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Phloem Sieve Plate in Cross-section
In this cross-sectional view of a sieve plate at 1000x the pores connecting two sieve tube members are visible. A section has to pass exactly above the sieve plate for it to be viewed in this way.
There is a second sieve tube with a fragment of a sieve plate in the upper middle third of this image. The section missed the sieve plate area of this sieve tube. Note the nucleated cells to the side of this sieve cell. These are likely companion cells.
Phloem is most easily located and identified in plant tissues by its close association with xylem in the vascular bundles of stems and leaves or vascular cylinders of roots. Watch for these in the microscopy modules!
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Vascular Tissues-The Phloem
Phloem is the vascular tissue plants use to transfer sugars from sites of production or storage to locations where energy is needed. In contrast to xylem, phloem tissues are living. Sieve tube members connect at specialized areas called seive plates. At maturity the seive tube members lose their nuclei and fill with a complex proteinaceous material called cell sap. Sugars are transported through the cell saps of adjacent cells.
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Phloem |
In losing their nuclei, sieve tube members lack the molecular control mechanisms most living cells possess. Nucleated cells adjacent to sieve tube members appear to take over the control of cellular functions within these phloem transport cells. These nucleated cells are appropriately called companion cells.
Sieve plates are specialized areas where materials flow through tiny pores from one sieve tube member to another. When phloem is damaged or torn, callose seals off these areas to prevent loss of cell sap.
Most phloem cells are parenchyma cell types. Sometimes phloem tissues contain sclerenchyma fibers for support. Now, find and roll over the lower, middle sieve plate.
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Sieve Plates
In longitudinal section at 400x magnification sieve plates can be seen in the phloem of some plants. In this Cucurbita stem two sieve plates are visible. You can see pores in the upper sieve plate because it is cut at the diagonal. The brown material near these sieve plates is believed to be coagulated P-protein or cell sap.
Just to the left of the upper sieve plate is an elongated nucleus of a parenchyma type cell that could be a companion cell. Remember, companion cells appear to control the cellular activities of the sieve tube members.
Now, to see a sieve plate in cross-section, roll over the companion cell nucleus.
