Section 1.1: Tree Anatomy & Functions

Outcomes

Students will:

  • Identify and describe the main parts of a tree (roots, trunk, and canopy).

  • Analyze the characteristics of cells in tree roots, trunk, and leaves.

  • Understand the structural and functional relationships among cells and tissues, including how cells and tissues contribute to the growth and health of the tree.

  • Comprehend how tree rings form annually, including how the size of each tree ring differs due to environmental conditions.

Key Terms

Adhesion / Capillary Action / Chlorophyll / Cohesion / Early Wood / Endodermis / Epidermis / Exodermis / Heartwood / Inner Bark / Late Wood / Mesophyll / Outer Bark / Osmosis / Parenchyma Cells / Pericycle / Phloem / Photosynthesis / Pith / Sapwood / Stoma / Transpiration / Tree Ring / Vascular Bundle / Vascular Cambium Layer / Vascular Tissue / Xylem

See content or Module Glossary for definitions

What Makes Up a Tree?

Trees are the largest plants on the planet. They provide us with oxygen, help stabilize the soil, and give life to the world’s wildlife. Trees are key members of the ecosystem and they are an excellent source of information about the environment. Through the TREE Program, you will get to investigate and find out what stories’ trees have to share, specifically looking at trembling aspen trees. For now, we start by looking at what trees are composed of. They can be divided into three main parts: the roots, the trunk, and the canopy.

  • Roots - Refers to all the tree parts below ground although roots can occasionally be above ground. Roots spread throughout the ground to anchor the tree in place and to gather water and nutrients. Roots also serve to store food for the tree throughout the winter. With some species, such as trembling aspen, trees reproduce through their roots, creating what are known as clone trees.

  • Trunk - Refers to the large column(s) of wood above ground but below the canopy. The trunk supports and elevates the canopy, as well as transports water and nutrients throughout the tree.

  • Canopy - Refers to the leaves and branches of the tree. The canopy positions the leaves in full view of the sun, allowing for photosynthesis, transpiration, and hormone production in the tree.

What Makes Up Tree Roots on the Cellular Level?

Tree roots are not only the anchor for the tree but essential in providing the tree with life. They soak up the necessary nutrients and water from the soil, feeding the growth cycle of the tree. Tree roots are composed of a complex and intertwined network of cells with two main types of structures: root hairs and lateral roots.

Root hairs are a small outgrowth stemming off the lateral roots. Root hairs are continually being created and typically last 2-3 weeks before they die off and their nutrients are taken up by the roots. Root hairs provide a lot of surface area to absorb water and nutrients into the root. Lateral roots are the other main root structure and are much bigger than the root hairs. The lateral roots contain larger amounts of tissue, called vascular tissue, to transport nutrients and water throughout the tree.

When roots are cross sectioned, we can see that they have five main types of cells: the epidermis, the exodermis, the cortex, the endodermis, and the vascular tissue (as shown in the Figure). The term tissue in this context refers to when cells are bundled together.

Figure 1 shows a cross-section of the cellular structure of roots. Image by CNX OpenStax.
Figure 1 shows a cross-section of the cellular structure of roots. Image by CNX OpenStax.

Types of Tree Root Tissue

  • Epidermis - The epidermis is the outermost tissue that is a physical barrier providing protection, insulation, as well as moisture and gas control. The epidermis also absorbs some nutrients and water. As the epidermis wears away and dies off, it is continually replaced by cells from the exodermis.

  • Exodermis - The exodermis tissue is right underneath the epidermis and it replaces epidermis tissue as it wears away and dies off. This is very similar to how the inner bark replenishes the outer bark on the trunk which is explained in upcoming sections.

  • Cortex - The cortex is a layer of cells that lies directly below the exodermis. The cortex transfers nutrients from the root hairs to the vascular tissue and is used for energy storage. The cortex separates the exodermis from the endodermis.

  • Endodermis - The endodermis is the innermost layer of the cells with thicker cell walls. Some of these cells are coated in a water-repellent substance called suberin, which helps keep the endodermis watertight. The endodermis serves to regulate water and nutrient movement between the soil and the vascular tissue.

  • Pericycle - The pericycle is composed of tissue that lies just inside the endodermis. The pericycle serves as internal support and protection for the root and it also forms new lateral roots by dividing rapidly in a specific location.

  • Vascular Tissue - Vascular tissue is a bundle of cells that transport water and nutrients from the roots to the rest of the tree.

How do Trees Drink and Gather Nutrients?

Trees drink by increasing the concentration of salts in their roots such that their roots have a higher salt concentration than the surrounding soil. When this happens, water flows into the roots to cause the root's salt concentration to be in equilibrium (be the same) with the soil's salt concentration. This process is known as osmosis. As the tree drinks, any minerals and nutrients that are dissolved in the water will flow into the roots as well. The roots separate these nutrients and minerals through cells called sieve cells and the nutrients and minerals then make their way up to the trunk to start feeding the rest of the plant.

What are the Main Parts of a Tree Trunk?

Much like the variety of shapes and sizes seen with humans, tree trunks vary from one species to the next. However, there are parts that are common to all trees and these are the bark, the vascular cambium layer, the sapwood, the heartwood, and the pith (as shown in Figure 2).

The bark of a tree serves as a physical barrier for protection, insulation, and moisture control. Bark is separated into outer and inner bark.

  • Outer bark is composed of dead cells, commonly referred to as cork. Outer bark is covered with fine oxygen-breathing pores called lenticels.

  • Inner bark is composed of cells that transport sap and nutrients throughout the tree. As these cells age, they become outer bark.

Figure 2 shows the anatomy of a tree trunk. Original image by Thomas Steiner.
Figure 2 shows the anatomy of a tree trunk. Original image by Thomas Steiner.

The vascular cambium layer is a thin layer of cells with no specific task yet. This is where majority of the trees outward growth occurs. These cells continually divide with a varying rate throughout the year, creating phloem cells on the bark side and xylem cells on the inside.

  • Phloem cells transport sap and other nutrients throughout the tree. These cells become phellem cells as they mature and die off.

  • Xylem cells transport water and minerals throughout the tree. These cells become the sapwood as they mature and die off.

Sapwood is composed of the xylem cells created by the layer surrounding it, the vascular cambium layer. Sapwood is responsible for water transport and storage through the tree’s daily water cycle. As these cells mature and die off, they harden and become more rot resistant, turning into heartwood.

Heartwood is a layer of wood composed of dead xylem cells and fiber bundles. Heartwood has a darker appearance than sapwood. Over time, the cells harden and become stronger, enabling the heartwood to be structural support for the tree.

The pith is the center most portion of the tree and is composed of soft spongy parenchyma cells (explained below). The pith is surrounded by a ring of xylem cells, which is then surrounded by a ring of phloem cells. This allows the pith to store and transport water and nutrients throughout the tree.

What Makes up a Tree Trunk on the Cellular Level?

As noted in the previous section, there are many different parts that make of a tree trunk. Within each of these parts, there are many different and uniquely specialized cells. The four main cells are xylem, phloem, parenchyma, and fiber bundles.

Figure 3 shows the inner workings of xylem and phloem. Image by KI3580.
Figure 3 shows the inner workings of xylem and phloem. Image by KI3580.
  • Xylem Cells - These cells are responsible for transporting water and minerals up from the roots throughout the tree. These cells combine together to form hollow tubes called vessel elements. It is important to note that xylem cells only transport water up (see Figure 3).

  • Phloem Cells - These trunk cells are responsible for transporting sap, which contains sugars and other nutrients, up and down throughout the tree. Phloem cells combine together to form hollow tubes called sieve tubes. These tubes are separated into smaller sections by sieve plates which allow for the bidirectional flow of nutrients. It is important to note that the phloem can transport nutrients up and down (see Figure 3).

  • Parenchyma Cells - Parenchyma cells provide support and nutrients to phloem and xylem cells and make up the bulk of plant cells.

  • Fiber Bundles - Fibers are long, slender cells that typically occur in bundles. These cells are mostly composed of cellulose, a tough material that makes up the cells walls and provides cell with structural support.

How do Trees Grow?

Trees expand and increase their size through the vascular cambium layer in their trunk. This layer continually divides at different rates throughout the year into phloem and xylem cells. The xylem cells are created on the inside of the tree and they are responsible for sapwood and heartwood growth. The phloem cells are created on the bark side of the tree and will quickly convert itself into the cork-like wood that we commonly associate as bark.

How do Tree Rings Form?

Tree rings form every year as the tree is living and growing. These rings will be different sizes depending on the factors experienced during the growing season. These factors include: temperature, pests, diseases, access to water, nutrients, sunlight, and more. When the conditions are just right for the tree, the vascular cambium layer (refer back to Figure 2) will rapidly divide and create sapwood and bark at a set rate. This initial rapid growth creates lightly coloured, less dense wood called early wood. However, depending on the trees' environmental conditions, the growth rate of the rings could change.

As the season progresses towards winter, trees spend less energy on growth and more energy creating an energy store for the winter. This later slow growth creates the darkly coloured, more dense wood called late wood. As winter occurs, a tree experiences little to no growth but once spring comes around, the ring formation cycle repeats. This cycle is what gives trees the alternating light and dark circular patterns we see and are what we call tree rings (see Figure below).

Figure 4 shows a cross section of a tree trunk, labelled with certain parts of the tree ring.
Figure 4 shows a cross section of a tree trunk, labelled with certain parts of the tree ring.

What Makes up a Tree Leaf on the Cellular Level?

The last main part of a tree are the leaves, which make up the canopy. As the roots are important for soaking in water and nutrients, leaves are important for soaking in sunlight, the other essential ingredient in giving plants life. In order to trap the sunlight and convert it so the tree can use it, tree leaves are composed of many specialized cells. The four main types of cells found in a leaf are the epidermis cells, mesophyll cells, stomata, and vascular bundles. These cells aid in photosynthesis, hormone production, and help move water to the outside of the leaf to be evaporated.

  • Epidermis - The epidermis is the outer layer of cells that provides a physical barrier for protection and insulation for the leaf. The epidermis acts like a skin and is coated in a wax-like substance called cuticle, that helps prevent unwanted water loss. See Figure 5 for a visual representation.

  • Mesophyll - These cells are composed of parenchyma cells in long tube-like arrangements. Mesophyll is split in two layers, the palisade mesophyll and the spongy mesophyll, and these cells make up the middle section of a leaf (see Figure 5).

Figure 5 shows the cellular structure of leaves. Image by Maksim.
Figure 5 shows the cellular structure of leaves. Image by Maksim.
  • Palisade Mesophyll - Palisade mesophyll is composed of tightly packed parenchyma tissue that contain large amounts of chloroplasts. Chloroplasts are small organelles filled with the photosynthetic pigment chlorophyll. Chlorophyll is responsible for photosynthesis, where energy from sunlight is converted to sugars. Since the chloroplasts need sunlight, the palisade mesophyll occupies the top inside portion of the leaves.

  • Spongy Mesophyll - Spongy mesophyll is composed of loosely packed parenchyma tissue and occupies the bottom inside portion of the leaves. By packing the spongy mesophyll loosely, gases have plenty of room to move between the chloroplasts and the stomata allowing the chloroplasts access to fresh carbon dioxide.

  • Stoma - As shown in Figure 5, stoma are little natural openings in the epidermis that allow for regulated gas exchange between the tree and the atmosphere. Specialized cells, called guard cells, open and close the stomata (a collection of stoma) when the leaf needs more carbon dioxide or when the tree needs to transpire.

  • Vascular Bundle - A vascular bundle is a bundle of xylem and phloem cells that are essentially the veins and arteries of the leaves. Vascular bundles supply nutrients and water to the leaves for photosynthesis and transpiration. They also transport the sugars of photosynthesis to the rest of the tree.

Trees breathe by opening the guard cells around the stomata on the underside of their leaves. With the stoma open, gases from within the leaves can exchange with the air outside through diffusion until the gas concentrations have reached equilibrium or a balance.

What is Transpiration and Capillary Action?

Transpiration is the evaporation of water out of a tree. When the tree transpires, water is moved from the roots and up the stem using the properties of water cohesion and adhesion. With cohesion, water molecules are being attracted to themselves. With adhesion, water molecules are being attracted to other surfaces.

As one water molecule lifts itself up slightly to adhere to the inside of a small tube, or in a plant's case, the stem and leaves, water molecules below lift themselves up to cohere to the initial water molecule. This process is referred to as capillary action and for a model of this process, see Figure 6.

Figure 6 shows capillary action in action. The water inside the tube has lifted itself higher than the water outside the tube. Capillary action is why test tubes have a meniscus line. Image by CNX OpenStax.
Figure 6 shows capillary action in action. The water inside the tube has lifted itself higher than the water outside the tube. Capillary action is why test tubes have a meniscus line. Image by CNX OpenStax.

Additional Resources & References for Section 1.1

Resources

References

Everett, T. H., Weber, L., and Berlyn, G. P. (2019). Tree plant. Retrieved from https://www.britannica.com/plant/tree/Tree-structure-and-growth

The Royal Parks. (2019). Why are trees so important? Retrieved from https://www.royalparks.org.uk/parks/the-regents-park/things-to-see-and-do/gardens-and-landscapes/tree-map/why-trees-are-important

U.S. Forest Service. (n.d.). Anatomy of a tree. Retrieved from https://www.fs.fed.us/learn/trees/anatomy-of-tree