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Figure 5-1. Woody Dicot Twig, External Anatomy. a. General View. b. Detail of Leaf Scar
and Axillary Bud.
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Laboratory 1
Woody Stems and Wood Anatomy
Introduction
In the last laboratory you studied external morphology, internal anatomy of herbaceous stems and
initiation of the vascular cambium. When the vascular cambium in dicots and gymnosperms stems starts
functioning, the stems develop the woody structure you will study more intensively today.
A. Winter Twigs
Woody plants in the winter condition can usually be identified to species by external morphological
characteristics of their twigs. Observe the twigs on demonstration, determine the leaf arrangement
(opposite, alternate, or whorled) and identify the following structures: terminal bud at the tip of each
main stem and branch, axillary or lateral buds on the sides of the branch, nodes (point of attachment of
leaves or scales to the stem), internodes (stem segment between two successive nodes), lenticels
(breaks in the bark that function in gas exchange), leaf scars (left where a former leaf stalk detached),
bundle scars (remains of vascular bundles in leaf scars), and terminal bud scale scars (remains of
previous year's terminal bud).
In most woody plants from seasonal climates, a terminal bud forms towards the end of every growing
season. Previous rings of terminal bud scale scars can therefore be used to determine the age of woody
seedlings or determine past twig growth rates. Use lab materials and your instructor’s help to identify the
structures listed. Label Figure 5-1.
B. Anatomy of a Young Woody Stem
Even in dicot stems that we normally consider herbaceous such as Medicago (Alfalfa) stem studied in the
previous lab the vascular cambium may become active enough to produce a complete ring of secondary
xylem and secondary phloem. Remember that a secondary tissue is one that is formed by a cambium
and adds to the diameter of a plant structure. Compare Figure 5-2 to the younger stages where only
primary tissues are present as illustrated in Figure 4-4 from lab 4. In this later stage, note that it becomes
more difficult to distinguish the original vascular bundles. Complete Q1 through Q4 on the answer sheet.
Note the relative amounts of secondary xylem and secondary phloem. The proportional production of
each will remain fairly constant within a species even if it lives for centuries, but the secondary xylem will
accumulate as wood, and much of the secondary phloem will be short-lived. Is this correct as modified or
is the point different? The original sentence reads "These ratios will remain fairly constant within a
species even if it lives for centuries" which raises the specter of a 10 foot diameter tree with an 8" phloem.
All of the tissues outside of the vascular cambium comprise the bark.
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Find and label these tissues in figure 5-2:

bark

vascular cambium

epidermis

secondary xylem (wood)

cortex

primary xylem

primary phloem

pith

secondary phloem
Complete Q5 on the answer sheet.
Figure 5-2. Mature Medicago Stem.
C. 2-5 Year Old Woody Dicot Stem
The basswood (Tilia) stem cross section in your slide box represents a stem segment that is 2-5 years
old. On this stem, the vascular cambium has produced several annual rings of secondary xylem (wood)
and quite a bit of secondary phloem (but the phloem is not in easily distinguished annual rings). Complete
Q6 and Q7 on the answer sheet.
Study your microscope slide and label figure 5-3 as completely as possible, emphasizing the following
parts.
1. Wood
Within the secondary xylem you should be able to distinguish

annual rings (complete Q8 on the answer sheet).

spring wood, the first wood in each ring,

summer wood, the last wood in each ring,

uniseriate xylem rays, one cell wide strands of cells running at right angles to the cut,

multiseriate xylem rays, rays that are more than one cell wide,

xylem vessels, large diameter thick walled cells.
The primary xylem is difficult to distinguish , but it has no xylem rays and occurs just outside of the pith.
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2. Bark
Bark is defined as everything outside of the vascular cambium. Going from vascular cambium to stem
surface, you should be able to identify:

secondary phloem

phloem rays (uniseriate, biseriate and dilated) - complete Q9 on the answer sheet.

cortex

periderm (see below)

epidermis (sometimes). Complete Q10 on the answer sheet.
Figure 5-3. Tilia stem cross section, cut early in the 5th year of growth.
3. Periderm
In the Tilia stem, the periderm is poorly developed and difficult to distinguish. However, you should still be
able to see that it is composed of radial rows of cells at the outer edge of the stem section. On the
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Sambucus (elderberry) and Pelargonium (geranium) stem cross-section slides on display you should be
able to see the periderm much better (Figures 5-4 and 5-5).
Figure 5-4. Periderm of Sambucus.
Figure 5-5. Periderm of Pelargonium.
The periderm is composed of three tissues, phellogen, phellem and phelloderm. The phellogen (cork
cambium) is composed of thin-walled parenchyma cells that divide and produce phellem (cork) cells to
the outside. The phellem cells are dead at maturity and contain suberin in their cell walls. This makes the
periderm impermeable to water Q11. The phellogen produces a limited amount of phelloderm to the
inside. These cells remain living until the phellogen dies, then all of the remaining cells of the periderm
die.
In larger, older stems new periderms arise toward the
inside of the first periderm, first arising from the inner
cortex and later from the living cells of the secondary
phloem. Subsequent periderms are lense-shaped
areas instead of a complete cylinder like the first one.
Figure 5-6 shows a cross section of walnut tree bark,
which is on demonstration. The dark bands in the bark
represent different periderms.
Figure 5-6. Walnut bark Cross Section.
4. Lenticels
Occasionally, local regions of phellogen within a
periderm become quite active (see the Sambucus
lenticel slide on demonstration and Figure 5-7). As a
result of this increased activity lenticels form where the
outer bark of a woody stem ruptures. These lenticels
Figure 5-7. Lenticels of Sambucus.
allow gas exchange between the stem (or root)
and surrounding air allowing CO2 into the stem
(where it may be used in photosynthesis) and
allowing O2 and water out. Complete Q12 on the
answer sheet.
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D. Woody gymnosperm stem cross section
Observe the cross section of a pine (Pinus) stem from your slide set (figure 5-8). Despite gymnosperms
not being flowering plants, most structures are the same as those of a woody dicot. Differences include
(1) xylem has neither fibers nor vessels, only tracheids (that is why they are in distinct radial rows)
(2) xylem rays in gymnosperms are nearly always only one cell wide (uniseriate)
(3) sieve cells in phloem have a nucleus and lack companion cells,
(4) phloem has no fibers
(5) elongate tubes (resin ducts) run lengthwise through the wood, cortex, and phloem. The resin is
usually under pressure and when released (usually by a wound), oozes out onto the stem
retarding growth of bacteria and fungi and repelling insects with its strong odor.
Figure 5-8. Gymnosperm Stem Cross Section, Cut in 3rd Year of Growth.
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1. Wood Sections
Observe prepared slides of pine (Pinus), oak (Quercus) and basswood (Tilia) woods. These represent the
three major wood types: non-porous, ring-porous and diffuse-porous, respectively. Gymnosperms
(softwoods) are usually non-porous (lack vessels) whereas dicots (hardwoods) are either ring porous or
diffuse porous (have vessels in one pattern or the other). Note that the actual hardness of the wood has
no necessary bearing on whether a species is called “hardwood” or “softwood”.
To develop your understanding of the three dimensional structure of wood, try to identify all three planes of
sectioning: transverse (cross), tangential, and radial. Keys to distinguishing among them include the
pattern of the annual rings and rays, and the appearance of the vessel elements, tracheids, and fibers.
The ribbon-like rays appear as strips running across parts of a radial section, but are cut at right angles by
a tangential section and appear as narrow stacks of cells.
In Figures 5-9, 5-10, and 5-11 label the wood type, plane of section and the following structures:

rays

vessels

tracheids

resin ducts.
In oak wood, note the contrast between spring wood vessels and summer wood vessels, and the
presence of giant rays as well as normal rays. Complete Q13 on the answer sheet.
a.
Figure 5-9. Pine Wood Sections
b.
c.
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a.
b.
c.
Figure 5-10. Basswood Wood Sections.
a.
b.
c.
d.
Figure 5-11. Oak Wood Sections.
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E. Wood Characteristics (Demonstrations)
1. Macroscopic Patterns
Identify all three planes of cuts (cross, tangential and radial) on woody stems and wood blocks. On some
wood you can see the xylem vessels ("pores") with the unaided eye. Identify each wood as ring porous,
nonporous or diffuse porous.
2. Tree Trunk
Study the cross cut tree trunk in some detail. Several locations in the wood are dated. Look for a color
contrast that distinguishes heartwood from sapwood. The dated section is from a conifer and little color
difference is apparent. Look for other sections in which the distinction is clear. Complete Q14 through
Q18 on the answer sheet.
3. Knots
Examine the knots that are on display. Differentiate between tight knots and loose knots. Complete
Q19 and Q20 on the answer sheet.
4. Angles of Branches
The angle of branch divergence from the dominant stem can vary considerably. Examine two such
branches cut down the middle to show annual ring connections (or lack of connections) between stems.
Notice that in branches that diverge at a narrow angle, the bark often is included in the space between the
stems. Branches that diverge at a greater angle (approaching 900) include no bark. What effect does that
have on branch strength or on whether or not trees will split? Complete Q21.
F. Experiment: Effects of Tyloses
1. Red Oak
(a) Take a piece of red oak (different groups may have different lengths) on which one end has been
shaved to allow it to fit inside a piece of rubber tubing. Connect the shaved end to the air nozzle
with the tubing. Be sure someone has a firm grip on the connection between the wood and the
tubing.
(b) Place the free end of red oak into a beaker containing 2 inches of water.
(c) Turn the air valve on slowly.
(d) Does air pass from one end of the wood to the other so that air bubbles into the water? Complete
Q22 on the answer sheet.
2. White Oak
Repeat steps (a) to (d) with a similarly prepared piece of white oak. Does air pass from one end of
the wood to the other so that air bubbles into the water? Complete Q23 on the answer sheet.
Compare wood of white and red oak with the demonstration microscopes and in the SEM (scanning
electron microscope) diagrams and see if you can see physical differences that explain your results.
Complete Q24 and Q25 on the answer sheet.
G. Experimental Determination of the Length of Xylem Vessels
(a) Use tubing to connect the vacuum nozzle with one end of a grapevine stem segment, making
sure that the connection to the vine is tight. Someone should have a firm hold on that connection.
(b) Immerse the open end of the vine segment into the paint.
(c) Slowly turn on the vacuum.
WARNING: Do not allow paint to enter vacuum nozzle. Sit and observe and do not leave
unattended.
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(d) Turn off vacuum as soon as paint enters the tubing or after 3 minutes if the paint doesn't reach
the tube.
(e) Remove the grapevine from the paint. Wash off any extra paint to keep from messing up the lab.
(f) If paint came out of the end of the grape vine into the tube, at least some of the water-conducting
vessels are as long as the stem segment. If paint did not enter the tube, cut the stem into 5 inch
segments beginning with the end that was in the paint. Number the segments as you cut them so
you can keep track of where each segment came from. Shave the cut ends with a sharp razor
blade and view under a dissecting microscope. Look for paint in the xylem vessels. How long is
the longest vessel with paint in it?
(g) Look at prepared grapevine under the dissecting microscope. Complete Q26 on the answer
sheet.
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giant ray
KEY WORDS
opposite
alternate
whorled
terminal bud
axillary bud
lateral bud
node
internode
lenticel
leaf scar
bundle scar
terminal bundle scale scar
annual ring
spring wood
summer wood
xylem ray
uniseriate ray
multiseriate ray
bark
phloem ray
dilated ray
phellogen
phellem
cork
phelloderm
lenticel
resin duct
non-porous
ring-porous
diffuse-porous
softwood
hardwood
transverse (cross) section
tangential section
radial section
heartwood
sapwood
tight knot
loose knot
tyloses (pl. tyloses)
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Answer Sheet, Laboratory 5
Q1.
List the two primary tissues that appear in Figure 5-2.
Q2.
List the secondary tissues.
Q3.
Which has more cells, secondary xylem (wood) or secondary phloem?
Q4.
Is bark exactly the same as secondary phloem? If not, how do the terms differ?
Q5.
not?
Q6.
Note the figure legend does not ask for “medullary ray” that we saw in the younger stem. Why
Can you tell how old the tree is from the slide?
Q7.
Actually, Q6 is a trick question. Where would you have to make a cross section of the tree to tell
how old it is?
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Q8.
Which annual ring is the oldest? The youngest?
Q9.
What function do the dilated phloem rays serve? Why would a woody stem that does not have
dilated phloem rays be more susceptible to pathogens?
Q10.
What normally happens to the epidermis by this time?
Q11.
How do you suppose it is significant that the first periderm forms about the time the epidermis
ruptures and begins to slough off?
Q12.
Is there any advantage to allowing water vapor to escape from the stem? Any disadvantage?
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Q13.
From the appearance of the thinner rays, did the big vessel elements expand before or after the
development of the rays?
Q14.
Q15.
alive?
Q16.
How does one determine the age of such a tree at the time it was cut?
What clues do the annual rings give about weather patterns during the time when the tree was
Locate the vascular cambium. Where are the epidermis and cortex of this stem?
Q17.
Identify heartwood and sapwood. What do you think is involved in the conversion of heartwood
to sapwood?
Q18.
Which of the rings still function in water conduction?
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Q19.
What does a knot represent?
Q20.
What is the explanation for the difference between tight knots and loose knots?
Q21.
If you are pruning branches on a tree in your yard, how does this information help you decide
which branches to cut and which to save?
Q22.
What anatomical features can you see that would account for the differences in airflow in red
and white oak wood?
Q23.
Based on you observations, why is white oak, rather than red oak, used for storing and aging
whiskey?
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Q24.
Based on you observations, why are red oak populations more susceptible to some bacterial
and fungal pathogens than white oak?
Q25.
If tyloses block xylem vessels, how can water get to the top of a white oak tree?
Q26.
How long is the longest grape xylem vessel you can find? How do your results compare to those
of the other tables?
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