Phloem - I

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Phloem - I
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Components of the Phloem

• Sieve Elements conducting cells which transport
  products of photosynthesis and other solutes
• Sieve tube members (in angiosperms) Fig B
• Sieve cells (in gymnosperms and lower vascular
  plants Fig A
• Companion cells specialized parenchyma
• Regular parenchyma
• Fibers
• Sclereids (on rare occasions)

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Phloem is generally external to the xylem in
monocot and dicot bundles
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Phloem in bundle of sunflower stem
Phloem
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Close up of phloem in corn
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Over stained phloem in Tilia
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Phloem in Ranunculus root
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Phloem in Smilax root
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Phloem in grass leaf
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Sieve Tube Member
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Sieve Tube Members
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P-protein bodies
Sieve Plate
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Electron micrographs of a sieve tube member and
sieve plate
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Sieve tube members in milkweed stem
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Sieve plate in cucumber
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Typical view of phloem
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P-protein plugs in cucumber
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P-Protein in sieve plate pores
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Sieve cell in pine
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Development of sieve tube member and related
companion cells
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Companion cells in milkweed
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STM and companion cells in cucumber
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Companion Cell
Sieve Tube Members
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Companion cells in leaf veins

• Ordinary companion cells
• Transfer cells
• Intermediary companion cells

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Ordinary Companion Cell
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Transfer Companion Cell
Sieve Tube Member
Companion Cell
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Intermediary Companion Cell
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Electron micrograph of a portion of common wall
between a companion cell (top) and a sieve
element (bottom).
Three companion cell plasmodesmata merging into
one pore in a sieve area
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Pressure Flow Hypothesis
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Aphids are used to study phloem translocation -
and used to prove the Pressure-Flow Hypothesis
The empty ovule technique is also used.
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Pathways of phloem loading
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Sucrose is actively loaded into the companion
cell through Sucrose-H symport that is dependent
on H-ATPase
Apoplast
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Polymer trapping helps explain how symplastic
phloem loading, which depends on diffusion,
allows for the accumulation of sugars against a
concentration gradient
Oligosaccharides are too large to diffuse back to
bundle sheath cells.
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