The Tree reference article from the English Wikipedia on 24-Jul-2004
(provided by Fixed Reference: snapshots of Wikipedia from wikipedia.org)

Tree

See the real Africa
This article is about the biological organisms known as trees. For other meanings of the word see tree (disambiguation).

Baobab tree in South-AfricaEnlarge

Baobab tree in South-Africa

A tree can be defined as a large perennial woody plant. Though there is no set definition of size, it is generally at least 6 m (20 ft) high at maturity, and with branches supported on a single main stem. Compared with most other forms of plants, trees are long-lived. A few species of trees grow to over 100 m (300 ft) tall and some live for several millennia. Trees are important components of the natural landscape and significant elements in landscaping.

Table of contents
1 Classification
2 Morphology
3 Major tree genera
4 Life stages
5 Cultural, religious & mythological aspects
6 See also

Classification

Trees occur in many diverse orders and families of plants, and thus show a wide variety of growth form, leaf types and shapes, bark, reproductive organs, etc. The earliest trees were tree ferns and horsetails, which grew in vast forests in the Carboniferous Period; tree ferns still survive, but the only surviving horsetails are not of tree form. Later, in the Triassic Period, conifers, ginkgos, cycads and other gymnosperms appeared, and subsequently flowering plants in the Cretaceous Period. Most species of trees today are flowering plants and conifers. The list below gives some examples of well known trees and how they are typically classified.

Morphology

The component parts of a tree are the roots, trunk(s), branches, twigs and leaves. Tree stems consist mainly of support and transport tissues (xylem and phloem). Wood consists of xylem cells, and bark is primarily made of phloem.

Trees may be broadly grouped into exogenous and endogenous trees according to the way in which their stem diameter grows. Exogenous trees, which comprise the great majority of modern trees (all conifers, and all broadleaf trees), grow by the addition of new wood outwards, immediately under the bark. Endogenous trees, mainly in the monocotyledons (e.g. palms), grow by addition of new material inwards.

As an exogenous tree grows, it creates growth rings. In temperate climates, these are commonly visible due to changes in the rate of growth with the temperature variation over the year. These can be counted to determine the age of the tree, and used to date cores or even wood taken from trees in the past; this is known as the science of dendrochronology. In tropical climates with near-constant climate, growth is continuous and does not form distinct rings, so age determination is impossible. Age determination is also impossible in endogenous trees.

The roots of a tree are generally embedded in earth, providing anchorage for the above-ground biomass and absorbing water and nutrients from the soil. Above ground, the trunk gives height to the leaf-bearing branches, aiding in competition with other plant species for sunlight. In many trees the arrangement of the branches optimize exposure of the leaves to sunlight.

Not all trees have all the plant organs mentioned above. For examples: most palm trees are not branched, the saguaro cactus of North America has no functional leaves, tree ferns do not have bark, etc. Based on their rough shape and size, all of these are nonetheless generally regarded as trees. Indeed, sometimes size is the most important consideration. 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. However, no sharp 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. Bamboos by contrast, do show most of the characteristics of trees, yet are perhaps strangely rarely called trees.

A small group of trees growing together is called a grove or copse, and a landscape covered by a large area of trees is called a forest. Several biotopes are defined largely by the trees that inhabit them, for example, rainforest and taiga; see ecozones. Large, but scattered trees with grassland (usually grazed or burned over periodically) in between is called savanna.

Major tree genera

Flowering plants (Magnoliophyta)

Dicotyledons (Magnoliopsida; broadleaf or hardwood trees)

[[NyssaceaeEnlarge

[[Nyssaceae

: a Dove tree in flower'']]

Monocotyledons (Liliopsida)

Conifers (Pinophyta; softwood trees)

The [[coniferEnlarge

The [[conifer

Coast Redwood, the tallest tree species on earth]]

Ginkgos (Ginkgophyta)

Cycads (Cycadophyta)

Ferns (Pterophyta)

Life stages

The life cycles of trees, especially conifers, are divided into the following stages in
forestry for survey and documentation purposes:
  1. Seed
  2. Seedling: the above ground part of the embryo that sprout from the seed
  3. Sapling: After the seedling reaches 1m tall, and until it reaches 7cm in stem diameter
  4. Pole: young trees from 7-30cm diameter
  5. Mature tree: over 30cm diameter, reproductive years begin
  6. Old tree: dominate old growth forest; height growth slows greatly, with majority of productivity in seed production
  7. Overmature: dieback and decay become common
  8. Snag: standing dead wood
  9. Log/debris: fallen dead wood

Tree diameters are measured at height of between 1.3-1.5m above the highest point on the ground at its base. The 7cm diameter definition is economically based, from the smallest saleable stem size (for paper production, etc), and the 30cm diameter is the smallest base diameter for sawlogs. Each stage may be uniquely perceptive to different pathogens and suitable for especially adapted arboreal animals.

Cultural, religious & mythological aspects

Trees often serve as important symbols in mythologies and religions. Examples are Yggdrasil in the Norse Mythology, the Christmas Tree that is derived from Germanicic mythology, the Tree of Knowledge of Judaism and Christianity, and the Bodhi tree in Buddhism. In some religions, such as Hinduism, trees are said to be the homes of tree spirits.

See also