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Mass Transport in Plants: the
Phloem
What is translocation?
Translocation is the movement
of nutrients around a plant. The
term includes the movement of
minerals, which can be dissolved
in water and transported in the
xylem, but usually refers to the
transport of sugars, amino acids,
and other organic molecules in
the phloem.
Translocation can occur in either
direction in the phloem – it is
bidirectional. It is an active
process, requiring energy, unlike
water transport in the xylem.
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Phloem as seen under a light
microscope
The mass flow hypothesis
The most widely accepted explanation of sap movement
in plants is the mass flow hypothesis (sometimes called
the pressure flow hypothesis).
According to the theory, sap moves through phloem vessels
due to differences in hydrostatic pressure.
Evidence for this effect
includes the excretion of sap,
or honeydew, by an aphid
when it taps a phloem vessel
to feed. The sap is forced
through the aphid’s body,
demonstrating that the sap in
the phloem is under pressure.
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Translocation of sugars
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Ringing Experiments
• Evidence for water movement in xylem
and sugar movement in phloem.
• Woody stems have an outer protective
layer of bark, with a ring of phloem inside
and a ring of xylem inside that.
• Outer layers of bark and phloem are
removed from a section of stem.
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Ringing Experiments
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• Stem above missing tissue ring
swells with liquid rich in sugars.
• Non-photosynthetic tissues below the
ring wither and die.
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Ringing experiments
Conclusions
• Sugars of phloem accumulate above ring
as they are unable to move down the
stem.
• Sugars don’t pass below the ring so
tissues die.
Phloem, rather than xylem, is responsible for
the translocation of sugars in the plant.
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Tracer experiments
• Radioactive carbon isotope 14C is used to make
14CO .
2
• The 14C is incorporated into the sugars produced
during photosynthesis.
• Autoradiography allows tracking of these sugars.
Radioactive areas correspond to areas of
phloem, not xylem.
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