Plant Anatomy

Vascular plants have an advanced form of vascular (conductive) tissue consisting of xylem and phloem tissues. These two tissues are arranged in a characteristic pattern that we shall soon examine. These tissues are typically surrounded by a tissue known as ground tissue. Each plant organ is covered by a single layer of cells known as the dermal tissue. The cells of these plant tissues typically have cellulosic walls, true nuclei, numerous chloroplasts, prominent vacuoles, and store starch. You should be able to observe these cellular structures in some of the cells you will observe today with your microscope.

The colors you will observe in specimens on prepared slides are artificial. The thin sections of plant organs have been stained with a series of dyes (green, red, and purple) that are absorbed by structures containing particular chemicals. The red dye, for instance, stains areas rich in fatty, oily, or waxy chemicals, whereas the green dye stains cellulose (a polysaccharide). Sections of living plant tissues would typically not have any color except yellow or green in the chloroplasts (chlorophyll is a green pigment, carotene is a yellow pigment) or red colors in the vacuole (anthocyanin pigments found typically only in flower or fruit tissues).

You should draw the outline shape of the objects you observe without fancy shading or color. Your drawing should be LARGE so that all of the important fine structures are clearly rendered. Do NOT draw in every cell you observe; if a region is composed of many cells, outline the region and draw only a few cells (5 or 6) in detail. DO NOT draw in hundreds of tiny imperfect circles!! Your instructor will demonstrate a good drawing early in the laboratory period. Every important structure should be labeled. In this exercise, the structures to be labeled are indicated in bold type. Spend no more than 20 minutes making each drawing.

A. The leaf. Obtain a prepared slide of a leaf cross section and examine it carefully with your microscope. Locate the three major layers of the leaf.

1. The epidermis surrounds the inner tissues. Notice how the upper epidermis has very few stomata compared to the lower epidermis. The epidermis is covered with a waxy material called cutin which prevents evaporation and water loss. The cutin picks up the red dye and should appear as a thin pinkish layer on the outer surface of the leaf. Thus, the only meaningful openings for gas exchange are the stomata surrounded by guard cells. The guard cell pairs work in a special manner involving light, hormones, and ion pumps to fill up with water by osmosis and open the stoma, or to lose water by osmosis and close the stoma.

2. The mesophyll consists of large cells (parenchyma) filling up the bulk of the leaf mass. This is subdivided into the upper palisade mesophyll and the lower spongy mesophyll layers. The palisade layer is a parallel array of columnar cells each containing many chloroplasts. The spongy layer has nearly isodiametric cells arranged in a loose network. Both areas of mesophyll carry out photosynthesis for the plant and need good gas exchange to do this. You will notice that each cell in the mesophyll is largely surrounded by an apparent gas space. The gases produced as waste in the cell (e.g.: oxygen) can be exchanged for essential gases (e.g.: carbon dioxide) in the gas space. The gas space, in turn, might exchange gases with the external atmosphere through the stomata. How might you test this hypothesis? Hint: PV=nRT. Do it!

3. Now return to your examination of the leaf cross section at the microscope. The veins pass through and across the section of leaf tissue on your slide. The veins consist of two bundles of elongated cells, each bundle like a handful of straws. The cells are bundled side to side, but are connected end to end along the length of the vein. The two bundles in each vein are distinct:
   1. The bundle of cells closer to the upper epidermis is the xylem tissue. This tissue consists mostly of dead, elongated cells, attached end to end, with the endwalls missing or perforated. The side walls of the xylem cells are heavily thickened with lignin (a brittle crystalline material giving mechanical strength). Lignin picks up the red dye so the walls appear pinkish. Water and minerals from the soil come into the leaf through these xylem cells. The water is brought there mostly by evaporative forces generated by gas exchange at the epidermis. Look at your plant in the Eosin Y. What evidence does it provide you for evaporative water loss?

   2. The bundle of cells closer to the lower epidermis is the phloem tissue. This tissue consists mostly of living, elongated cells, attached end to end, with perforated endwalls. The sidewalls of phloem cells are relatively thin and pick up only the green dye. The phloem carries the chemical products of photosynthesis and other chemical reactions from the mesophyll tissues to the rest of the plant via the stem.

4. Diagram a portion of the cross section of a leaf. You need only show the layers and a few cells (less than 10) in each region. Label your diagram completely.

1. The epidermis. You will notice that there is an outer layer of epidermis. Of course the stem is usually round, so there is no inner and outer or upper and lower distinctions. The epidermis, like that in the leaf, is responsible for preventing water loss except through stomata. The stem epidermis will also have guard cells that regulate loss through the stomata, they are rare, however. Is there any cutin?

2. The cortical parenchyma filling up the stem volume is roughly equivalent to the mesophyll parenchyma of a leaf. In many species the stem is green and the outer layers of the cortex contain the chloroplasts necessary for photosynthesis. The outer cortex area may also contain some cortical collenchyma. These cells have unevenly thickened walls and are responsible for mechanical support.
   Embedded in the cortex are veins or vascular bundles as discussed below. Near the center of the stem cross section is more parenchyma. This inner area is called the pith region.

3. The vascular bundles are equivalent to the veins of a leaf. Since these are oriented primarily up and down the length of the stem, the stem cross section shows only slices of these elongated cells. Each bundle consists of two major tissues as in the leaf. The cells with green cell walls located toward the epidermis are the phloem cells. The cells with red cell walls grouped toward the center of the stem are the xylem cells. The tissues are separated by the cambium and surrounded by fibers. How do you know that xylem actually conducts materials up the plant? Examine your stem in Eosin Y appropriately for the answer.

4. How might you be sure that it was xylem (not phloem or other tissues) that carried the dye? Do test that and make a sketch of the results.

5. Diagram the cross section of a typical dicot stem. You need only show the layers and a few (less than 10) cells in each region. Label your diagram by connecting lines to your drawing from the labels.

1. The root epidermis is equivalent to that in the stem and leaf, but both cutin and stomata are absent in young portions of the root.
   Why would root tips not need protection from water loss?
   Why would stomata not be of any use to a root?
   What cellular process in a root would require gas exchange?

2. The ground parenchyma is represented by only an outer layer of cortex. This region typically lacks collenchyma.
   Why would a root not need the mechanical support offered by collenchyma?

3. The vascular tissue is partially coalesced into a solid vascular cylinder. In the cross section you see a circle representing these tissues. The cells again are very elongated and you observe only slices of them. The central portion of tissue comprises the coalesced xylem areas.
   How can you tell that this area is xylem?
   Near the periphery (edge) of this central disc of vascular tissue you will find discrete (uncoalesced) bundles of phloem.
   How can you tell that these areas are phloem?

4. Between the vascular tissues and the cortex is a single layer of cells known as the endodermis. In these cells, a portion of the radial walls, both longitudinal and tranverse, contains lignin and suberin (waterproofing substances). This band around the wall is called the Casparian strip. What color does the Casparian strip stain?
   Between the endodermis and the vascular tissues is the pericycle, the origin of branch roots and root bark. This endodermis is critical to active transport and uptake of minerals from the soil water. Aside from mechanical anchorage, selective mineral uptake is the single most important function of the root.

5. Design and carry out an experiment to test endodermis function. Here are some hints to get you started. The endodermis is thought to perform selective mineral uptake via active transport. Active transport involves mineral-specific membrane transport proteins. Eosin Y is an artificial dye, not a soil mineral. If you recall what you did with Eosin Y before, now do something that involves the root and tests endodermis physiology (function). How can you be reasonably sure it was the endodermis that is responsible for selective mineral uptake? Do that test.

6. Diagram the cross section of a root. You need only show the layers and a few (less than 10) cells in each region. Label your diagram by connecting lines to your drawing from the labels. Hand in all drawings with your abstract for this exercise.

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