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.