WB01343_.gif (599 bytes)   Secondary Phloem   WB01343_.gif (599 bytes)
Secondary Phloem has the same origin as secondary xylem, namely, the vascular cambium. Cells displacedwpe21.jpg (5545 bytes) towards the outside of the vascular cambium differentiate as phloem. Secondary phloem can remain active over several growth cycles. Secondary phloem, like secondary xylem, is a complex tissue. It always has sieve elements which are analogous to tracheary elements. However, parenchyma is also present, and sclerenchyma may also be visible.

There are two kinds of Sieve Elements, Sieve Cells and
Sieve Tube Members.
Sieve Tube Members are highly specialized for translocation. They are moderately elongate and have horizontal or oblique end walls with sieve plates. Sieve plates contain large sieve pores. The pores are lined with callose which regulates the diameter of the pores. Callose plugs the pores when the cells are ruptured. The pores can also be closed slowly in response to major changes in the environment, like the onset of winter in the Temperate Zone or a dry season in the Tropics. Pores closed in this manner may open when growing conditions become favorable.

Sieve Tubes are formed by the vertical union of several sieve tube members. This sounds familiar!

Sieve Cells are also specialized for translocation but they lack sieve plates. However, sieve pores are more numerous where sieve cells overlap.  Sieve Cells have a narrower diameter compared to Sieve Tube Members. Sieve Cells are also more highly elongated compared to sieve tube members.

Callose is also associated with the sieve pores of Sieve Cells.
The cell walls of most sieve elements are similar to those of parenchyma cells, and it is often difficult to distinguish between these in cross sections unless a sieve plate or sieve pores can be seen. Callose is preferentially stained by aniline blue. Consequently, sieve elements may be located by their reaction to this stain. However, other cells also react positively with this stain. Callose stained with aniline blue fluoresces under near ultra violet and violet light. This provides a more certain method to locate sieve elements. Callose is also present in their cytoplasm and its fluorescence aids in the location of sieve elements.

Sieve tube members have companion cells, and sieve cells have albuminous cells associated with them. These are also hard to distinguish from phloem parenchyma, especially in cross sections. Starch is usually absent from albuminous cells and this can provide a way to distinguish them and phloem parenchyma.

Rays are present in secondary phloem and are usually continuous with rays in the secondary xylem. Some species develop dilated rays in their secondary phloem. Tilia americana and Hibiscus tiliaceus (hau) are two prime examples for the presence of dilated rays. Dilated rays are principally composed by parenchyma cells which result from localized anticlinal divisions.

Secondary Phloem of Gymnosperms

You should recall that the secondary xylem of gymnosperms like Norfolk Island Pine is simple compared to angiosperms like hau (Hibiscus tiliaceus). The same is true for secondary phloem.
  1. Study cross sections of Podocarpus stems stained with phloroglucinol.
  2. Note the presence of sclerenchyma fibers which stain positively for lignin in the secondary phloem. These occur in tangential bands that lie parallel to the surface of the stem.
  3. What is their most likely function?
    PodoSecPhlomLab.jpg (104724 bytes)
  4. Follow cell files radially to locate the vascular cambium. Can you observe major differences in the appearance of the unlignified cells?
  5. Locate the rays. Are they continuous with those of the xylem? What is their width? Do they appear to be uniseriate or multiseriate? Can you be sure of this from a cross section?
  6. Examine a longitudinal section and try to locate sieve areas.
  7. Observe commercial slides (and/or fresh sections) of Pinus stems and compare its secondary phloem with that of Podocarpus. What major difference do you see between the two immediately after staining with phloroglucinol? sp-4-400.jpg (23268 bytes)

Important NOTE: Some of the commercial slides
have been stained with Lacmoid.  WB00678_.gif (615 bytes)

  1. Lignified cell walls will stain blue.

  2. Unlignified cell walls may not stain at all.

  3. Sieve pores, however, stain blue. sp-8-crop.jpg (23405 bytes)

  4. This accentuates them in secondary phloem.

We will label Lacmoid-stained slides for you!

The densely stained cells seen in long sections of commercial slides are phloem parenchyma. Note the presence of Amyloplasts in them. Phloem parenchyma
can also contain crystals.

  1. Examine a longitudinal section(s) and locate the vascular cambium, secondary xylem and secondary phloem. We have some stained with traditional stains and others stained with lacmoid. How can you tell which tissue is which????
  2. Locate the sieve areas!

Secondary Phloem in Gymnosperms (cont.)


Podocarpus stem cross section at the interface of Secondary Phloem & Xylem stained with Phloroglucinol


Pinus Stem cross-section showing the Epidermis, Cortex, Secondary Phloem & Vascular Cambium.


Pinus Secondary Phloem & Xylem


Secondary Phloem of Pinus


Longitudinal section showing the Secondary Xylem & Phloem of Pinus with typical staining.


Secondary Xylem and Phloem of Pinus stained with Lacmoid

Secondary Phloem in Angiosperms

We will use Tilia americana and hau as our principal examples of secondary phloem for angiosperms.

  1. Observe commercial slides of Tilia stems that show secondary phloem.
  2. Note the large dilated rays.
  3. Trace a dilated ray back to the vascular cambium and verify that it is continuous with xylem rays.
  4. Why do you think there are no dilated rays in the secondary xylem?
  5. What might be the function(s) of dilated rays?
  6. Are all of the rays dilated in the secondary phloem?
  7. Note the alternating layers of secondary phloem cells produced between the rays.

The vascular cambium alternatively produces bands of fibers, phloem parenchyma and sieve tube members plus companion cells.

  1. Try to distinguish the companion cells in cross sections.HauStmXSLMLab.jpg (177756 bytes)
  2. Use your polarizers to accentuate the phloem fibers.
  3. What is the function of secondary phloem fibers?

Various human civilizations learned to make cloth & rope from bark fibers like those found in Tilia and hau (Hibiscus tiliaceus).

Hau was not used for the production of kapa by Hawaiians. However they did use the "inner bark" (Secondary Phloem) of Wauke (Broussonetia papyrifera) as the source of fibers for kapa. Wauke is also known as Paper Mulberry .

  1. Observe cross sections of  hau and locate the fibers in the secondary phloem. Stain with phloroglucinol & note the color of the Phloem Fibers.
  2. Observe cross sections of wauke as well! Stain with phloroglucinol & note the color of the Phloem Fibers.
  3. Compare fresh sections of hau   with  wauke & Tilia americana!
  4. Locate evidence of Periderm formation in these slides. Periderm is called "outer bark".
  5. Where is the first phellogen formed?
  6. Where does it develop as the stem enlarges?
  7. Examine demo slides which show periderm development in the secondary phloem of older stems.
  8. Locate Periderm in demo slides of hau and wauke!


Secondary Phloem in Tilia americana


Overview of secondary Phloem in Tilia


Epidermis, Cortex and Secondary Phloem in Tilia. The Periderm has begun to develop! Locate this area!


Cross sections of Tilia stem showing the entire Secondary Phloem, Vascular Cambium & Secondary Xylem


Same as above but showing the vascular cambium and young secondary xylem.


Sieve Tube Member differentiating in Secondary Phloem


Find the Companion Cells!

Secondary Phloem of Tilia americana viewed
with crossed polarizers. What accounts for the birefringence or the lack of birefringence in this image??????

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