24-03-2015, 08:32 PM
Trees around a lake.
The Trembling Aspen in its autumn colours
Strangler fig tree in Costa Rica. Locally known as Guanacaste.
A tree is a tall plant with a trunk and branches made of wood. It can live for many years. The four main parts of a tree are the roots, the trunk, the branches, and the leaves.
The roots of a tree are usually under the ground. One case for which this is not true are the roots of the mangrove tree.[1] A single tree has many roots. The roots carry food and water from the ground through the trunk and branches to the leaves of the tree. They can also breathe in air.[1] Sometimes, roots are specialized into aerial roots, which can also provide support, as is the case with the Banyan tree.
The trunk is the main body of the tree. The trunk is covered with bark which protects it from damage. Branches grow from the trunk. They spread out so that the leaves can get more sunlight.
The leaves of a tree are green most of the time, but they can come in many colours, shapes and sizes. The leaves take in sunlight and use water and food from the roots to make the tree grow, and to reproduce.
Trees and shrubs take in water and carbon dioxide and give out oxygen with sunlight to form sugars. This is the opposite of what animals do in respiration. Plants also do some respiration using oxygen the way animals do. They need oxygen as well as carbon dioxide to live.
Parts of trees
Beech leaves.
Tree roots anchor the structure and provide water and nutrients. The ground has eroded away around the roots of this young pine tree.
The dark lines between the centre and the bark are medullary rays, which allow nutrients to flow across the tree trunk.
The parts of a tree are the roots, trunk(s), branches, twigs and leaves. Tree stems are mainly made of support and transport tissues (xylem and phloem). Wood consists of xylem cells, and bark is made of phloem and other tissues external to the vascular cambium.
Growth of the trunk
As a tree grows, it may produce growth rings as new wood is laid down around the old wood.It may live to be a thousand years old. In areas with seasonal climate, wood produced at different times of the year may alternate light and dark rings. In temperate climates, and tropical climates with a single wet-dry season alternation, the growth rings are annual, each pair of light and dark rings being one year of growth. In areas with two wet and dry seasons each year, there may be two pairs of light and dark rings each year; and in some (mainly semi-desert regions with irregular rainfall), there may be a new growth ring with each rainfall.[2]
In tropical rainforest regions, with constant year-round climate, growth is continuous. Growth rings are not visible and there is no change in the wood texture. In species with annual rings, these rings can be counted to find the age of the tree. This way, wood taken from trees in the past can be dated, because the patterns of ring thickness are very distinctive. This is dendrochronology. Very few tropical trees can be accurately dated in this manner.
Roots
The roots of a tree are generally down in earth, providing anchorage for the parts above ground, and taking in water and nutrients from the soil. Most trees need help from a fungus for better uptake of nutriens: this is mycorrhiza. Most of a tree's biomass comes from carbon dioxide absorbed from the atmosphere (see photosynthesis). Above ground, the trunk gives height to the leaf-bearing branches, competing with other plant species for sunlight. In many trees, the order of the branches makes exposure of the leaves to sunlight better.
Exceptions
Not all trees have all the organs or parts as mentioned above. For example, most palm trees are not branched, the saguaro cactus of North America has no functional leaves, tree ferns do not produce bark, etc. Based on their general shape and size, all of these are nonetheless generally regarded as trees. Trees can vary very much. A plant form that is similar to a tree, but generally having smaller, multiple trunks and/or branches that arise near the ground, is called a shrub (or a bush). Even though that is true, no precise differentiation between shrubs and trees is possible. Given their small size, bonsai plants would not technically be 'trees', but one should not confuse reference to the form of a species with the size or shape of individual specimens. A spruce seedling does not fit the definition of a tree, but all spruces are trees.