VASCULAR CAMBIUM & SECONDARY XYLEM  

Rays constitute the radial system in secondary xylem and phloem. As the name suggests rays run parallel to radii that pass through the center of the axis. They are largely composed of parenchyma cells which are usually elongated perpendicular to the long axis of the stem. Thus, we could call them radial (ray) parenchyma.

The term axial is applied to cells which are oriented parallel to the axis of elongation. Thus, we could study axial parenchyma. The axial vascular system is composed of xylem and phloem, plus parenchyma. Each of these contains axial parenchyma, specialized vascular cells and fibers.

The radial system is simple by comparison. Occasionally, tracheids, laticifers and secretory ducts may be present in both systems (axial & radial).

The axial and radial systems can be studied with two types of longitudinal sections.

• Radial sections are longitudinal sections cut parallel to the rays.

• Tangential sections are cut perpendicular to the rays.

• See if you can identify the planes of section for the cross-shaped image on the right.

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Plane of Section for hau (Hibiscus tiliaceus)

Take a cylinder of hau (Hibiscus tiliaceus) stem.

Peel off the Bark.

It separates at the Vascular Cambium which is a point of weakness.

Why is it weak?

The Bark consists of Secondary Phloem & Periderm.

Make an X on one of the Transverse Faces.

Cut a Longitudinal Section near the periphery of the stem that is perpendicular to one of the lines of the X.

This will produce a thin "sliver".

What kind of section is this?

Cover the cut surface of the "sliver" with Toluidine Blue.

Wait a minute and Blot off the excess stain.

Examine under a Dissecting Scope and Locate the Rays.

Are they very tall? Are they uniseriate or multiseriate?

Cover the other cut surface with Phloroglucinol.

Let this sit while you examine the section stained with Tol. Blue.

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Cut a Longitudinal Section Parallel to one of the lines of the X.

What kind of section is this?

Cover one cut surface with Toluidine Blue and the other with Phloroglucinol & treat both as stated above.

Examine under a Dissecting scope and Locate the Rays.

What kind of information can you get from this kind of section?

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Aristolochia Slide D:

In order to refresh your memory, examine transverse
sections of Aristolochia ("D") slides. Transverse sections reveal the relationship of the cambial cells to their immediate derivatives which become secondary xylem or phloem. The young xylem and phloem cells form radial files because of their origin from the tangentially dividing cambial" initials".


In older xylem the radial seriation is disturbed because of    
non-uniform cell enlargement. There is, for example, a striking difference in width between the vessels and the cambial cells.
The vessels greatly disturb the original radial file of cells produced close to the cambial zone.

Secondary phloem shows a more orderly arrangement because its cells do not enlarge greatly. The cambial cells   
are relatively undifferentiated. The phloem and xylem
derivatives gradually acquire their characteristic traits. It is instructive to follow the radial files of cells from the cambium into the mature tissues and to recognize the differentiation processes that involve changes in size, wall structure, and cells contents.

• Study tangential sections of the vascular cambium from
  Robinia. Both the axial and the radial systems originate in
  the cambium. The tangential section clearly reveals the   
  difference between the initials of the two systems.

• Examine tangential sections of the vascular cambium of Juglans (Walnut).

  Is this storied or non-storied?

  What else can you determine by observing the vascular cambium?

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Ring-porous wood has wide vessels (pores) concentrated in the early wood and smaller vessels in the late wood.

Diffuse-porous wood has vessels of relatively uniform size distributed evenly throughout growth intervals.

• Examine thin wood macrosections from several plants and note the following:
  
  a] growth rings,
  
  b] size and orientation of rays
  
  c] diffuse or ring porous

• Compare several kinds of wood, using microthin sections.

Certain woods have conspicuously wide pores which are actually the vessel members.

Other woods do not have vessels but have tracheids.

If vessels are present, they may be filled with tyloses. What are tyloses?

In some woods rays are only one cell wide (uniseriate).

In others the rays are several cells wide (multiseriate) and may be visible to the naked eye.

Tangential Section of storied wood which has Uniseriate & Multiseriate Rays

Growth increments or rings vary in width. The variation is due to the interaction of genes and the environment.

The features listed above are, in combination, responsible for the pattern of wood "grain".

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The concept of Axial & Radial Systems in Wood

A comparison of the transverse and longitudinal sections
of woods reveals that certain cells are oriented with their long axes parallel with the long axis of the stem. These cells constitute the axial system. Other cells are oriented at right angles to the axial system. These cells constitute the radial system composed of rays.

 

• Observe commercial slides of pine wood which clearly illustrate the differences between the two.

• Note especially the difference in appearance of the rays
  in the three kinds of sections. Rays are the most useful
  structures for distinguishing between the radial and tangential
  sections.

Radial Section

Transverse Section

Tangential Section

This is a good time to consult the world famous
Fold A Wood Consortium for Cosmic Consciousness!

Click on the button to bring up a full-sized image of Pinus wood.
Print it & trim away the white areas.
Fold & secure with tape.
Identify the three planes of section.

Bordered Pits

Pine wood clearly reveals the complicated structure of conifer bordered pits. A Torus is present for conifer bordered pits but is absent in angiosperms.

A clear concept of the parts of the pits and their interrelation should be obtained by the study of pit-pairs in face views (radial sections) and in sectional views (transverse and tangential sections).

The Fence Analogy for Rays

I think of the rays as fences.

The radial section allows you to see the fences from the side. Thus you can see the arrangement of the different fence posts as well as the height of the fence. However, you can’t see its thickness.

The tangential view is like looking at the fence from one of its ends. You can see its height and thickness but you can’t see the sides of the fence or is length.

If you take a few steps to the right you can see the end of the fence and its side at an oblique angle.

Radial View of a Fence	Tangential view of a Fence	Oblique view of a Fence

What would a Transverse view of a uniseriate or a multiseriate fence/ray resemble?

Uniseriate

Multiseriate

Gymnosperm Wood

• Examine slides of transverse, radial, and tangential sections, and  macerated wood.
  
  The following questions may be used as a guide to your study.

• Examine macerated pine wood. This will help you answer the questions above.

• Examine slides of transverse, radial, and tangential sections.

• Examine prepared slides or freshly macerated material.

  Identify the cell types present. Try especially to differentiate between vessel members, tracheids, and libriform fibers.

  Note also the "tails" on some vessel members. This feature indicates the occurrence of intrusive growth. What is that?

A Large Vessel Member with Tracheids and Fibers	Member; Note the simple Perforation Plate	A Vessel Member (Left) with a Fiber-Trachied (Middle) and axial Parenchyma (Right)
A Trachied: Note the shape and presence of many Pits.	A Trachied: Note the many Pits	Libriform Fibers: Note the shape and diameter.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

"Fold A Wood"

Cut out the cross shaped wood images below and fold them to make a cube. Identify the different Planes of Section!

SECONDARY XYLEM

In the previous labs we have looked at the origin and development of primary and secondary vascular tissues. We will examine details of secondary xylem for some "typical" angiosperms and gymnosperms.

Tilia americana (basswood).

• Examine prepared slides of transverse, radial, and tangential sections. Pay particular attention to the obvious differences between oak and basswood.

Name The Plane of Section!!! Oh PLEASE!!!!!!

Study prepared slides of transverse, radial, and tangential sections and  macerated wood.

Transverse Section

Radial Section