Download The structure and function of Xylem and phloem tissue

The structure and function of
Xylem and phloem tissue
Objectives
To understand and be able to
describe the structure of xylem
and phloem as seen under a
microscope
The structure and function of
Xylem and phloem tissue
• Xylem tissue transports water, inorganic ions
and a few small organic molecules from the
roots upwards
• Phloem tissue transports organic substances
made by the plant, such as sucrose, from
where they are made to the rest of the plant,
such as leaves to storage organs
• Xylem and phloem usually are held close
together in a vascular bundle
Xylem
• Made from dead vessel
elements (cells) aligned end
to end to form a continuous
tube called a xylem vessel
• Thick walls impregnated
with lignin - waterproofs
the cells and and is strong
to provide structural
support.
• End walls and cell contents
decay as xylem develops.
• Lignin is laid down in various
patterns.
• Pits in lignin allow
communication between
adjacent cells.
Why is the
flow of
water not
impeded?
Xylem Images
Xylem Structure
Made from dead cells aligned end to end in a
continuous column
Function
Xylem tubes are narrow
Lignified walls
Pits in lignified walls
Lignin first laid down in spiral/reticulate format
Phloem Structure
Reduced cytoplasm and no nucleus
Companion cells have numerous
mitochondria
Plasmodesmata link the cytoplasm of the
sieve tube cell and the companion cell
Function
The
structure
and function
of xylem and
phloem
• Consists of sieve tube elements and
companion cells.
• Sieve tube elements have very little
cytoplasm and no nucleus but they are
living cells.
• They line up end to end, and are
connected by a sieve plate.
• Sieve tube elements have thin walls and
are usually 5 or 6 sided.
• Each sieve tube element lies closely to a
Companion cell.
• These are small cells with a large
nucleus, dense cytoplasm and numerous
mitochondria to produce ATP for the
active loading of sucrose into the sieve
tube elements.
• Cytoplasm of the companion cell is linked
to the sieve tube element by gaps in the
cell walls called plasmodesmata.
Phloem
Phloem Images