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Plant Root

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Plant Root

Have you ever heard the expression “putting down your roots”? Well, although you may not have physical roots, it means that you are establishing yourself in one place, as a plant would do by putting down roots! Roots are important plant organs that help them stay put and carry out a number of other functions as well. Most plants (besides nonvascular plants) contain these special organs.

Plant roots definition

Roots are plant organs that are typically part of the belowground network of the plant. Although, as we will learn later, plant roots can also be aboveground, for extra support. Roots help provide support, or anchorage, to plants since they cannot move and must be able to maintain themselves in variable conditions. Most importantly, however, roots help absorb water and nutrients the plant needs to produce food and maintain its health.

The plant root is an organ that helps to anchor and absorb water and nutrients from the soil.

Plant root structure

The structure of plant roots and their growth vary dependent on the plant species, as is true for the other plant organs. However, a basic structure that characterizes plant roots can be discussed to understand more about the tissues and cells that make up the root.

Root growth

Starting at the tip of the root, there is a root cap, which covers and protects the actively growing root in the soil. Above the root cap, the plant root is split into different zones, from the tip upwards: zone of cell division, zone of elongation, and zone of differentiation.

  1. Zone of cell division: the zone of cell division is right above the root cap (which is protecting it) and is made up of meristem. Meristems are areas of cell division, and they give rise to new plant organs. Each root has its own root apical meristem or area of cell division which helps it extend into the soil.

  2. Zone of elongation: this is an area of rapid cell growth above the meristem.

  3. Zone of differentiation: this is an area above the zone of elongation where the cell division and growth have stopped, and cells become different types of cells that will constitute the different root tissues.

The root apical meristem is the zone of cell division that supplies the roots with new cells as they continue to grow. The roots will continue to go throughout the plant's life to head in the direction where water is readily available. These zones only make up the very small fragment at the beginning of the root, beyond these zones, the root has the three types of plant tissue that also appear in leaves and stems: ground tissue, vascular tissue, and epidermal tissue.

Root anatomy

Epidermal tissues in roots make up the outer layers of cells. Inside the root past, the epidermis is the cortex (Fig. 1). The cortex is made up of plant ground tissue and includes a few different layers: the mesocortex or “middle cortex” and the endodermis. The middle cortex is usually a site for storage, while the endodermis is the separation between the cortex and the middle of the root known as the stele.

The endodermis has a Casparian strip which forces water and other molecules entering the root to first go through the selectively permeable membrane of plant cells. Thus, this helps protect roots from harmful pathogens or chemicals. In the stele are the vascular bundles and other ground tissue. The very middle of the cell is composed of the pith, which is also a storage site for sugars. Some plants (eudicots) do not have a pith.

Plant Root, Cross-section of monocot root, StudySmarterFigure 1: Cross-section of a monocot plant root (Smilax). Source: Fayette A. Reynolds, Berkshire Community College Bioscience Image Library via flickr.com, edited.

Primary versus secondary growth

In the root primary growth refers to the growth of roots, in particular getting longer, and the addition of new roots. Primary growth is the main growth of herbaceous plants. Woody plants may undergo another round of growth known as secondary growth. Secondary growth is lateral growth that increases the width of a plant root or stem. This occurs by the growth of the vascular tissue and produces layers that cause the cortex to shed and be replaced with a layer known as the periderm. Knowing all the names of root layers is not necessary for the AP test, but it's good to know that different plants have different root tissues!

Function of roots in plants

Roots help to anchor plants in the ground. Plants inhabit many different biomes, from the desert to the tropical rainforest, meaning they face all kinds of conditions. Having a root system has ensured plants the protection they need in different climates. Typically, root systems are more extensive than the shoot system (stem and leaves). In addition to providing stability, roots, being a novel characteristic of vascular plants, absorb water, minerals, and nutrients from the soil which travel through the xylem into other parts of the plant.

Roots are also important because they,

  • form symbiotic relationships with bacteria and fungi to help the plant get nutrients or minerals,

  • act as storage of nutrients/food and,

  • sequester carbon in plants and in the soil.

Types of roots in plants & examples

Taproot system

In angiosperm (flowering) plants, the radicle is the first root to arise after the seed germinates. If this radicle develops into the main central root, known as a tap root, and all the other roots develop off of the taproot, the system is called a taproot system. Taproot systems can grow to be quite extensive, making them efficient absorption systems. Taproots themselves can grow deep into the soil, anchoring the plant, and also sometimes acting as a nutrient and food storage system for the plant. Examples of taproots that humans use for food include carrots and sugar beets (Fig. 2).

Fibrous root system

In the fibrous root systems, there is no main taproot, and the primary root does not last. Instead, the roots are formed from low nodes on the stem, and are often, thin, roughly the same size, and form mat-like structures. The fibrous root systems are the roots found in ferns and monocotyledons (corn, wheat, palm trees). Monocots are said to have “adventitious root systems” which means they develop from the stem near the soil surface (Fig. 2).

Prop roots

Prop roots are a type of adventitious root that helps provide extra support for the stem, typically aerial support. Prop root examples include the roots of red mangrove trees, the Banyan tree, and corn plants (see Fig. 2)

Plant Root, Examples of plant root types, carrots, grass roots, and Banyan tree, StudySmarterFigure 2: Different types of roots: carrots are taproots (left), grass has fibrous roots (middle), and a Banyan tree has prop roots for support (right). Source: pixabay.com.

Helping the environment, some plant roots provide structural support to their surrounding environment in areas where the substrate- soil, sand, rocks, etc.- is loose and easily eroded. Many beach dunes are held together by the presence of the root systems of the dune plants!

Root adaptations to environmental pressures

Different root systems may be found in the same environment, as plants have evolved different ways of obtaining resources. Largely depending on the climate, the quality of the soil, and the needs of the plant, roots can vary between species.

Environmental pressures don't necessarily favor one type of root system. For example, living in an arid desert means that plants have to find a way to get and absorb water. Some cacti have shallow fibrous root systems to absorb the most water when rainfall occurs. Other plants (e.g., the Mesquite tree) have a taproot system that extends deep below the surface to reach underground water stores.

Root nodules

In a group of plants known as the legumes, special nodules, or chambers, exist to allow for the presence of nitrogen-fixing bacteria in the roots. This symbiosis is important to the plants since the bacteria make nitrogen readily available in a form accessible to the legume plants where it might not be otherwise. We eat lots of plants from this family including lentils, beans, and chickpeas.

Root hairs

Root hairs are outgrowths from a type of epidermal cells in the roots of almost all vascular plants, and they increase the surface area of the root system significantly (Fig. 3). Being extensive and having adaptations to increase surface area is important since the more surface area, the more water will diffuse into plant cells via osmosis.

Root hairs are modified epidermal cells that greatly increase the surface area of the roots, allowing them to absorb more water and minerals.

Osmosis is the diffusion of water from an area of low solute concentration to an area of high solute concentration, take a refresher on Osmosis if you need it!

Lenticels

Lenticels are openings in the epidermis that allow for gas exchange with the tissues below (Fig. 3). Unlike stomata, lenticels remain open and do not close. Some plant roots (woody dicots) may have them to increase the presence of gases, such as oxygen in the roots (remember oxygen is still needed by plants for cellular respiration!).

Pneumatophores are special roots that grow upwards out of the water in plants that are coastal or live in swampy places. These roots have lenticels on them to remedy the lack of gas exchange in submerged roots.

Plant Root, Photographs of Root Hairs and Lenticels, StudySmarterFigure 3: On the left, is a photo of a root hair; on the right, is the lenticel of a root. Source: Fayette A. Reynolds, Berkshire Community College Bioscience Image Library via flickr.com, edited.

Plant Root - Key takeaways

  • Roots anchor the plant and absorb water and minerals from the soil.
  • At the tip of the root is where the growth and cell division take place. The root cap protects this area.
  • Above the root cap is the zone of cell division which has the undifferentiated, dividing meristem tissue.
  • The taproot system contains the main root that the other roots grow off of. The fibrous root system consists of thinner roots that do not grow off of the main root but instead form from the lower nodes of the stem.
  • Roots have special adaptations that allow them to be more efficient, such as root hairs for a higher surface area.

Frequently Asked Questions about Plant Root

A plant root is an organ that helps to anchor the plant and absorb water and nutrients from the soil. Typically, they are the belowground part of the plant, but some roots (i.e., prop roots) may appear above ground and be modified from nodes on the plant stem. Roots are also important because they provide storage, form mutualistic relationships with fungi and bacteria, and contribute to carbon sequestration. 

No, not all plants have roots. Nonvascular plants (mosses, liverworts, and hornworts), because of the lack of vascular systems do not have roots. They may however have root-like structures called rhizoids to help with stability and water and nutrient absorption. 


Plant roots take many different forms depending on the environment and the needs of the plant. 


Examples of plant roots include: 

  1. Carrots, sugar beets, and cassava roots (taproots)- which are important for storage in plants.
  2. Long, thin roots of grasses (fibrous roots)- which help to absorb water and anchor both the plant and the substrate around the plant (help to prevent erosion). 
  3. Prop roots which help to support the plant including those of corn plants and mangrove trees. 


Types of plants roots include: 

  1. Taproot system
  2. Fibrous root system 
    1. Prop roots

The taproot system is when the primary root in the developing plant becomes the main root from which all of the other roots originate.


In the fibrous root system, there is no taproot and the roots are formed from low nodes on the stems of plants. These are sometimes also called adventitious roots. All ferns and monocots have fibrous root systems. 


Prop roots are a type of fibrous root that provides aerial support for the plant and are often exposed. 

Yes, plants can live without roots as not all plants have roots. In fact, nonvascular plants (mosses, etc.) do not have roots.


 Although roots greatly increase water absorption ability and provide structural support for plants, smaller plants may be able to survive without root systems if given proper water and mineral supply (i.e., plant cuttings, parts of the plant taken from the shoot system, can survive without roots and grow new ones when possible). 

Final Plant Root Quiz

Question

What is the purpose of the root?

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Answer

Absorb water and nutrients

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Question

A root is a....

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Answer

mostly belowground plant organ. 

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Question

T or F: Roots have vascular tissue, epidermal tissue, and ground tissue. 

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Answer

True

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Question

What is the part that protects the actively growing part of the root called?

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Answer

A root cap.

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Question

Which is not a zone in the part of root tip associated with growth?

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Answer

Zone of specialization

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Question

Meristem tissue is tissue that contains cells that are _______ and are undergoing _______.

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Answer

Undifferentiated and cell division

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The ________ forces water and minerals to go through a selectively permeable membrane, protecting the plant from toxins and pathogens. 

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Answer

Casparian strip

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Secondary growth is lateral growth meaning it increases the ______ of the plant root.

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Answer

Width

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Question

Primary growth in roots includes:

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Answer

Getting longer

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T or F: All plants have roots.

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False

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Question

Which of the following is NOT a type of root system?

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Thin root system

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Question

The taproot system occurs when the primary root, known as the ______, develops into the main root from which the lateral roots grow. 

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Answer

Radicle

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Question

Which of the following is NOT a characteristic of fibrous root systems?

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Answer

They develop from the radicle or primary root

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Question

____ _____ are modified epidermal cells that greatly increase the surface area of the roots, allowing them to absorb more water and minerals.


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Root hairs 

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Question

_________ are openings in the epidermis that allow for gas exchange with the tissues below and remain open.


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Answer

Lenticels

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Question

Pneumatophores are roots that contain lenticels and may characterize plants whose roots are...

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Answer

submerged in oxygen-poor water (i.e., swamps)

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Question

In a group of plants known as the legumes, _____ _______exist to allow for the presence of nitrogen-fixing bacteria in the roots. This is an example of a ________.

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Answer

Root nodules; Symbiosis

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Question

What is the term for the middle, or center, part of the root that contains the vascular bundles and other ground tissue?

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Answer

Stele

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