TREE PARTS – The parts of a tree are described using labelled photos – trunk , bole, branch, twig, crown, roots, mycorrhiza, bark, leaves, needles and buds. Reproductive parts of a tree are described elsewhere. To learn more about flowers and fruits of Broadleaf trees click HERE. To learn more about pollen cones and seed cones of Conifers click HERE.
A tree is defined as a perennial, woody plant with a single main stem – the trunk or bole – bearing branches clear of the ground. The total structure above the main stem, including the branches and leaves, is called the crown. The words twig and shoot are both used for small, thin branches of woody plants.
THE TREE – Trees are adapted to grow taller as rapidly as possible as they compete for access to sunlight. The main stem grows at the fastest rate. Branches grow at a slower rate. Branch initiation and growth is controlled by the leading tip. Once the tree reaches its maximum height, lower branches are allowed to grow and curve upwards giving the typical tree shape, as shown by this mature Lime tree. For more information on similar trees and other possible tree shapes click HERE
Most tree roots are found in the upper 50cm of the soil. Some trees can have roots that extend well beyond their crown, often reaching 2-3 times its diameter. Below ground the roots grow and branch to form a complex network which exist in a symbiotic relationship with fungi.
A leaf is the main photosynthetic organ of a tree. Broadleaf trees generally have wide leaves with veins. Broadleaf trees bear flowers and fruits not cones. Conifers have leaves that are needle-like or scale-like. They bear cones not fruit. The word ‘conifer’ means cone-bearing.
Deciduous trees shed their leaves in autumn and grow a new set in spring. Most broadleaf trees are deciduous but holly, holm oak, cherry laurel and portugal laurel are evergreen trees and keep their leaves all year. Most conifers are evergreen but the european larch, swamp cypress and dawn redwood are deciduous.
THE TRUNK, BRANCHES AND SHOOTS – Trees grow in two ways – growth of the shoot tip is called primary growth – growth that increases the thickness of the shoot is called secondary growth. Primary Growth is initiated by rapidly dividing cells in the shoot tip. Subsequently these cells elongate and the stem gets longer. Secondary Growth takes place once the primary growth has elongated the growing stems. It converts the green shoots into woody twigs, adds growth rings to the trunk and adds bark to the bole.
The sapwood, which is a layer of living xylem tissue is responsible for transporting water and nutrients from the roots to the rest of the tree. These xylem cells are arranged in annual rings, with each ring representing one year’s growth. As new layers of xylem are formed, older layers become inactive and become heartwood which provides structural support.
Secondary growth is carried out by two layers of cells. The vascular cambium has cells that divide inwards to produce xylem cells – which carry water and outwards to produce phloem cells – which carry sugars and nutrients. The cork cambium has cells that divide to produce a layer of bark, which is dead cork. The way the bark is produced results in two different bark types – smooth/peeling bark or ridged/plated bark. To see 12 examples of the former and 15 of the latter click HERE
The active xylem (sapwood) can be quite thick, often several centimeters or more. The xylem cells formed in the spring growth season are larger and lighter in colour than those formed in the summer growth period. The contrast between the two is what creates the visible ring pattern.
The phloem layer is much thinner than the xylem layers, ranging from a few cell layers to several millimeters or up to a centimeter or more in old trees. The primary function of the phloem is to transport the sugars produced in the leaves through photosynthesis to other parts of the tree for growth and storage.
ROOTS AND FUNGI – Trees may have two different types of root. Some trees have predominantly shallow root systems that spread horizontally just below the surface. Other trees have deep taproots or deep-reaching roots that extend several feet or more into the ground.
Most tree roots are found in the upper 50cm of soil, where they can access oxygen and nutrients. Some have tap roots which go much deeper. Many trees have roots that extend well beyond their canopy spread, often reaching 2-3 times the diameter of the tree’s canopy. One of the function of roots is to anchor the tree to the soil but in stormy weather they can be blown over, as shown here. The roots absorb water from the soil and transport it to the rest of the tree and also absorb essential nutrients, such as nitrogen, phosphorus, and potassium, from the soil.
Roots form symbiotic relationships with beneficial soil microorganisms called mycorrhizal fungi. Mycorrhiza means fungal root. The fungus cells are in the form of hyphae cells which can join and branch and elongate at their tips to produce a complex network of underground filaments called a mycelium. The hyphae also enter the root cells and form bush-like structures called arbuscles where nutrient exchange takes place. The hyphae extend into the surrounding soil, absorb nutrients and water and bring them back to the plant. In return, the plant provides the fungi with carbohydrates, which the fungi use for energy. Hyphae tip growth involves the production of vesicles. The contents of the vesicles are used to break down and digest the substrate on which the fungus is growing. Mushrooms are the fruiting bodies of underground mycelia and usually only appear in autumn when wetter, cooler conditions aid spore dispersal and germination.
BROADLEAF TREE LEAVES – A leaf is the main photosynthetic organ of a tree. This is a process in which carbon dioxide from the air is combined with water in the presence of light to produce sugars and oxygen. Leaves have evolved to absorb the maximum amount of sunlight. They have surface pores which allow carbon dioxide to enter and oxygen to leave. At the same time they must minimise water loss and so they may have wax-covered surfaces and may control the time that the pores are open.
The broadleaf tree leaf stalk is referred to as a petiole by botanists. The petiole connects the leaf to the shoot. If the leaf has no petiole and sits right on the shoot it is known as sessile. The leaf blade is divided down the centre by the midrib. This is the central or main vein of the leaf. The veins are tubes that carry water and nutrients to and from the leaf surface. The network of veins reaches every part of the leaf surface. The leaf blade is also referred to as the lamina. The edge of the lamina is called the leaf margin. The shape of the leaf is determined by how the margin and the veins develop and is under precise genetic control.The birch leaves shown here are simple leaves. For more information on leaves click HERE.
The leaf margin may be smooth, toothed or lobed. There are growth points along the margin which are aligned with veins. Smooth margins grow at a uniform rate but toothed leaves have parts of the margin that are distant from the growth points and lag behind resulting in the formation of teeth. If the growth between the tips is inhibited even more, a lobe is formed. A lobe is a rounded or pointed segment of a leaf that is separated from other segments by a gap that does not reach the midrib of the leaf. This is a simple leaf.
There are two types of Broadleaf tree leaves. They may be simple or compound. A simple leaf is made up of one piece. It is undivided. A compound leaf is divided into several leaflets. Pinnate is a term used to describe a compound leaf that has leaflets arranged in 2 rows either side of a midrib like a feather as in this ash tree leaf. Note that there is no bud where each leaflet joins the midrib but there is a bud where the leaf joins the shoot. For more examples of pinnate leaves click HERE
The Horse Chestnut leaf is also a compound leaf but it is referred to as a palmate leaf with 5 leaflets radiating from a single growth point. For more examples of palmate leaves click HERE
CONIFER LEAVES – Conifer trees evolved long before broadleaf trees. 200 million years ago the forests were dominated by conifers with some tree ferns, cycads and ginkgos. With the appearance of broadleaf trees 140 million years ago, conifers became less dominant and retreated to habitats that gave them an advantage over the rapidly diversifying angiosperms. Today’s distribution reflects this as they are found in areas with short growing seasons – northern latitudes or high altitudes – or arid areas that inhibit broad leaf trees. Conifers underwent adaptations that led to the development of specialised needle-like or scale-like leaves with waxy surfaces to minimise water loss.
Needles
The leaves of Pines, ‘true’ Cedars, Larches, Spruces, Firs and the Common Juniper are long and thin and needle-like and are simply called needles. Pine needles are bundled together, usually in clusters of 2,3 or 5. The number in the bundle can help to ID the tree. The needles are joined at the base near the shoot and the bundle of needles is called a fascicle. Where the fascicle joins the shoot there is a basal sheath which is initially transparent. Individual Spruce and Fir needles are arranged in ranks on either side of the shoot. Usually the needles are in flat ranks but in others they may stick out in all directions. Larch and ‘true’ Cedar needles are in clusters of 20 to 45 on short shoots called spurs. The leaves of the Common Juniper are needle-like and finely pointed. For examples of conifer trees with needles click HERE
Scales
Conifer leaves may also be shaped like a scale on a fish or reptile. These scale-leaves are closely pressed against the stem and overlap each other, forming a compact arrangement. They serve a similar function as needle-like leaves, reducing water loss and providing protection against harsh environmental conditions. Cypresses have overlapping scale-like leaves that cover the shoot as in the Lawson Cypress. Sometimes these leaves are just called scale-leaves. Some conifers have main shoots that are covered in long, dark green leaves that resemble scales as in the Giant Sequoia. Some Cypresses have scale leaves that are very closely pressed to the shoot as on the Italian Cypress. The Coast Redwood has scale-leaves on the main shoot and flat needles on the side shoots. For examples of conifer trees with needles click HERE
BROADLEAF TREE BUDS – Apical dominance is a phenomenon in plants where the growth of the main, or apical, bud at the top of the plant inhibits the growth of lateral (axillary) buds along the stem. This phenomenon is essential for shaping the growth and structure of many plants, particularly trees and shrubs. Apical dominance is primarily controlled by the plant hormone auxin and has several key features. The apical bud, which is located at the tip of the main stem or branch, produces a higher concentration of auxin compared to lateral buds. This elevated auxin level inhibits the growth and development of the lateral buds further down the stem. By suppressing the growth of lateral buds, apical dominance encourages the plant to grow upward or outward, promoting vertical growth and maintaining the plant’s dominant central stem. This is especially important for competing for light in densely vegetated environments. Under certain conditions lateral buds can be released from apical dominance. This can result in the development of new branches or side shoots, altering the plant’s growth pattern.
In Britain most tree growth occurs in spring and summer. During the growing season the tree produces leaves and new buds ready for next years growth. These buds remain dormant during the winter and open in the following spring. Inside each bud are tiny preformed leaves, shoots or flowers. Terminal buds are formed at the end of the shoot. Lateral buds are formed at the junction of the leaf and shoot called the axil. (Lateral buds are also called axiliary buds). This is an ash tree shoot at the beginning of winter. For examples of different winter buds click HERE.
During the summer growth period, the ash tree terminal bud produces new growth and the original lateral buds, that were on either side of the terminal bud, open and grow new leaves each with a new lateral bud in its axil. The new growth ends with the formation of a two new leaves (along with new lateral buds) and a new terminal bud at the end of the shoot. In the terminal bud is the apical meristem. This is a 0.1mm diameter dome that consists of a few hundred cells which divide repeatedly in spring to produce new growth cells. Subsequently these cells elongate and the stem gets longer. Each axillary bud (lateral bud) includes its own apical meristem which may specialize in producing either vegetative shoots (leaves, thorns, branches) or reproductive shoots (flowers).