Objectives: By the end of this laboratory you should be able to:

1. recognize and describe the primary meristems in a dicot shoot tip.

2. recognize and know the functions of the tissues /cell types in a dicot stem cross section.

3. recognize and know the functions of the tissues/ cell types in a monocot stem cross section.

Herbaceous Dicots: On most herbaceous Dicot plants that have the capacity to grow and branch continuously (indeterminate growth), there is a terminal bud containing an apical meristem at the tip of each shoot. Cell division in the apical meristem allows the shoot to grow in length. The soft tissue formed by growth of an apical meristem is called primary tissue. You will look at three slides from an herbaceous dicot stem, composed of soft (non-woody) primary tissues.

Obtain a slide of a longitudinal section (l.s.) ,or lengthwise cut, of a Coleus stem tip. Look at the slide before putting it under the microscope in order to get an idea of the size of the specimen. Under the microscope, locate the apical meristem at the very tip of the shoot. Notice the cells are all small and densely stained; they do not have the large central vacuole of mature cells. Division of cells in the apical meristem produces three other primary meristems :

1. the protoderm, which divides further to produce the epidermis. The protoderm is the outside layer of cells on the very young parts of the stem tip.

2. the ground meristem, which produces the parenchyma cells of the cortex and pith. The cortex is the region of the stem between the epidermis and the vascular bundles, and the pith is the tissue in the center of the stem.

3. the procambium which produces the vascular tissues (xylem and phloem). The procambium is usually visible as dark strands in the very young leaves and stem.

Note also the leaf primordia, which are very young, undeveloped leaves. There may be one or two pairs of leaf primordia close to the apical meristem on your slide. Look for bud primordia, bumps of densely stained cells in the axils of the larger leaves on your slide. These will become the axillary or lateral buds.

Label Figure 1 with the terms in bold type.

Obtain a slide of a dicot stem cross section, usually buttercup or alfalfa. Again, note the size of the specimen before putting it on the microscope. If your slide has both dicot and monocot stem sections, the dicot will be the smaller section(s).

On low or medium power, locate:

1. the epidermis- the outermost layer of cells. In some preparations, the cuticle, a waxy layer secreted by the epidermal cells, is visible.

2. the cortex- may be composed of parenchyma cells, and/or collenchyma cells, which have thickened walls, and function as a supportive layer. The cortex is usually several layers thick.

3. the vascular bundles- arranged in a single ring interior to the cortex.

4. the pith- composed of large, thin- walled parenchyma cells. The vacuoles of these cells may be filled with storage compounds. The cells in the pith are more loosely arranged than the more exterior layers of cells.

Label Figure 2 with the terms in bold type.

On medium and high power , look at the vascular bundles in more detail. Find:

1. Primary xylem- The largest and innermost cells in the bundle, usually arranged in a string or row, are the major conducting cells of the xylem. These large cells are the vessel elements. They are elongated. have thickened cell walls, flattened and perforated end walls, and are dead at maturity. The empty lumen of the cell can conduct water unobstructed by any living parts. Smaller thick walled cells in the xylem may be tracheids, which are also dead conducting cells. Parenchyma cells , which are thin walled and still living, may also be present.

2. Vascular Cambium- These flattened, thin-walled cells mark the boundary between the xylem and the phloem. Try to find a layer that looks like a "break" or discontinuity between the inner and outer parts of the vascular bundle. The cambial cells are usually lightly stained and greenish or bluish in color. The cambium also extends between the bundles, and may be visible as an indistinct line dividing the cortex and the pith. It is only one or two cells in width. In many herbaceous dicots, the vascular cambium divides several times to produce a small amount of secondary xylem and phloem. This woody tissue helps support the stem and increases its diameter. If you snap off an older portion of a begonia or geranium, you may be able to see this secondary tissue.

3. Phloem- this food conducting tissue is also composed of thin-walled cells. The larger cells in the phloem are the sieve tube elements. They are the actual conducting cells. These elongated cells are alive, but have no nucleus. Occasionally, the perforated end wall, called a sieve plate, is visible. Smaller cells in the phloem may be companion cells, which are thought to help sustain or control the sieve tube members, and parenchyma cells, which are often used for food storage. Note- you probably cannot distinguish between these cell types in the phloem on our slides, but know they are present and what their functions are.

4. Bundle cap- The bundle cap is the outermost portion of the vascular bundle. It is composed of sclerenchyma cells, known as fibers. These cells are very elongated, narrow, and have thick but elastic cell walls. Fibers help support the stem, giving it strength and flexibility. The fibers of some plants are removed from the stems to make twine (burlap), rope (hemp) and thread (linen). Label Figure 3 with the terms given above.

Before leaving the dicot stem x.s., notice there may be chloroplasts in the outer cortical cells. Many herbaceous stems are green and photosynthesize. Also, much of the support in the stem comes from the turgor pressure of the parenchyma cells found throughout. As long as the vacuoles are filled with water, the plant remains upright. What does a plant look like when it looses turgor pressure?

What is this condition called?

What other two cell types help support the stem?

Activity 4-Monocot stem x.s. We usually use a very young corn (maize) stem as an example. Obtain a slide of a monocot stem x.s. On low and medium power, look at the arrangement of vascular bundles and parenchyma tissue. Notice there is no definable cortex and pith in the monocot stem. Why?

Label the epidermis, vascular bundles, and ground tissue (parenchyma cells surrounding the bundles) in figure 4.

On high power, look more closely at the vascular bundles. As in the dicot stem, the xylem is located on the interior half of the bundle and the phloem on the exterior half. Notice there are only 2 or 3 vessel elements in the xylem, but they are very large in diameter. Often, the tissue around the earliest vessel element has torn apart, due to stretching, leaving a space called a lacuna. This space may be filled with air or water in a living stem.

The phloem cells are also larger in the monocot than in the dicot stem. The sieve tube elements are large and thin -walled, often staining a light blue or green. Look for a sieve plate with its perforations. The companion cells are the much smaller cells in between. Phloem parenchyma may also be present.

Note the sclerenchyma cells of the bundle not only form a cap but extend all the way around, enclosing it in a bundle sheath of fibers. On bundles near the periphery of the stem, the fibrous sheath extends outward to the epidermis. This forms the tough rind found on mature corn and bamboo stems. The entire vascular bundles from long, linear monocot leaves are removed and used as sources of fiber to make twine, sacks, rugs, and fiberboard.

Label the parts on figure 5.

1. Name and describe the 3 types of meristem tissues that result from the first divisions of the apical meristem.

2. How does the arrangement of vascular bundles differ in monocot and dicot stems?

3. What is the function of the vascular cambium?

4. Is there a vascular cambium in monocot stems?

5. Which stem has bundles that look like monkey faces?

6. Which stem has a cortex and pith?