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Transcript
21.2 The Vascular System
KEY CONCEPT
The vascular system allows for the transport of
water, minerals, and sugars.
21.2 The Vascular System
Water and dissolved minerals move through xylem.
• Xylem contains specialized cells.
– vessel elements are short and wide
– tracheid cells are long and narrow
– xylem cells die at maturity
tracheid
vessel
element
21.2 The Vascular System
• The cohesion-tension theory explains water movement.
– Plants passively transport water through the xylem.
– Cohesion is the tendency of water molecules to bond
with each other.
– Adhesion is the
tendency of water
molecules to bond
with other
substances.
21.2 The Vascular System
• Water travels from roots to the top of trees.
– absorption occurs at roots
– cohesion and adhesion in xylem
– transpiration at leaves
21.2 The Vascular System
• Transpiration is the loss of water
vapor through leaves.
– water vapor exits leaf stomata
– helps pull water to the top
branches
21.2 The Vascular System
Phloem carries sugars from photosynthesis throughout
the plant.
• Phloem contains specialized cells.
– sieve tube elements have
holes at ends
– companion cells help sieve
tube elements
– unlike xylem, phloem tissue is
alive
21.2 The Vascular System
• The Pressure-flow model explains sugar movement.
– plants actively transport sugar from the source
– sugar flows to the sink due to pressure differences
phloem
xylem
sugars
1 Sugars move from their
source, such as
photosynthesizing
leaves, into the phloem.
3 The sugars move into
the sink, such as root
or fruit, where the are
stored.
water
2 Water moves from
the xylem into the
phloem by osmosis,
due to the higher
concentration of the
sugars in the phloem.
The water flow helps
move sugars through
the phloem.