Laboratory 1 Woody Stems and Wood Anatomy




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Figure 5-1. Woody Dicot Twig, External Anatomy. a. General View. b. Detail of Leaf Scar and Axillary Bud.



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.

Find and label these tissues in figure 5-2:


  • bark

  • epidermis

  • cortex

  • primary phloem

  • secondary phloem

  • vascular cambium

  • secondary xylem (wood)

  • primary xylem

  • pith

C
Figure 5-2. Mature Medicago Stem.

omplete Q5 on the answer sheet.

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.

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.

3.P


Figure 5-3. Tilia stem cross section, cut early in the 5th year of growth.

eriderm


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 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 5-5. Periderm of Pelargonium.



Figure 5-4. Periderm of Sambucus.

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.

I
Figure 5-6. Walnut bark Cross Section.

n 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.

4.Lenticels


O
Figure 5-7. Lenticels of Sambucus.
ccasionally, 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 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.


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. e
    longate 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.

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.







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


  1. 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.

  2. Place the free end of red oak into a beaker containing 2 inches of water.

  3. Turn the air valve on slowly.

  4. 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


  1. 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.

  2. Immerse the open end of the vine segment into the paint.

  3. Slowly turn on the vacuum.

WARNING: Do not allow paint to enter vacuum nozzle. Sit and observe and do not leave unattended.

  1. Turn off vacuum as soon as paint enters the tubing or after 3 minutes if the paint doesn't reach the tube.

  2. Remove the grapevine from the paint. Wash off any extra paint to keep from messing up the lab.

  3. 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?

  4. Look at prepared grapevine under the dissecting microscope. Complete Q26 on the answer sheet.

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

giant ray

heartwood

sapwood


tight knot

loose knot



tyloses (pl. tyloses)

Answer Sheet, Laboratory 5


  1. List the two primary tissues that appear in Figure 5-2.

  2. List the secondary tissues.

  3. Which has more cells, secondary xylem (wood) or secondary phloem?

  4. Is bark exactly the same as secondary phloem? If not, how do the terms differ?

  5. Note the figure legend does not ask for “medullary ray” that we saw in the younger stem. Why not?

  6. Can you tell how old the tree is from the slide?

  7. Actually, Q6 is a trick question. Where would you have to make a cross section of the tree to tell how old it is?

  8. Which annual ring is the oldest? The youngest?

  9. 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?

  10. What normally happens to the epidermis by this time?

  11. How do you suppose it is significant that the first periderm forms about the time the epidermis ruptures and begins to slough off?

  12. Is there any advantage to allowing water vapor to escape from the stem? Any disadvantage?

  13. From the appearance of the thinner rays, did the big vessel elements expand before or after the development of the rays?

  14. How does one determine the age of such a tree at the time it was cut?

  15. What clues do the annual rings give about weather patterns during the time when the tree was alive?

  16. Locate the vascular cambium. Where are the epidermis and cortex of this stem?

  17. Identify heartwood and sapwood. What do you think is involved in the conversion of heartwood to sapwood?

  18. Which of the rings still function in water conduction?

  19. What does a knot represent?

  20. What is the explanation for the difference between tight knots and loose knots?

  21. 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?

  22. What anatomical features can you see that would account for the differences in airflow in red and white oak wood?

  23. Based on you observations, why is white oak, rather than red oak, used for storing and aging whiskey?

  24. Based on you observations, why are red oak populations more susceptible to some bacterial and fungal pathogens than white oak?

  25. If tyloses block xylem vessels, how can water get to the top of a white oak tree?

  26. 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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