STEMS II 

• Locate these major structures in commercial slides and fresh sections.

• Stain with Phloroglucinol and Toluidine Blue.

• Don’t forget to use your polarizers.

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• Locate the major structures and regions in commercial slides

Vascular Bundle & interfascicular zone	Active Vascular Cambium	Induction of cambial activity in the interfascicular region

Slide C:

Two- three year old stems (slide C) exhibit considerable secondary growth.

Vascular Bundles in a 3 year-old stem

Pith in a 3 year-old stem

 

 

 

 

The vascular cambium and the cork cambium (phellogen) produce secondary growth.

These produce internal pressures due to the production and enlargement of the new cells they produce. This is especially true for secondary xylem.

During enlargement, the ring of perivascular (extraxylary) fibers often becomes broken. The gaps in the ring are filled with parenchyma cells, some of which develop into thick-walled sclereids. In this case, pressure has induced meristematic activity in some cells.

Results of secondary growth on the stem fibers	Effects of secondary growth on the phloem

The epidermis is still intact except in a few places where lentils are forming from the phellogen. Since the interfascicular cambium is composed only of ray initials, the vascular bundles remain distinct, and are separated by wide rays of parenchyma.

The xylem shows two growth layers.

Many pith cells are crushed, as a consequence both of movement of the vine and of inward pressure of developing xylem tissue.

The fact that many vines have isolated vascular bundles or wide areas of parenchyma in the xylem suggests that this might have functional significance. Considering the manner in which vines grow, can you imagine what function this tissue organization may have?

Slide D:

Older stems (slide D) clearly show periderm formation. In Aristolochia the periderm develops first in isolated vertical strips. Thus, as seen in cross sections, parts of the stem's circumference have an intact epidermis while other parts have a well developed periderm with several layers of cork cells (phellem), cork cambium (phellogen) and a relatively wide phelloderm.

The Phellogen (Cork Cambium) produces Phellem (Cork) to the outside and Phellogen towards the Inside.

The Phellogen can be located by following files of vacuolate Phellem & Phellogen cells towards the centrally located meristematic cells.

Remember that meristematic cells are densely cytoplasmic and lack prominent vacuoles.

The phelloderm is usually not prominent and may consist of a single layer of parenchyma. Aristolochia has an uncharacteristically large phelloderm.

Note the lenticels in the periderm. These are areas of hyperactive cell division compared to the rest of the phellogen. Cells in the lenticils have less suberin and are loosely aggregated. This allows for gas exchange. Other parts of the periderm are relatively impervious to gases

The xylem shows several growth layers. As the individual wedges of vascular tissues increase in size, new rays develop within them.The pith and some of the inner parts of the interfascicular areas are almost completely crushed.

Coleus Stems

The development of secondary growth in stems is readily seen by making a series of cross sections from a Coleus stem.

The large primary bundles are conveniently located in the corners of its square stem. By taking sections from more mature parts of the stem the pattern of development is readily discerned!

Examine the series of photos below, then observe fresh sections.

Developmental Sequence of Vascular Differentiation in Coleus

Coleus and other mints (Lamiaceae)  have large primary vascular bundles that occur in each corner of the stem. The bundles have Bast Fibers, Phloem and Xylem. A Vascular Cambium is present between the xylem and phloem. It divides periclinally to produce files of cells in two directions. Cells displaced towards the interior become xylem. Those displaced towards the periphery become phloem. Cambial activity spreads to the interfascicular parenchyma. This spreads laterally until a complete ring of cambium encompasses the stem and unites the bundles.

Growth from the vascular cambium increases the girth or diameter of the stem. The vascular cambium of Coleus usually produces more xylem than phloem. The thick-walled xylem cells accumulate like loaves of bread which enlarge as they mature and have a hard crust after they are baked. These files of sturdy cells create pressure towards the inside and outside of the plant. The thin-walled cells of the pith and phloem are crushed as the pressure increases.

The vascular cambium replenishes the phloem. However, the amount of active phloem may be limited to a small zone.

Fibers may be present in the secondary phloem. These protect the sieve elements and allow for the expansion of the active phloem.

The Phellogen (Cork Cambium) is another lateral meristem. It usually develops from subepidermal cells in stems. It divides periclinally to produce rows of cork (Phellem) cells towards the exterior. Cork cells are dead at maturity and have suberized walls.

Early stages in Periderm Development

Suberin is a lot like cutin. It protects the stem because It is impervious to water and most pathogens, and provides some insulation. It stains positively with Phloroglucinol.

• Look in the outer cortex of Coleus for these early stages of periderm formation. Periderm forms the outer bark. We will have more to say about this later.

• Study unstained sections of Eucalyptus bark and observe the layers of Phellem (Cork).

• Deploy your Polarizers!!!!!

• Stain with Phloroglucinol which stains Suberin as well as Lignin.

1. Dermal Layer (Epidermis – Periderm (Phellogen [Cork Cambium] – Phellem [Cork])

2. Cortex (Collenchyma – Sclerenchyma – Parenchyma)

3. Vascular Tissue (Phloem Fibers – Phloem – Vascular Cambium – Xylem

4. Interfascicular Region (Parenchyma – Vascular Cambium)

5. Pith (Parenchyma – Other Cell Types)

• Study the primary Vascular Bundles. There may already be some secondary growth but evidence of the primary bundles is usually present.

  • Primary xylem will jut into the pith due to the production of secondary xylem.

  • Secondary xylem is usually more linear in its organization and may have a distinctly different cell composition compared to primary xylem.

  • Phloem fibers can also mark the location of primary vascular bundles.

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Woody dicot: Tilia (basswood) stem.

Tilia sp.

Tilia flowers

Tilia fruits

Tilia americana is the model plant that is routinely
studied for the stem anatomy of woody angiosperms. Its characteristics encompass a good sample of anatomical features for this kind of plant.  

We will study secondary xylem and phloem in depth later in the course. Do not worry about the details. My goal is to simply introduce the anatomical basics for these secondary tissues.

• Study commercial slides of stems in several
  stages of development.

• Work from the outside towards the inside of the stem!

The following details may be observed:

1. In younger stages the epidermis is present.

2. In older stems it is replaced by a periderm containing several layers of tannin-filled cork cells. Tannins are brown and impart a dark color to the Cork. The smallest periderm cells that contain  protoplasts with nuclei are the innermost cells in each radial file of cork cells. These constitute the phellogen or cork cambium.

3. The Cortex is composed of collenchyma and parenchyma. The large cells with red-staining contents are mucilage cells.

4. The Vascular Tissues form a continuous ring.

5. The Phloem in older stems appears to be divided into
   two kinds of wedges with opposite orientations.

• The wedge that points towards the outside contains sieve-tube members, companion cells, phloem parenchyma and phloem fibers.

• The fibers occur in tangential bands (parallel to the surface) alternating with bands that contain the rest of the phloem components.

• The outermost fiber bundles are the primary phloem fibers, the others arose from the vascular cambium and are part of the secondary phloem.

• The other Wedge is principally composed of
  parenchyma. These are dilated phloem rays.
  Dilated means broadened or expanded.

• In addition to the dilated rays, there are narrow undilated rays.

• Note the continuity of the rays from the phloem to the xylem.

• The stems in slides labeled "young stem" are just starting to form secondary xylem. 

• In older sections from one to several increments of
  secondary xylem occur, and the primary xylem is next to the pith.

• The secondary xylem of Tilia contains vessel elements (widest cells), tracheids, fibers (narrow cells with relatively thick walls), and xylem parenchyma (small dark cells).

• The Pith contains thick-walled cells with dark globules of tannin.

• The large "spaces" in the pith, which may be filled with red-staining material, are mucilage ducts

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Woody Gymnosperms:
Araucaria, Podocarpus or Pinus stems.

Pinus strobus

Araucaria sp.

Podocarpus sp.

Secondary Xylem and Phloem of Podocarpus

• View cross sections stained with Phloroglucinol
  
• The vascular tissues are similar to what you saw with Pinus.
• However, the secondary phloem contains fibers. Why is this noteworthy?

Secondary Phloem and Xylem of Podocarpus stained with Phloroglucinol