We know what a tree is – but there is no precise definition which is completely recognized of what a tree is in ordinary language or in botany.
In the broadest viewpoint, a tree is any plant formed with an elongated trunk which supports branches and leaves above the ground. Trees are usually defined by height, but the minimum height of a tree is loosely defined, as shrubs are smaller plants ranging in height from 18 inches to 33 feet.
A more common, but narrow definition of tree indicates that the tree’s primary growth is upward from the growing tip and has a woody trunk formed by secondary growth, which thickens each year as it grows outward. Trees are also defined by use – a plant that yields lumber, which can be used for milling.
The standard tree is made up of three separate components – the trunk, the roots and the crown. The trunk or the stem of a tree or shrub is the main organ of the plant that provides the rigid woody structure. It supports the crown and functions as a conduit to carry water and nutrients between the roots and crown. A tree may have a single trunk or have multiple trunks.
The above-ground structure of the tree has the following components.
- Bole: The area between the base of the trunk to the first branch.
- Trunk: The main structure of the tree from which branches grow.
- Branch: A part of the tree that grows from the trunk in a typically horizontal, upward direction. Large branches are called boughs and the smaller branches are called twigs. The branches support foliage growth.
- Foliage: A broad term meaning the leaves of a tree, but can include needles found on pines and scale-like material found on junipers. Foliage is normally green.
- Crown: The top of the tree.
Trunk and Branches
Trees have the more permanent structure of a trunk and branches. This is the main difference that separates trees from plants. Trees having wood at their core are what makes them more resilient, helps give them their incredible lifespans and making its branches more suitable for artistic pruning.
The main purpose of the trunk is to raise the leaves above the ground, enabling the tree to reach the light and survive. It also performs the task of transporting water and nutrients from the roots to the above ground parts of the tree and to distribute the food produced by the leaves to all other parts including the roots.
A cut across a tree trunk provides an inside view of the different layers.
- Pith: The center of the trunk and the initial provider of nutrients of the sapling. Pith, or medulla, is a tissue in the stems of vascular plants. Pith is composed of soft, spongy parenchyma cells, which store and transport nutrients throughout the plant
- Heartwood: The densest, heaviest part of the trunk and its wood is commonly the darkest in color. The inactive xylem cells of the heartwood’s function chiefly to give strength to and support the tree and also is important in the tree physiology because it is a storehouse for sugars, dyes and oils.
- Sapwood: Xylem, or sapwood, is the third layer of vascular tissue. It is made up of thick-walled cells which, like the phloem, transport sap from the roots to the crown. Xylem represents the young wood of the tree and is light in color. As xylem cells age they become inactive and become the tree’s heartwood.
- Cambium: An outer growth layer that creates both sapwood and phloem, which in turn increases the tree’s thickness. In climates with distinct seasons, the tree adds a new cambium layer each growing season. This expands the girth or the trunk (and branches) and produces annual rings which can be interpreted from a coring or by felling to determine the tree’s age.
- Phloem (inner bark): The next layer after the cambium tissue and before the bark. Its purpose is to move sap that has been transformed from leaf photosynthesis and that contains sugar, around the tree and back down to the roots.
- Bark: The outer layer that protects the tree and varies from species to species. A tree’s trunk and branches have a covering of bark. The bark derives from dead phloem cells which are shed outward. It has multiple functions, insulating the tree from excessive heat, which may include fire resistance, or cold and provides protection against insects and diseases. Bark may be very thin or up to 6 inches thick as in the cork oak; it can be of variable color; it may contain tannins and the surface may range from smooth to deeply furrowed. Depending upon the species, dead bark can flake or peel off in patches or strips. Bark characteristics provide key to tree identification and exhibits a distinctive appearance in the garden.
- Growth Rings: Each year of growth creates a new ring of sapwood that is visible.
- Medullary ray (wood ray): Living cells that run across the growth rings, that allow the transportation of sap radially.
A complex root system exists beneath the ground in order for a tree to gain nourishment from the soil. Awareness of all parts and functions of the tree root system is vital for proper care of your trees.
The purpose of tree roots is to:
- Uptake water, oxygen, and minerals
- Transport water and minerals toward the inner trunk
- Support the tree structurally
- Promote growth
Large trees have over 30 miles of roots, with 5-million root tips, including many small colonies of beneficial fungus. The work of the root system is fueled by sugar from the leaves, using oxygen found between soil particles. One-celled projections, or root hairs, do the work of absorbing water and minerals from the soil. Beneficial fungae, called mycorrhizae, live on feeder roots and also contribute to the collection process.
The length and depth of root growth depends on the amount of space given the plant. Conducting roots may extend outward as far as 2 or more times the height of the tree without barriers and disturbances. Conducting roots then converge into brace roots, which are thick and grow laterally, providing much of the necessary support for the tree. Roots continue to extend as-needed, growing in an ever-widening disk around the tree.
The crown, which consists of the leaves and branches at the top of a tree, plays an important role in filtering dust and other particles from the air. It also helps cool the air by providing shade and reduces the impact of raindrops on the soil below.
The leaves produce food for a tree. They contain chlorophyll, which facilitates photosynthesis and gives leaves their green color. Through a process called photosynthesis, leaves use the sun’s energy to convert carbon dioxide from the atmosphere and water from the soil into sugar and oxygen. The sugar, which is the tree’s food, is either used or stored in the branches, trunk and roots. The oxygen is released into the atmosphere.
Leaves are structures specialized for photosynthesis and are arranged on the tree in such a way as to maximize their exposure to light without shading each other. Trees have evolved leaves in a wide range of shapes and sizes, in response to environmental pressures including climate and predation. They can be broad or needle-like, simple or compound, lobed or entire, smooth or hairy, delicate or tough, deciduous or evergreen.
Contact Arborist Now to care for your valuable trees in Alameda, Hillsborough and Greater San Francisco Area. As Albert Einstein noted, looking deep into nature, you will understand everything better – we are hopeful that this brief overview of tree anatomy provides you with a better understanding of these magnificent structures.