missed.
During each growing season, a layer of bark is also added, but it is added to the inside of the bark. It would seem, then, that since a tree enlarges in diameter each year, the outer layers of bark must stretch. But what actually happens is that the outer layers of bark become dry and, instead of stretching, they crack.. This accounts for the scaly appearance of the bark of most trees.
On a cross- sectional surface we can see the growth rings. These are the concentric layers of wood added each season to the diameter of the trunk. The rings are usually quite distinct because in the temperature climates, the wood formed during the early part of the growing season is different from the wood formed later. The wood formed in the spring when growth is more rapid is called earlywood or springwood, and is characterized by cells which are larger and thin – walled, making a rather porous layer of wood. Slower growth later in the growing season produces latewood or summerwood, which has smaller thick-walled cells, forming relatively more dense wood.
Besides, on the surface of hardwoods, fine lines can be seen radiating from the centre of the tree outward. These are wood rays made up of cells oriented horizontally in the tree instead of vertically, as the majority of the cells are. The horizontal orientation of ray cells helps to conduct food materials laterally in the tree.
XII. Answer the following questions:
1. In what three directions does a tree grow? 2. Where does the secondary growth occur? 3. What part of a tree produces cells? 4. How can one know the age of a tree ? 5. Why are the annual rings well seen in temperate climate? 6. What are wood rays? What is their function?