Post on 04-Jan-2016
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Phloem - I
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)
Phloem is generally external to the xylem in monocot and dicot bundles
Phloem
Phloem in bundle of sunflower stem
Close up of phloem in corn
Over stained phloem in Tilia
Phloem in Ranunculus root
Phloem in Smilax root
Phloem in grass leaf
Sieve Tube
Member
Sieve Tube Members
Sieve Plate
P-protein bodies
Electron micrographs of a sieve tube member and sieve plate
Sieve tube members in milkweed stem
Sieve plate in cucumber
Typical view of phloem
P-protein plugs in cucumber
P-Protein in sieve plate pores
Sieve cell in pine
Development of sieve tube member and related companion cells
Companion cells in milkweed
STM and companion cells in cucumber
Sieve Tube Members
Companion Cell
Companion cells in leaf veins
• Ordinary companion cells
• Transfer cells
• Intermediary companion cells
Ordinary Companion Cell
Transfer Companion Cell
Companion Cell
Sieve Tube Member
Intermediary Companion Cell
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
Pressure Flow Hypothesis
Aphids are used to study phloem translocation - and used to prove the Pressure-Flow Hypothesis
The empty ovule technique is also used.
Pathways of phloem loading
Sucrose is actively loaded into the companion cell through Sucrose-H+ symport that is dependent on H+-ATPase
Apoplast
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.