The Root: Structure and
Development
Chapter 24
Roots
• Root- the descending axis of a plant,
normally occurs below ground.
• Root Functions
– Anchorage.
– Absorption of water, minerals,
inorganic ions.
– Storage.
– Conduction.
• Two types of root systems
– Taproot- a stout, tapering main root
from which smaller lateral roots arise;
deep; gymnosperms & dicots.
– Fibrous- arise from stem similar to
lateral roots; shallow; monocots.
Root Penetration
• Root depth and distance
depend on soil:
– Moisture
– Temperature
– Composition
• Feeder Roots- those
involved in uptake of water
and minerals, occur usually
in the upper 1 meter of the
soil.
• Rootcap- a thimblelike mass
of parenchyma cells that
covers the root and secretes
mucigel.
• The epidermis in young
roots absorbs water and
minerals.
• Root Hairs- tubular
extensions of epidermis
that facilitate absorption.
Primary Development
of the Root Tip
• Region of cell division
– ~1 mm in length.
– Apical meristem.
• Region of cell elongation
– ~2 mm in length.
– Cells elongate, functional
xylem starts to develop and
phloem is mature.
• Region of maturation
– ~2 mm-?? mm in length.
– Root hairs are produced,
functional xylem and
phloem are present.
Eudicot versus Monocot Roots
• Eudicots
– Vascular cylinder solid.
– Cortical cells sometimes
retained for life, but some shed
due to secondary growth.
• Monocots
– Vascular cylinder in ring with
pith.
– Cortical cells retained for life,
because they lack secondary
growth.
Cortex and Vascular Cylinder
• Cortex- ground tissue (cells between the vascular cylinder
and the epidermis) occupies the greatest area of the root.
• Vascular cylinder- xylem, phloem, and pericycle.
• Pericycle- 1 or more layers of nonvascular cells
surrounding the vascular tissue of the root.
• Intercellular spaces- empty spaces among cortical cells;
essential for aeration of the root cells.
• Epidermis- the outermost layer of cells (the root).
• Exodermis- the outermost layer of the cortex, 1 or more
cell layers in depth; casparian strip in walls.
• Endodermis- the innermost layer of the cortex, lacks
intercellular spaces; casparian strip in walls.
Casparian Strip
• Casparian Strip- a bandlike region of the primary cell wall
containing suberin and lignin. Impermeable to water and
ions, prevents water loss and attack by microorganisms.
Lateral Roots Arise from the Pericycle
Effects of Secondary Growth on
the Primary Body of the Root
• Secondary Growth in roots consists of:
– 1) Secondary vascular tissues (secondary xylem
and phloem) from vascular cambium.
– 2) Periderm, mostly cork tissue from cork
cambium.
Root Modifications
• Aerial Roots- roots produced
from above ground structures.
• Air Roots- negatively
gravitropic roots that aerate
the root system.
• Fleshy Roots- specialized
storage organs consisting of
parenchyma and vascular
tissue. Carrot, sweet potato,
sugarbeet.
Fleshy Rootan adaptation
for food storage.
Sugarbeet
Sweet Potato