Roots

What is a root?

  • A root is a plant organ that performs the function of absorbing water and minerals, and/or anchoring 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 be underground, but some plants have aboveground roots

  • Some ancient plants, such as mosses, have root-like rhizoids that are not true roots. Rhizoids lack vascular tissue; they may anchor but are not efficient at absorbing water

What are the functions of roots?

  • Roots of most plants perform two functions: absorption and anchorage

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

  • Roots secure the plant body to the soil substrate, to allow for absorption

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 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

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)

LABEL

  • Epidermis

  • Cortex

  • Pith

  • Xylem

  • Phloem

  • Endodermis

  • Pericycle

Above: Cross-section of a monocot root

EuDicot root, cross section (xs)

LABEL

  • Epidermis

  • Cortex

  • Xylem

  • Phloem

  • Cambium

  • Endodermis

  • Pericycle

Above: Cross-section of a EuDicot root

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