Stems

Materials

1. Dormant twigs of buckeye (Aesculus) or similar woody twigs with large buds

2. Prepared slides of cross sections of young and older

alfalfa (Medicago) stems, 2- to 3-year-old basswood or

linden (Tilia) stems, and corn (Zea mays) stems 3. Live Begonia or Coleus stems

4. Sharp single-edged razor blades

5. Live 2-year-old basswood or linden (Tilia) twigs 6. Watch glasses containing tap water

7. Sets of seven dropper bottles containing gentian violet

stain, eosin stain, phoroglucinol stain, clove oil, xylene

95% ethyl alcohol, and balsam or Permount in xylene 8. Disks of white filter paper (9 centimeters in diameter) 9. Stock bottle of 95% ethyl alcohol 10. Models and charts of both dicot and monocot stems 11. Cake of paraffin

Some Suggested Learning Goals

1. Know the externally visible parts of a woody twig in its winter condition, and know their derivations or functions.

2. Know the locations and functions in a young alfalfa stem of epidermis, cortex, primary phloem, primary xylem, pith, and what additional tissues are produced in older alfalfa stems.

3. Understand the differences and similarities between alfalfa and Begonia stems.

4. Be able to locate and identify the following in a basswood or linden stem, and know the function of each: cork, cork cambium, phelloderm, cortex, primary phloem, secondary phloem, vascular cambium, secondary xylem, primary xylem, broad phloem ray, narrow phloem ray, broad xylem ray, narrow xylem ray, fibers, tracheids, vessels, annual ring of xylem, and pith.

5. Know the differences between dicot and monocot stems, and the parts and functions of a vascular bundle of corn.

Introduction

Stems and roots are connected to each other and they share most of the same tissues, but the arrangement of the tissues may be different. In addition, a pith composed of parenchyma cells is usually seen in the center of dicot stems (pith was absent from the dicot roots examined in the previ­ous exercise). Roots generally have an endodermis and a pericycle, both of which are usually not seen in stems.

A. External Form of a Woody Twig

Examine the woody dicot twig provided. Note at the tip of the twig the terminal bud protected by bud scales. Locate a node (leaf attachment region of stem), and examine the leaf scar(s) present. How many bundle scars are present in each leaf scar? (Bundle scars usually appear as tiny bumps where strands of xylem and phloem broke when the leaf fell from the stem; if necessary, use your dissecting micro­scope to locate them.) Groups of small, narrow scars (bud scale scars) often extend around the twig; these sometimes indistinct scars are created when the terminal bud scales of a previous terminal bud fall off. Are any such groups of scars present? If so, how old is your twig?

Note the small, slightly raised lenticels, located mostly in the outer bark of internodes (regions of stem between nodes). Lenticels consist of spongy, slightly larger cork cells that may or may not have waxy suberin; they function in permitting exchange of gases (e.g., oxygen, carbon diox­ide) between the interior of the stem and the air.

Leaves are usually attached to the stem at an angle. The region immediately above the leaf base and the stem is called an axil. Are the axillary buds (there is normally one axillary bud in each axil) similar to the terminal buds? If not, how do they differ?

B. Herbaceous Dicot Stems

Select a slide showing a cross section of a young alfalfa stem (Medicago, young xs), and examine it under low power. Identify the tissues that have been produced by the apical meristem. These tissues include the epidermis, which consists of a single layer of cells that covers all of the exte­rior of plant organs; the cortex, consisting of thin-walled parenchyma cells that function in food storage; primary xylem, whose vessels and tracheids conduct water and min­erals; primary phloem, whose sieve-tube members and as­sociated companion cells function in conduction of food in solution; and pith, whose parenchyma cells, like those of the cortex, function in food storage. How does the arrange­ment of the primary xylem and phloem in this stem differ from their arrangement in the buttercup root you examined in the previous exercise?

Examine an older section of alfalfa stem (Medicago, old xs). Note that the xylem and phloem are a little more extensive, due to the addition of secondary xylem and sec­ondary phloem by the vascular cambium, a narrow layer of brick-shaped, meristematic cells that develops between the primary xylem and phloem.

With the aid of a sharp razor blade, cut paper-thin cross sections of a Begonia, Coleus, or other provided stem. Mount in a drop of phoroglucinol (or water) on a slide, and add a coverslip. Examine with the lowest power of your microscope. What similarities between this and alfalfa stems can you detect?

C. Internal Structure of a Woody Stem

Your instructor may have you make your own microscope slide of a woody stem, OR he or she may choose to have you examine previously prepared slides exclusively. If you are to make your own preparation, one procedure is as follows:

Using a sharp razor blade, cut paper-thin cross sections of a 2- or 3-year-old basswood or linden (Tilia) twig. (Cau­tion: The wood is hard; try to brace the twig before cutting so that the razor blade doesn't slip and injure you. You may need to practice a little to get the sections thin enough.) Float the sections in water. Transfer the two thinnest sec­tions, without allowing them to become dry, to a drop of water on a slide. Have gentian violet stain ready, and blot off the water with filter paper. Immediately add a drop of the stain to each section. After about 30 seconds, blot off the gentian violet stain, and add a drop of 95% alcohol to each section. Wait about 1 minute, blot, and add a drop of eosin stain. After 1 more minute, add alcohol again, blot off, and add a drop of clove oil to each section. The slide should now be ready for the addition of a coverslip and mi­croscopic study.

If you have a good slide and want to preserve it perma­nently, you may do so by following the clove oil with a drop or two of xylene. Then add balsam and place the cov­erslip on top. After that, the slide will probably take a few days to dry. It can then be stored indefinitely. After you have examined your stained sections, turn to a prepared slide of the same plant (Tilia xs) and compare it with the slide(s) you have made.

Contrast the linden stem with the alfalfa and Begonia or Coleus stems. Note that the phloem in the basswood or linden stem is quite complex, and that there are several ad­ditional tissues. Focus on the outermost part of the bass­wood stem first. Note that by the time the stem is 2 or 3 years old, the epidermis has been lost, and the cells that are sloughing off to the outside are cork cells produced by a cork cambium that has developed toward the outer part of the original cortex. The cork cambium, consisting of a nar­row band of meristematic cells, also produces phelloderm

cells toward the inside of the stem. The phelloderm cells re­semble the cells of the cortex.

To the interior of the cortex is a cylinder of phloem, which, as previously mentioned, is quite complex in bass­wood or linden stems. In cross section, it appears as a cir­cular band of wedges alternating with tapering trapezoids of banded tissue. The wedges include relatively large parenchyma cells that are the flared-out tops of broad rays. The rays function in lateral conduction of water, food, and other materials throughout the stem. The part of the ray in the phloem is referred to as a phloem ray, while the part of the same ray in the xylem is called a xylem ray. In cross section, basswood or linden stems reveal that the broad rays are usually two or three cells wide in the xylem but flare out and become many cells wide in the phloem. Narrow rays, however, are usually one cell wide in both the xylem and phloem. The banded, tapering trapezoids consist of thin-walled sieve-tube members and companion cells (usu­ally stained green) between bands of thick-walled fiber cells that give strength to the stem. The fibers (usually stained red or purple) are often slightly larger at the outside edge of each trapezoid; the larger fibers were produced by the apical meristem as part of the primary phloem, while most of the phloem visible in this slide is secondary phloem produced by the vascular cambium, a narrow band of brick­shaped cells at the base of the wedges and trapezoids.

The vascular cambium also produces secondary xylem toward the interior. Note that the xylem (wood) appears to have been produced in bands or rings. In fact, each year's production of xylem by the vascular cambium is referred to as an annual ring. In basswood or linden stems, each an­nual ring consists of larger vessels that are produced when the cambium first becomes active in the spring, and then progressively smaller vessels and tracheids produced throughout the remainder of the growing season. Note, again, that the broad phloem rays become broad xylem rays once they cross the vascular cambium, and that there are several narrow phloem rays (becoming narrow xylem rays in the xylem), usually one cell wide, between the broad rays. The large, thin-walled parenchyma cells in the center constitute the pith.

D. Monocot Stems

The alfalfa, Begonia or Coleus, and basswood stems are all dicot (dicotyledonous) stems. Examine a slide of corn (Zea mays xs) stem, representative of monocots (monocotyledo­nous plants). Note that the xylem and phloem are in vascu­lar bundles that are scattered throughout the stem instead of being in a ring as they are in dicot stems. Notice, also, that all the tissues are primary; there are no secondary tissues because no cambium is present to produce them. This also results in there being no separation between cortex and pith, and the parenchyma cells throughout which the vascu­lar bundles are scattered are referred to as fundamental tis­sue instead.

Drawings to Be Submitted

1. Label the provided drawing of the woody twig. The labels should include TERMINAL BUD, AXILLARY BUD, NODE, INTERNODE, BUNDLE SCAR(S), GIRDLE, LENTICEL(S), and BUD SCALE(S).

2. On the illustration of the alfalfa stem provided, label the following: EPIDERMIS, CORTEX, XYLEM, PHLOEM, PITH, and VASCULAR BUNDLE.

3. Label the following on the drawing of a wedge-shaped portion of a cross section of a linden stem provided: CORK, CORK CAMBIUM, PHELLODERM, CORTEX, PHLOEM, VASCULAR CAMBIUM,

ANNUAL RING OF XYLEM, PRIMARY XYLEM, SECONDARY XYLEM, PITH, BROAD PHLOEM RAY, BROAD XYLEM RAY, NARROW PHLOEM RAY, NARROW XYLEM RAY, PRIMARY PHLOEM, and SECONDARY PHLOEM.

4. Label the illustration of the monocot (corn) stem provided. Indicate VASCULAR BUNDLE, EPIDERMIS, and FUNDAMENTAL TISSUE on the wedge-shaped portion of the cross section. Label BUNDLE SHEATH CELL(S), SIEVE-TUBE MEMBER, COMPANION CELL, VESSEL MEMBER, TRACHEID, XYLEM, and PHLOEM on the enlargement of a single vascular bundle.

 

Questions

1. What protects the buds of dormant twigs?

2. What are bundle scars?_______________________________________________________________

3. Where, specifically, are axillary buds located?

4. What structures associated with gas exchange are found throughout stem internodes?

5. What is the difference between bud scale scars and leaf scars?

6. Which tissue separates cortex from pith in an older alfalfa stem? What is the function of this tissue?

7. What is the primary function of cortex and pith?

8. Which tissue conducts water and minerals in solution?

9. If you saw cross sections of Begonia or Coleus and alfalfa stems side by side, what differences would be obvious?

10. Which stains are used to make the tissues of your handmade linden (basswood) slide more readily visible?

11. If you wished to make your handmade linden (basswood) slide permanent, which additional substances would you use?

 

12. Which two tissues are produced by the cork cambium, and which two tissues are produced by the vascular cambium?

 

1. Where are axillary buds located?

2. What are the small bumps of parenchyma tissue on the surface of the internodes called?

3. How is a bundle scar formed?___________________________________________________________

4. What is the function of a lenticel?

5. Which of the stems in this exercise has the most complex phloem?

6. What stains are used in making your own linden (basswood) slide?

7. In addition to cork, what tissue is usually produced by the cork cambium?

8. How are vascular bundles arranged in a monocot stem?

9. Which of the stems featured in this laboratory exercise is (are) NOT (a) dicot(s)?

10. To make your own microscope slide of a linden (basswood) stem permanent, what substance  would you add just before placing a coverslip on it?