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What is a root?
What is a root?
A root is a vascular plant organ that absorbs water and minerals and/or anchors the plant to the soil substrate.
Roots have a root cap and endogenous lateral root growth (lateral roots are formed inside the root)
Roots tend to grow underground and are positively gravitropic (growing toward gravity), although some specialized roots are aboveground. Roots are also hydrotropic (growing toward moisture)
Some ancient plants, such as mosses, have root-like rhizoids that are not true roots. Rhizoids lack vascular tissue; they may anchor and absorb water
What are the functions of roots?
What are the functions of roots?
Roots of most plants perform two functions: absorption and anchorage
Absorption
Absorption
Vascular tissues move water (and minerals) from the soil, through plant body to areas of photosynthesis (e.g. leaves)
Roots do not absorb "food" from the soil; Sugars are a plant's food and made in the leaves
Anchorage
Anchorage
Roots secure the plant body to the soil substrate, to allow for absorption
Water Movement in Roots
Water Movement in Roots
Apoplastic movement: water movement through cell walls and inter-cellular space of the cortex
Symplastic movement: water movement through cellular membranes and living cells of the cortex
Roots have an endodermis with thickenings called Casparian strips
These thickenings prevent apoplastic movement into the xylem
This allows for regulation of water uptake by the plant since the cellular membrane can control the movement of water through the endodermis
Lateral Root Development
Lateral Root Development
Lateral roots are created by a meristem called the pericycle, located just under the endodermis
The pericycle creates a new (lateral) root apical meristem (RAM)
This new RAM grow through the endodermis, cortex, and epidermis to emerge into the environment as a lateral root
In woody plants that produce secondary growth in their roots, the pericycle also forms the vascular cambium, and then the cork cambium of the root
Lateral root growth is endogenous: side roots are created inside of the parent root and emerge through the cortex and epidermis
n.b. Contrast this with how lateral stems (branches) form; from axillary buds on the outside of the stem. This is called exogenous growth
Laboratory
Laboratory
Allium Root Tip, longitudinal section (ls)
Allium Root Tip, longitudinal section (ls)
LABEL
Apical meristem
Procambium
Ground meristem
Protoderm
Region of cell division (RCD)
Region of elongation (ROE)
Above: Longitudinal section of the root tip of Onion (Allium)
Monocot root, cross section (xs)
Monocot root, cross section (xs)
LABEL
Epidermis
Cortex
Pith
Xylem
Phloem
Endodermis
Pericycle
Above: Cross-section of a monocot root
EuDicot root, cross section (xs)
EuDicot root, cross section (xs)
LABEL
Epidermis
Cortex
Xylem
Phloem
Cambium
Endodermis
Pericycle
Above: Cross-section of a EuDicot root
Questions for Thought
Questions for Thought
What is germination, when does it begin, and when does it end?
How does a root develop through primary growth?
What are the zones of a young root as you go proximally along the root, starting at the root cap?
Where does water enter roots, and how does it travel to the xylem?
How does internal and external root structure differ in monocots and eudicots?
During secondary growth in roots, newly formed wood will replace the endodermis. Why doesn't this affect water absorption in the roots?
How do lateral roots form, and how is this different from stems?
What are some root adaptations in an arid environment?
What are some root adaptations in a water-logged environment, and how are these related to root respiration?
What are mycorrhizae, and how are they crucial to plant vigor?
What are root nodules, and how does this give these plant a competitive edge?
How is an adventitious root different from a radicle?
See also the stem section for comparison questions
Additional Resources
Additional Resources
How plants build the microbiome they need to survive in a tough environment (Phys.org 8Jul2025)
└Johnson et al. (2025) Functional team selection as a framework for local adaptation in plants and their belowground microbiomes
Beneficial root microbes can help sustain rice yields in unfertilized fields (phys.org 7Jul2025)
└Adachi et al. (2025) Field dynamics of the root endosphere microbiome assembly in paddy rice cultivated under no fertilizer input
Study reveals how wheat roots are quietly engineering their microbiomes (Phys.org 23Jun2025)
└Yang et al. (2025) Eight Years in the Soil: Temporal Dynamics of Wheat-Associated Bacterial Communities Under Dryland and Irrigated Conditions
Unexpectedly deep roots in plants unearth new questions about soil carbon storage (Phys.org 17Jun2025)
└Lu et al. (2025) A continental scale analysis reveals widespread root bimodality
Smart 'switch' in plants allows them to redirect roots to find water (Phys.org 12Jun2025)
└Roy et al. (2025) Redox-regulated Aux/IAA multimerization modulates auxin responses
A three-way interaction that hatches potato root parasites (Phys.org 4Jun2025)
└Akiyama et al. (2025) Solanoeclepin C, a root-secreted molecule converted by rhizosphere microbes to hatching factors for potato cyst nematodes
A root development gene that's older than root development (Phys.org 25May2025)
└Iwata et al. (2025) Evolutionary-conserved RLF, a cytochrome b5-like heme-binding protein, regulates organ development in Marchantia polymorpha
The Evolution of Tree Roots Nearly Ended Life on Earth (Syfy Wire 19Nov2022; Smart et al. 2022)
Digging Deep Reveals the Intricate World of Roots (NatGeo 2015)
Time-lapse video of roots growing (Duke University, Feb 2021)
The power of your garden's hidden half: roots (BBC Future 17Oct2022)
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