1. Phloem - I

2. 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)

3. Phloem is generally external to the xylem in monocot and dicot bundles

4. Phloem in bundle of sunflower stem

5. Close up of phloem in corn

6. Over stained phloem in Tilia

7. Phloem in Ranunculus root

8. Phloem in Smilax root

9. Phloem in grass leaf

10. Sieve Tube Member

11. Sieve Tube Members

12. P-protein bodies
Sieve Plate

13. Electron micrographs of a sieve tube member and sieve plate

14. Sieve tube members in milkweed stem

15. Sieve plate in cucumber

16. Typical view of phloem

17. P-protein plugs in cucumber

18. P-Protein in sieve plate pores

19. Sieve cell in pine

20. Development of sieve tube member and related companion cells

21. Companion cells in milkweed

22. STM and companion cells in cucumber

23. Companion Cell
Sieve Tube Members

24. Companion cells in leaf veins
Ordinary companion cells
Transfer cells
Intermediary companion cells

25. Ordinary Companion Cell

26. Transfer Companion Cell
Sieve Tube Member
Companion Cell

27. Intermediary Companion Cell

28. 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

29. Pressure Flow Hypothesis

32. Aphids are used to study phloem translocation - and used to prove the Pressure-Flow Hypothesis
The empty ovule technique is also used.

33. Pathways of phloem loading

34. Sucrose is actively loaded into the companion cell through Sucrose-H+ symport that is dependent on H+-ATPase
Apoplast

35. 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.