The purpose of this lab is to continue our study of individual cell types, in this case Tracheary and Sieve Elements. In later labs we will study more closely the ways in which these cell types relate to other cells in the same tissues.

Cells in the primary xylem develop while the organ is still elongating. Thus, they must be sturdy enough to form an uninterrupted conduit for water, and extensible enough to avoid being ruptured. Indeed the initial strands of tracheary elements are ripped to pieces during elongation.

Primary xylem contains tracheary elements, which show a centrifugal sequence of secondary wall patterns. These are annular (hoop-like), helical (spiral), scalariform (ladder-like) and reticulate (net-like). The patterns progress from the center -> outside (Centrifugal), and from the simple to the complex. Furthermore, the relative area of secondary wall also increases progressively. As the area of secondary wall increases it becomes impossible to classify tracheary elements as one of the preceding types.

The relatively small areas of thin primary wall that remain are called pits. These can be broadly elliptical (scalariform), or circular. The circular pits can be simple or bordered. Bordered pits predominate on most tracheary elements. The pattern and types of pits are important for translocation and are also useful for wood identification.


Treachery Elements in the Primary Xylem

A variety of young stems (stems without secondary growth) will be used to show many of the variations in xylem development. Helical thickenings are the most common and may show various degrees of pitch and coiling.

The extendibility of the helical thickened tracheary elements is readily demonstrated by making a circular incision in a petiole or stem of Ricinus (Castor Bean) and then breaking the material in two. The helical thickenings become exposed and can be extended by pulling.

This is also well demonstrated by Rose Flowered Jatropha.

Suitable material for the study of primary xylem are Coleus, Castor Bean (Ricinus), Widelia, and geranium stems, or petioles of celery (Apium) or kukui leaves.

Protoxylem with Annular Rings & Helical Thickenings (Below)

It is imperative that you learn the typical staining reactions of
Toluidine Blue & Phloroglucinol.

The Xylem and Sclerenchyma should stain Blue-Green while the Phloem stains Pink with Toluidine Blue. The Phloem may be unstained or may become pink.

Xylem and Sclerenchyma stain red-orange with Phloroglucinol while the Phloem is always unstained.

Vascular Tissues of Coleus Stained with Toluidine Blue: The Fibers and Xylem stained blue due to the presence of Lignin. The Phloem was unstained!

Vascular Tissues of Coleus Stained with Phloroglucinol: The Fibers and Xylem stained Red due to the presence of Lignin.
The Phloem is always unstained!

Coleus Stem Cross Section stained with Phloroglucinol: Note the Vascular Bundles in the corners!

Coleus Stem Cross Section stained with Phloroglucinol & Viewed with Crossed Polarizers


Tracheary Elements in Secondary Xylem

The basic difference between tracheids and vessel members is the presence of a perforation plate on the end walls of vessel members and its absence on tracheids. The perforation plate has openings that are larger than the pits that are present in tracheids. A linear series of vessel members is called a vessel.

The secondary xylem (wood) is highly complex and will be studied in greater detail in later labs. For now it is sufficient to be introduced to the basic difference between Tracheids and Vessel Members.

Compare the xylem in Podocarpus and Coffea. Can you see any differences in the size of cell diameters within each? In other words, which is more homogenous in cross section?

Xylem & Phloem of Podocarpus stained with Phloroglucinol

Xylem & Phloem of Coffea stained with phloroglucinol & viewed with crossed polarizers.

Radial section of Pinewood viewed with crossed polarizers.

Bordered Pits in Pine Tracheids

Macerated Pine Wood

Tilia Xylem Cross Section

Vessel Member from Oak (Quercus)

Highly magnified picture of a Vessel, viewed with Polarized Light:

The absence of transverse end-walls makes this a Vessel Member with a
Simple Perforation Plate


The detailed structure of sieve elements in the phloem cannot be observed easily without the use of special staining techniques. Consequently, some of the material used in this exercise will be fresh. Sections of living material are usually more difficult to interpret than commercial slides. Therefore certain prepared slides will be used for orientation, and to demonstrate the arrangement of cells in the phloem, as well as the associations of phloem & xylem.

Primary Phloem of squash (Cucurbita)

Cucumber Vascular Bundle: It has Phloem on two sides of the Xylem

Cucumber Vascular Bundle showing the Phloem. The dark cells are Companion Cells & the largest cells are Sieve Tube Members.

High-power study shows the three components of the phloem tissue: Sieve Elements (here Sieve Tube Members), Companion Cells (small cells accompanying the sieve elements), and Phloem Parenchyma cells (intermediate in size between sieve elements and companion cells). The end walls of the sieve elements seen from the surface in cross sections, bear highly differentiated Sieve Areas. These end walls are called Sieve Plates. The protoplasts of adjacent sieve tube members form a continuum through the sieve plates.

These connections are the Connecting Strands. Each is encased in Callose, a carbohydrate wall substance chemically distinct from the cellulose that lines the sieve pores.

Staining shows that sieve elements appear end to end in longitudinal series and thus, form Sieve Tubes.

The lateral walls of sieve tubes bear relatively undifferentiated Sieve Areas. The pores in the sieve areas are much larger than typical pits and resemble those in the sieve plate.

Top view of a Sieve Plate from a commercial slide at high magnification

Commercial slide of Phloem seen in Longitudinal section: Note the Red-Stained material which contains Callose. also note the Sieve Plates

In order to demonstrate Callose in fresh material we will use
free hand longitudinal sections of Cucurbita stained with Aniline Blue.

Aniline Blue preferentially
stains callose.

Furthermore, stained callose emits fluorescence under ultra-violet and violet light.

Staining Procedure

It is sufficient to stain the sections with Aniline Blue, wait 5 min.,
with water and observe.

I am including a more detailed protocol for your reference.

Detailed protocol for Aniline Blue Staining

Place sections in IKI for 3 minutes,

Rinse with water

Stain 5 minutes with 0.1% aqueous aniline blue.

Wash briefly with IKI

Mount in water.

The callose will stain blue. However, Aniline Blue will also stain other materials in the section so you need to locate the xylem which is auto-fluorescent, then the phloem.

Look for concentrations of the stain in the phloem region, and locate the presence of sieve plates in the highly stained areas.

Callose accumulates at the Sieve Plates due to the pressure that exists in the Phloem.

Overall view of a longitudinal section of cucumber stem stained with Aniline Blue and seen with Violet Fluorescence. The cell walls of the Xylem are auto fluorescent while the fluorescence of the Phloem is due to Callose which has stained with Aniline Blue

The sieve plates will be the most fluorescent areas because callose accumulates there normally and becomes more concentrated after wounding.

The sieve plates vary in their orientation. Some are perpendicular to the long axis of the stem while others may have 45O angles of inclination. The latter can be seen in face view in longitudinal sections.

This allows you to see the sieve pores.