Glossary - Tree
According to the classical definition, a plant is a tree if it meets these criteria:
- Be large: it must theoretically be able to grow to a height of at least 5 to 7 metres (this varies depending on the author of the definitions consulted)
- Be ramified (have branches), but starting only at a certain height above the ground
- Have a single trunk
- Have a self-supporting trunk: hold itself upright
- Have a trunk made of true wood, consisting of lignin, a polymer that gives the tree its great rigidity
- Have a long life expectancy
According to the modern definition (Hallé), it is a large plant that has evolved into a form that lifts itself off the ground so as to reach above its rivals in the search for light. As such, a plant is a tree that:
- is usually perennial (lives at least two years)
- has one or more vertically growing trunks
- has a structure and anatomy (one or more trunks) sufficiently rigid to lift itself above other plants competing for light
- has photosynthetic capability of some kind: usually leaves on branches, or large leaves if the tree has no branches; branches that are capable of photosynthesis if the leaves are too small for the job or if there are no leaves
- grows to a great height, in comparison with humans
