Woody Dicot Vine - Aristolochia (Dutchman's pipe).

Vines like Aristolochia commonly have wide interfascicular areas in the primary state of growth and correspondingly wide rays in the secondary body.

Examine slides of four stages of development
(youngest = A, oldest = D).

Also study slides prepared from living stems of Aristolochia. This genus is largely native to North and South America but is also found in the Mediterranean.

The youngest stems (slide A) are still undergoing primary growth. The epidermis is the surface layer. Just beneath this there are 10-15 layers of collenchyma and parenchyma cells that form the outer cortex. The innermost of these cell layers is slightly larger than the others and constitutes a starch sheath.

Inside this are 10-15 layers of cells with smaller diameters. These will develop into perivascular fibers. Perivascular means surrounds the vascular system. Inside this one sees immature vascular bundles.

The bundles (fascicles) are separated by parenchyma that comprises the interfascicular areas.

The center of the stem is pith composed of large parenchyma cells.

Young Stem stained with Toluidine Blue	Vascular Bundle stained with Toluidine Blue	Stem stained with Phloroglucinol & viewed with crossed polarizers

One-year-old stems (slide B) have completed primary growth. Tissues barely evident in the youngest sections have now matured.

The vascular cambium should be prominent between the primary xylem and phloem, and the interfascicular cambium is just forming.

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.

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 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 lenticels 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.

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?

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

The Phellogen can be located by following the two files of vacuolate 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.

Note the lenticels in the periderm. These are areas of hyperactive cell division compared to the rest of the Phellogen.

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.

• Study a series fresh sections of Coleus or another related plant stained with Phloroglucinol or  Toluidine Blue.

• Be sure to use your polarizers.

• Compare fresh sections with a commercial slide of Coleus stem.

What accounts for the Layers that you see in the Periderm?

These questions are really getting to be a major annoyance! Can't a guy or gal enjoy some microscopy without all of these nagging questions!

Optional

We can study the transition from primary to secondary growth by studying Cotton (Gossypium tomentosm) and/or Coffee (Coffea arabica).

Hawaiian cotton is indigenous to these islands but is probably part of a larger taxon that is widely distributed in the Pacific. It does not produce the finest fibers but it has important traits that have been used in cotton breeding programs. This supports the concept of population/species preservation in that valuable genes may exist in natural populations. If wild populations are destroyed, the beneficial genes are also destroyed.

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.

The following details may be observed:

In younger stages the epidermis is present.

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 cells are the phellogen or cork cambium.

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

The Vascular Tissues form a continuous ring.

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

The initial events in vascular development for gymnosperms is similar to that of Tilia.

However,  Secondary Xylem in gymnosperms is more homogenous than that in most Angiosperms.

The only Tracheary Elements present are Tracheids. However, in environments which have distinct seasons (Cold/Warm or Wet/Dry) large thin-walled tracheids are produced during optimal growth periods while narrower, thick-walled cells are produced under stressful conditions.

This produces "Growth Rings". Furthermore, there are more pits in the thin-walled tracheids compared to the thick-walled tracheids. Consequently, there is some degree of structure/function variation in the xylem. What does this last sentence mean???

Observe the following from commercial slides and fresh sections stained with Phloroglucinol.

The outermost phloem occurs just inside the resin canals. The sieve cells in the outer part of the phloem are crushed. The functional phloem occurs next to the vascular cambium.

The secondary xylem stains red due to the presence of lignin. It is fairly homogenous in appearance, but differences in diameter and wall thickness produce alternating growth rings.

The pith occupies the center of the stem.