12 April 2015
Xylem structure and function
WILF
Describe the structure of xylem tissue to include:
Vessels; Tracheids; fibres and xylem parenchyma
Explain why xylem is dead tissue
Explain the role of xylem in support in the plant
Role of xylem
• Absorbs
– Water
– Mineral salts
– Oxygen
• Resistance to forces
Arrangement of xylem in the stem
Collenchyma
Vascular bundle
Phloem
Cambium
Parenchyma
Xylem
How xylem and phloem are formed in the growing plant
Structure of xylem tissue
Elongated dead cells that
form tubes.
They are made up of
• Tracheids
– Tapered cells with sloping
end walls
• Xylem vessels
• Shorter and fatter than
tracheids.
• Each are perforated by
pits to allow sideways
movement of water and
minerals
• Fibres and xylem
parenchyma
Provide support
• Xylem are lignified
• Impermeable to
water and other
solutes.
• Form continual
tubes.
Location of xylem tissue
• This differs in the stem, leaf and root.
• Stem:
– Vascular bundles near the edge of the stem
• Root:
– Vascular system in a central position
• Leaf:
– Vascular bundles in the midrib and other leaf
veins
Stem
• Ring around the edge gives resistance to
bending
In the root, resistance to the pulling
force of the wind
Resistance to tearing in the leaf
Notes from the Student Guide
The vascular tissues are made up of:
xylem tissue, which transports water and mineral salts from the roots to the
leaves.
• vessels - the main conducting cells
• tracheids - also conduct water but are not as well adapted as vessels for this
function
• fibres – they have no role in water transport but function in support
• xylem parenchyma - this acts as packing tissue.
Vessels and tracheids are dead cells and they form a system of tubes through which
water can travel.
The cells are dead because lignin has been deposited on the cellulose cell walls
rendering them impermeable to water and solutes. These cells also provide
mechanical strength and support to the plant.
The distribution of xylem tissue differs in primary stems, leaves and roots.
• In stems it occurs as part of the peripheral vascular bundles. This organisation
gives flexible support but also resistance to bending strain.
• In leaves the arrangement of vascular tissues in the midrib and network of veins
also gives flexible strength and resistance to tearing strains.
• In roots the central arrangement is ideal for resistance to vertical stresses (pull)
and so helps the anchorage of the plant.