Understanding the Parts of a Plant: The Root System

by | Botany

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The root system is like an underground network for a plant, providing essential services to the above-ground parts that we see.

Roots are almost always underground, but these hidden systems are responsible for a range of functions from securing the plant in the soil to absorbing water and nutrients. You can think of the root system as the foundation of a building: it’s what keeps the plant stable, nourished, and capable of growth throughout its life cycle.

Perennials, which re-sprout multiple years in a row, often die back to the roots when the weather gets too cold. The soft above-ground tissue is nowhere to be seen, but underground, the roots continue working at a slowed pace, acting as a kind of life support for the plant until conditions are met for re-sprouting.

Parts of a Plant – Series Navigation

Before we begin: this is a free resource provided by HerbSpeak. If you find this guide helpful, please share it, or tell someone else about the website. It helps an independent author! This is a multi-part series broken into multiple sections, as follows:

  1. Parts of a Plant: Introduction & Basic Terminology
  2. Parts of a Plant: Roots (you are here!)
  3. Parts of a Plant: Stems
  4. Parts of a Plant: Leaves
  5. Parts of a Plant: Flowers
  6. Parts of a Plant: Fruits, Seeds, and Cones
  7. Parts of a Plant: Breaking the Rules

Each section is designed to be able to be read standalone, so you don’t have to worry about the order or where you start from. If you find yourself confused about some terminology used, however, the introductory “terminology” page may help you clear things up about words and concepts here. Jump to sections that you want to learn more about or follow along for self-study reference.

Now, let’s dig into root systems.

Types of Roots

As much as pants manufacturers would like us to believe it, there is very little in life that is one-size-fits-all. Root systems are one big example of this, coming in various shapes and structures to support different kinds of plants. Unfortunately, we don’t get to see this part of the plant above-ground often, so it can be easy to forget how many types of roots there are.

Two of the most common types are fibrous roots and taproots, but plants have evolved many other types of root systems to adapt to unique growing conditions. Among those unique roots are adventitious roots, water roots, root parasites, rhizomes, aerial roots, and tuberous roots.

Image above: Onion roots (Fibrous) VS. carrot roots (Taproot)

Fibrous Roots

Fibrous roots form a dense network of small roots that spread out horizontally in the soil. These root systems are only seen in plants classified as monocots.

These root systems create a complex, interwoven network of relatively thin roots that spread out horizontally near the soil surface. This intricate web is excellent at holding soil in place, making fibrous roots crucial in combating soil erosion.

Because these types of roots remain close to the soil surface, fibrous roots are experts at quickly absorbing moisture from rainfall, which makes it an excellent type of root system in floodplains, and wetlands. Plants commonly equipped with fibrous roots include flowering plants like grasses, rice, and clover.

Taproots

Taproots, however, have a large dominant root that grows vertically into the soil. All plants considered a dicot will also have a taproot. This primary root is flanked by smaller lateral roots, but it doesn’t expand rapidly outwards like fibrous roots. Plants like carrots and dandelions sport a taproot system.

This root system design allows taproot systems to delve deeper into the soil, tapping into water sources that are inaccessible to other root types. It’s a survival strategy plants use to adapt to challenging environments like drier soils with lower water tables. Because some taproots of plants like carrots and beets store so many nutrients, they’ve also become an essential part of human diets. Not all taproots are edible, however, nor is every taproot nutritious for humans.

Image above: lotus (Water roots) VS. Pothos (Aerial roots)

Water Roots

Water roots, as the name suggests, are specialized for life in aquatic environments. Found in plants like water lilies and mangroves, these roots may float on the water surface or extend into the water from the soil or trunk. If you’ve ever kayaked or paddleboarded on a lake with lilies, there’s a chance you’ve seen these roots. With an otherworldly look to them, they can leave you wondering whether it’s even a part of a plant.

Water roots can absorb dissolved oxygen directly from water. This is a vital feature since aquatic soils are often low in oxygen. In mangroves, you’ll notice these roots growing straight up out of the water, a type of root known as a “pneumatophore” – or a specialized kind of aerial root – to help the plant absorb oxygen from the air.

Furthermore, water roots typically have large air-spaces in the roots, known as aerenchyma, giving these roots a sponge-like appearance when cut in half.

Aerial Roots

These roots often serve specific functions tailored to the challenges of their environment, often serving to absorb oxygen and moisture. In climbing plants, aerial roots serve more than just nutrient absorption or oxygen intake; they can act as a support trellis. Climbing plants like pothos use aerial roots to grip onto surfaces, which helps them climb and spread over walls and trees.

Image above: Potato (Tuberous) VS. Mother-of-thousands, Kalanchoe spp. (Adventitious)

Tuberous Roots

Tuberous roots not only do their usual job of absorbing nutrients, but also act like a storage warehouse for the plant. These roots, like the ones found in sweet potatoes, store water and nutrients in a swollen, thickened structure. The plant can then tap into these reserves during drought or the dormant season. Tuberous roots differ from tubers (like potatoes), which are modified stems, not roots.

Adventitious Roots

These roots take every advantage they can get, which means you will find them sprouting unexpectedly from plant parts that don’t typically produce roots, such as stems or leaves.

Seen in many houseplants like mother-of-thousands or spider plants, adventitious roots provide additional support and can help a plant spread, climb, or deal with environmental stress. This is the primary method of propagation for plants that spread vegetatively – or, through their leaf/stem tissue. In the case of flood-tolerant plants, adventitious roots can emerge from submerged stems to help the plant survive waterlogged conditions. Corn is a common example used when discussing adventitious roots.

Image above: Turmeric (Rhizome) VS. Dodder, Cuscuta spp. (Parasitic)

Rhizomes

Rhizomes are not roots but underground stems that can produce both shoots and roots. Familiar examples include ginger and iris plants. Rhizomes serve as a storage unit for the plant to store extra nutrients and water, but it can also help plants propagate themselves.

These roots spread horizontally under the soil, able to sprout a new plant when the conditions are right. This enables plants – like grasses – to colonize new areas quickly. Rhizomes are not technically true roots, distinguished by nodes and internodes like stems, but they do travel underground.

Root Parasites

Living off someone else’s hard work isn’t exclusive to animals; some plants do it too. Root parasites like dodder and mistletoe connect their roots to the roots of host plants, tapping into their nutrient supply. These parasitic roots have specialized structures that penetrate the host’s vascular system to draw out nutrients and water. While humans are out watching movies about supernatural creatures of the night, the real vampires are living right at their feet. While some root parasites can photosynthesize on their own, many are entirely dependent on their host for survival.

Root Functions

Understanding the functions of a plant’s root system is like peeking behind the curtain and discovering a hidden world of complex and beautifully coordination biological system. While there is still much more that we have to learn about these hidden worlds, we know that it’s a communications hub and nutrient uptake source all in one.

Addressing Fungal Networks and the “Wood-Wide-Web”

Let’s first address to proverbial elephant in the room. A lot of popular science headlines make the roots’ ability to communicate sound like something mystical and idyllic. There’s likely some truth to these pieces, but also a fair amount of extrapolation and storytelling that goes into it. It’s important to know where those lines are, so we’ll talk about what we know (and don’t know) about root functionality in the following sections.

The idea that mycorrhizae, or fungal networks, communicate altruistically in a forest for the greater good of the local ecosystem is a popular theory. (1) It’s a great idea and makes our human hearts soar with the possibility of altruistic good in the ecosystem.

The facts are, however, that we don’t know. Nothing is certain, because there’s a lot of difficulty in creating a scientifically sound study that can be reproduced across multiple forest communities and habitats. “If you ask me if in the future, we will be showing that trees actually can communicate, I would not be surprised,” says Dr. Tamir Klein, a plant ecophysiologist interviewed by the New York Times. There is both support for, and against, this theory, making it a complex subject.

The truth of the matter likely lies somewhere in the middle, and likely isn’t universal for fungi, trees, or even nutrient types. While there’s a lot of evidence that agrees that there are mutually symbiotic relationships between certain plants and fungal networks, there are also parasitic fungi – and possibly even parasitic plants – that change the balance of the relationship on a per-plant, per-fungi basis.

Furthermore, it’s worth pointing out that most of these studies have been done on trees, not herbaceous or woody plants in the understory.

The complexity of the subject is just as, if not more, nuanced than the relationships between flowers and their pollinators. Ultimately, there just needs to be more research into root system relationships.

There is difficulty in admitting that we don’t know something. Humans have an element of pride that can be difficult to overcome in those moments of uncertainty. We want to have all the answers, even more than we want others to know that. One of the biggest lessons in science, however, is that we know a lot less than we think we do.

At the end of the day: as long as it doesn’t influence the scientific process or spread outright misinformation in education, there’s no harm in believing in a little extra good in the world.

Anchorage: Keeping the Plants Rooted in Place

Anchorage is the first line of defense for a plant, much like the role of an anchor for a ship. The root system provides the mechanical support needed to keep the plant upright and stable in the soil. This is especially important as the plant grows in size and complexity, bearing the weight of leaves, flowers, and fruits.

The roots spread into the soil, gripping it tightly with varying lengths and widths of root tissue, including root hairs. This grip allows the plant to withstand environmental pressures – such as wind and rain – that could otherwise uproot it or cause it to topple over and shift in the soil.

The design of the root system, whether fibrous or taproot, can significantly influence how well a plant is anchored in different soil types and habitats.

Fibrous roots, with their web-like structure close to the surface, are excellent for soil stability and resisting erosion, making them better able to withstand steep slopes. Because they are able to increase soil stability, they can play a role in stabilizing certain habitats, such as estuaries and river floodplains. (2)

Taproots, on the other hand, dive deep into the soil, offering a different kind of stability by anchoring the plant to deeper soil layers. These roots have an advantage in arid environments, where water is further from the surface, and the plant must tap into that source of moisture to sustain itself. These plants also tend to survive better in nutrient-poor soils, as the taproot will reach lower layers of soil that typically have more nutrient availability.

Nutrient Absorption

The roots are responsible for absorbing water and essential nutrients from the soil to sustain the plant’s growth and overall health. Elements like nitrogen, phosphorus, and potassium are vital for various biological processes, from protein synthesis to energy transfer. Think of the root system as the plant’s grocery store, but one that operates 24/7.

The surface of roots has tiny hair-like extensions called root hairs, which increase the root’s surface area, making nutrient absorption more efficient. These hairs penetrate the microscopic spaces in the soil to draw out nutrients, almost like a network of underground miners extracting valuable resources.

Nutrient Storage

Some root systems are built-in storage units for the plant, designed to hoard nutrients and water for use when the plant needs it most. This is particularly beneficial for plants living in harsh environments, or if the plant must go dormant for a long time to survive. This adaptive strategy enables the plant to survive and even thrive in conditions that might otherwise lead to its demise. Carrots are a familiar example of this type of root function. The part that we eat is the part that has swollen with stored starches and nutrients.

Image credit: https://www.ck12.org/c/biology/root-structure/lesson/Root-Structure-Advanced-BIO-ADV/

The Anatomy of the Root System

The photo above showcases the different parts of a root system under a microscope. It’s important to note the differences in shapes and sizes here. If it’s your first time looking under a microscope in a long time, it might not be easy to discern the different areas, so we’ll also cover each individual part of the root’s anatomy separately.

Root Cap

Starting at the very tip of a root, you’ll find the root cap. This protective feature is like a helmet for the root’s sensitive growing area. It shields the inner cells from damage as the root pushes through soil, rocks, or other obstacles. The root cap also secretes a slimy substance that acts as a lubricant, making it easier for the root to penetrate the soil.

Meristem

Right behind the root cap lies the meristem. If the root were a factory, the meristem would be the production line. This is where the magic happens—or, to put it in more scientific terms, this is where cellular mitosis occurs at an accelerated rate. The meristematic zone serves as a sort of “nursery” for the new cells that will populate the rest of the root structure. This non-stop activity is what allows the root to grow in length and regenerate cells that get damaged or lost.

The cells in the meristematic zone are unspecialized meaning they haven’t been assigned specific roles like ‘root hair’ or ‘root cap’ yet. These cells are moldable and flexible, and it’s this versatility that allows them to later specialize into different types of cells.

The meristem, like any other part of the plant, doesn’t operate in isolation. It’s regulated by a variety of plant hormones, mainly auxins, which dictate the pace and direction of growth. This is similar to the functionality of a plant stem, which we’ll discuss later in the Parts of a Plant series.

Once these newly minted cells are ready, they move on to the elongation zone.

Elongation Zone

Located just beyond the meristematic zone, the elongation zone is where the cells undergo a significant transformation—growing in size rather than dividing. After the cells are produced in the meristematic zone, they enter the elongation zone still immature and unspecialized.

Here in the elongation zone, they absorb water and expand rapidly, making the root longer. This is vital for reaching deeper soil layers that may contain essential nutrients and water reserves, effectively making the root system more resilient and resourceful.

Maturation Zone

As cells stretch and grow in the elongation zone, they stretch begin to take on specific roles as they enter what is called the maturation zone. This might be forming root hairs for nutrient absorption or integrating into the plant’s vascular system for transporting water and nutrients, whatever is needed at the time. This zone is the furthest away from the root cap, and you will begin to see root hairs here.

By the time cells reach this zone, they’re fully differentiated, meaning they’ve taken on specific roles. This section of the root is not just about growth but targeted, strategic growth, fine-tuned over millions of years.

Root Hairs

Originating in the maturation zone, these slender, microscopic structures are extensions of individual epidermal cells. Root hairs are singular cells, but they can accomplish big things. We could take a hint or two from these little cells.

Root hairs increase the surface area of the root and make it much easier for the plant to absorb water and essential nutrients like nitrogen, phosphorus, and potassium from the soil. These vital resources are pushed back into the main root system, which then sends them up to the rest of the plant.

These small root hairs also contribute to the plant’s ability to stabilize in soil. The mucilage secreted by root hairs helps bind soil particles together, which aids in water retention and prevents soil erosion. It’s like adding a layer of glue that holds the soil particles in place.

Additionally, they play a vital role in forging alliances with soil microorganisms. We know that root hairs are instrumental in forming symbiotic relationships with certain types of fungi strands, known as mycorrhizae, as well as different types of bacteria.

These fungi and bacteria attach to the root hairs and assist in nutrient absorption, essentially extending the root’s reach even further, becoming “fungal roots.” Beyond this, however, things get a little uncertain based on the earlier discussion about the wood-wide-web. The extent of our knowledge here isn’t comprehensive or absolute. We do know that many plants depend on this connection for healthy vigor, but it’s unclear exactly what – and how – everything is happening.

Cortex

The cortex is the main body of the root, and it’s located between the root’s outer layer, known as the epidermis, and its innermost vascular tissue. The primary role of the cortex is to store nutrients and carbohydrates produced by the plant. Think of this as the root’s pantry or storage unit.

When the plant goes through periods of stress, such as drought or nutrient scarcity, the resources stored in the cortex can be mobilized to support vital functions. In some plants – such as aquatic plants – the cortex also aids in aeration.

These aquatic plants grow water roots, with specialized cells known as ‘aerenchyma’, providing air spaces in the cortex. These air spaces allow the plant to survive in waterlogged conditions by facilitating the transport of oxygen from the plant’s above-ground parts down to the root system, giving it a sponge-like appearance.

Vascular Cylinder

Situated in the maturation zone, the vascular cylinder is the core of the root where the plant’s vascular tissues—xylem and phloem—are located.

The xylem is responsible for transporting water and nutrients from the root up to the rest of the plant, while the phloem moves food produced through photosynthesis from the leaves down to the roots for storage or growth.

See also on HerbSpeak: How Do Plants Absorb Water? 

Xylem and Phloem

It’s easy to think about the xylem and phloem as a sort of highway system. The xylem is dedicated to the transport of water and dissolved minerals from the root to the rest of the plant. These cells have thick walls fortified with lignin, an organic polymer that provides structural support.

Right next to the xylem is the phloem which transports essential nutrients, like sugars. The phloem ensures these nutrients are distributed wherever they are needed, from the root tips to the growing buds and leaves.

Pericycle

Surrounding the xylem and phloem is a layer of cells called the pericycle. The function of these cells are to create a protective structure to the xylem and phloem inside, helping stabilize the structure of the plant and keep it upright.

It’s from the pericycle that lateral roots emerge, branching out to explore more soil territory. These lateral roots enable the plant to tap into more extensive resources.

Endodermis

Finally, we have the endodermis, a single layer of cells that surrounds the vascular cylinder, which is a smaller circle within the root. This is very closely located to the pericycle. The endodermis functions like a sort of customs checkpoint, regulating what gets in and out of the vascular cylinder thanks to what is known as the Casparian strip—a waxy barrier that blocks passive flow of substances. This ensures that only essential nutrients and water are allowed to pass into the xylem and phloem, maintaining the plant’s overall health.

References
References
  1. New York Times, Are Trees Talking Underground? For Scientists, It’s In Dispute, https://www.nytimes.com/2022/11/07/science/trees-fungi-talking.html
  2. Steffens B, Rasmussen A. The Physiology of Adventitious Roots. Plant Physiol. 2016 Feb;170(2):603-17. doi: 10.1104/pp.15.01360. Epub 2015 Dec 23. PMID: 26697895; PMCID: PMC4734560.

 

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About Destynnie K. Berard
I am a lifelong naturalist who believes a good sense of humor is essential to staying happy. ★ After traveling for years, I settled in New England, falling in love with the diverse landscape the Northeast has to offer, and began pursuing conservation in earnest. ★ My career background is in enterprise marketing and communications, which provides me with a unique perspective of ecological relationships.

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