Justine Lambert, age 11, of Calgary, Alberta, Canada, for his question:
HOW DO TREE BRANCHES GROW THICKER?
Most trees start life as seeds. Once they develop they are called seedlings. After the young tree reaches a height of about six feet and its trunk becomes one to two inches thick, it is called a sapling. Trees need a tremendous amount of water as they grow. A large apple tree, for example, will absorb as much as 95 gallons of water from the soil every day.
Branches and the trunk of a broadleaf or needleleaf tree continue to grow higher, longer and thicker as long as the tree lives.
Just under a tree's bark is some material called the cambium tissue. This tissue causes branches and trunks to thicken. It uses the sugar manufactured by the leaves of the tree to make new plant tissue.
On its outside, the cambium makes new phloem tissue, or inner bark. On its inside, it makes new xylem, or wood.
Each year a new layer of thickness is added to each branch.
A tree's wood is made up largely of cellulose, a tough substance that is actually made from sugar. The xylem has two kinds of wood: sapwood and heartwood. The wood nearest the cambium is the sapwood. It is the living part of the xylem and is made up of tiny pipelines that carry sap from the roots to the leaves.
In tropical areas, the sapwood grows thicker on the branches and trunk throughout the year. In cooler climates, the cambium usually makes a new layer of sapwood only early in summer.
As a tree grows older, the wood nearest the center of the branches and trunk dies. This dead wood is the heartwood. It helps support the tree.
When trees make a new layer of wood once each year, the layers form a series of annual rings. Each ring represents one year's growth. After such a tree has been cut down, you can count the annual rings and find out how long the tree lived. You can also do the same thing after large branches have been cut down.
Most trees reproduce sexually. Seeds are produced only after pollen from the male part of a tree flower comes in contact with the female
. part of the flower.
Many trees have flowers with both male and female parts. The pollen from the male part can simply drop onto the female part to start the process which will develop seeds.
Trees grow taller only at the tips of their trunks and branches. Each year, the tip of each branch develops a bud that contains a green stem called a shoot. The shoots grow to make the trunk and branches taller.