phloem - i. components of the phloem sieve elements: conducting cells which transport products of...
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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
Phloem in bundle of sunflower stem
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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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Sieve Plate
P-protein bodies
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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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Sieve Tube Members
Companion Cell
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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
Companion Cell
Sieve Tube Member
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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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