Trees are the oldest living organisms on Earth, in fact they don't die but must be killed by disease, insects or people. They have been known to live up to 5,000 years and grow over 360 feet tall. Trees also freshen our air supply by removing carbon dioxide and producing oxygen, lower the air temperature by evaporating water into the air, prevent erosion by stabilizing the soil and provide numerous creatures---including humans---with shelter. Trees come in all shapes and sizes, but there are a few basic aspects that link all trees together.
To be a tree, it must be self-supporting, typically have a single stem and have a layer known as a cambium. The cambium is a layer of actively dividing cells found within the stems and roots. Without it, a plant is not a tree even though it may look like one. The palm tree is a good example. Because it lacks a cambium it is actually an herbaceous plant and not a woody plant like trees, vines and shrubs.
Tree, Shrub or Vine?
Plants are either herbaceous or woody. As stated, vines, trees and shrubs are woody plants. Shrubs and trees typically differ in two major areas: number of stems and height. Trees usually have a single stem, the trunk, and grow over 15 feet tall. Shrubs usually have multiple stems and grow less than 16 feet tall. A woody plant is a vine if it cannot support itself.
No matter how different they may look, all trees have the same basic parts. The three main parts are the crown, trunk and roots. The crown is the leaves or needles and branches of the tree. Roots are basically underground branches that keep the tree in the ground and store nutrients for later use. The trunk supports the crown and transports food between the roots and the crown.
Layers of a Tree
A tree's branches and trunk are made up of several layers. The outer layer is the bark, which serves as protection from insects and disease. Underneath that is the phloem, or inner bark, layer. The phloem carries sugar made in the leaves or needles to the branches, trunk and roots. The next layer is the xylem or sapwood layer. It carries tree sap, which is water plus nitrogen and mineral nutrients, to different parts of the tree. The next layer is the cambium, a layer of cells that produces xylem and phloem cells. The center of the tree is heartwood. Heartwood is old sapwood. It serves as a support structure for the tree.
Types of Trees
The two major types of trees, gymnosperms and angiosperms, are defined by their seeds. Angiosperms have seeds that are enclosed in an ovule; gymnosperms do not. Since they lack the ovule, gymnosperms usually enclose their seeds in cones, or conifers, which is why they are also known as coniferous trees. Most evergreens are gymnosperms. Angiosperms are also known as deciduous trees and hardwood trees, because their wood is harder than that of coniferous trees.