| Secondary Phloem is produced by the Vascular
Cambium and can form a continuous cylinder of tissue opposite secondary xylem in
woody plants. It is a complex tissue which contains Sieve Elements. Sieve Cells (SC) are characteristic for Gymnosperms, while Angiosperms produce Sieve Tube Members (STM). The latter form Sieve Tubes when joined end to end. Sieve Tube Members have Sieve Plates on their end walls. These contain large Sieve Pores. Sieve Pores are also present on the lateral walls of Sieve Tube Members. Sieve Cells do not have Sieve Plates but do have Sieve Pores. These may be more concentrated where they overlap with other Sieve Cells but they do not have Sieve Plates. Sieve Cells are generally longer than STM and are generally smaller in diameter. |
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|
 Sieve Plates in a Radial Section of Tilia. |
| Companion Cells are derived from the same
cell which forms a Sieve Tube Member. They control the
physiological processes involved in phloem transport. Albuminous Cells perform a similar function for Sieve Cells. However, they are not a direct descendent of the cell which produces the Sieve Cell. Albuminous Cells are present in the Rays of Pinus. Companion Cells and Albuminous Cells resemble Parenchyma cells. Furthermore, Parenchyma cells are also present in Secondary Phloem. Consequently, it is sometimes difficult to distinguish Companion Cells and Albuminous Cells from Phloem Parenchyma. Parenchyma Cells in Pinus often contain Tannins which appear dark on commercial slides. |
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| Finally,
Secondary Phloem may contain Fibers. These may occur as individual cells
scattered in the Phloem or they may occur in clusters or layers. Fibers help to strengthen the Phloem and can protect its thin-walled cells from the pressure which develops from secondary growth. We will start with gymnosperm phloem and proceed to angiosperm phloem. |
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