Lab 10 Plant Tissues

Lab 10
Plant Tissues

In this laboratory period you will view slides of representative sections of plants from which you will distinguish the major groups of plant tissues. Just as your body is composed of many types of tissues forming the organs of your body, the organs of a plant body are composed of many tissue types. The major organs of a plants and respective functions are as follows:

1. leaf - food production by the process of photosynthesis.

2. stem - conduction of food and water between leaves and roots and mechanical support of the plant.

3. root - absorption of water, salts, and minerals; anchorage of the plant, and storage of food.

Each of these areas requires several types of tissues to adequately execute its function.

Question 1: List the major organs and respective functions of the plant body:

The tissues of plants can be divided into three major groups, as follows:

1. meristematic tissue - young, undeveloped tissues; the major function of which is to grow, divide and increase the plant length and/or width.

2. epidermal tissues - surface tissues; the major function of which is to aid the particular plant area in dealing with its environment.

3. fundamental tissue - the bulk of plant tissues; major functions include storing, transporting, and producing of food and water, and providing mechanical support.

Question 2: List the three major groups of plant tissues and respective functions:

LAB OBJECTIVE:

1. To observe and learn to identify representative plant tissues and their functions in plants.

PROCEDURE:
Examine the following plant tissues using the slides as indicated.

I. Meristematic Tissue: Obtain the slide labeled "Coleus Stem Tip". Meristematic tissues are found at the growing tips of stems and roots. These are regions of primary growth for plants. Refer to figure 1. Scan the slide using the 4X and 10X objectives and locate the area represented by figure 1. Change to the 40X objective and view the cells in the area labeled corpus (body). These cells are typical meristematic cells. Note that they are quite small, have tiny cell walls, and a large nucleus. These characteristics should help you remember the tissue.

Question 3: Where are meristematic tissues found?

Question 4: What is meant by the term "primary growth"?

Question 5: List three characteristics that distinguish the cells of meristematic tissue.

II. Epidermal Tissue: You will observe epidermal tissue from two different plant organs so that you may see the great diversity that the epidermis of plants can exhibit. Examine the leaf epidermis, which functions in protection; and root epidermis, which functions in absorption.

A. Leaf Epidermis:
Obtain the slide labeled "Tradescantia (spider-wort) Epidermis". Observe the slide using the 10x objective. Note there are areas appearing much like small green "lips". Observe one of these areas under 40X. Refer to figure 2. The leaf epidermis serves a protective function. Its outer surface is coated with a natural wax called cutin forming a layer called the cuticle. (This is not observable in this slide. Refer to figure 4 and locate this layer.) The cuticle protects the leaf by preventing the loss of water by evaporation. It also prevents the leaf from breathing which would cause considerable problems if it were not for the specialized epidermal structures you are viewing. The cells that appear much like lips are called guard cells. These cells, by changing their shape, open and close pores in the leaf stomata. Note the large green-staining nucleus in each cell and the many small green-stained "dots". These smaller structures are chloroplast. Note that in the surrounding cells a nucleus is present but no chloroplasts. The guard cells are the only photosynthetic epidermal cells and this is thought to be vital to their ability to open and close the stoma.

Question 6: What is the function of the guard cell?

Question 7: What is the cuticle composed of?

Question 8: What is the function of the leaf epidermis?

B. Root Epidermis:
Obtain the slide labeled "Root Hairs". The slide has a whole root tip mounted on it. Observe the slide using the 10X objective. Notice the small projections all over the surface of the root. These are root hairs. The epidermal cells of the root have extensions of their outer surface that project out into the soil. The epidermal root cells absorb water, salts, and minerals from the soil. The root hairs increase the surface area available for absorption. Refer to figure 3. Try to focus on the individual root hairs and observe that they are extensions of the epidermal cells. Often the cell nucleus is located on the root hair. This slide will be difficult to focus, as the root is relatively thick.

Question 9: What are root hairs?

Question 10: Would you expect to find a waxy cuticle on the root epidermis like the one on the leaf epidermis? Explain.

III. Fundamental Tissue: These tissues are considered the "work horses" of plant tissues, and are highly diversified. Due to lab time limitations, your instructor may choose to delete some of the following information.

A. Parenchyma and Chlorenchyma:
Parenchyma cells primarily function in food and water storage, while chlorenchyma cells have the same function in addition to containing chloroplast. Chlorenchyma cells are found in the green parts of plants.

Question 11: Distinguish between the terms parenchyma and chlorenchyma.

1. Chlorenchyma: Obtain the slide labeled "Ligustrum (privet) Leaf". This is a cross section of a typical leaf. Refer to figure 4. Observe the slide on 4X, then the 10X objective. The tissue inside the leaf is called the mesophyll of the leaf. This tissue shows two different arrangements; closely packed rectangular cells forming the palisade layer and loosely arranged, irregularly shaped cells forming the spongy layer. Locate the lower epidermal layer of the leaf. Observe this on 40X. Note the cuticle. Scan along the epidermis until you can locate guard cells and stomata.

Question 12: What is the mesophyll of the leaf?

Question 13: What are the two names of the two layers of chlorenchyma inside the leaf?

2. Parechyma: Obtain the slide labeled "Ranunculus (buttercup) Mature Root". Scan this slide on 4X. Note the red cross-like structure in the center of this cross section. This is the vascular tissue (xylem and phloem) of the root. Refer to figure 5. This tissue you are to observe is in the area labeled cortex. Locate this area and view it under 40X. Note the cells have thin primary walls. Note also that many of the cells contain small purple-stained bodies. These are grains of starch. In the living root these parenchyma tissue cells were storing starch as food for the plant.

Question 14: What is the function of root parenchyma tissue?

Question 15: Why do these root parenchyma cells not contain chloroplast?

B. Collenchyma:
Obtain the slide labeled "Typical Monocot Stem, Corn". This is a cross section of the stem of corn (Zea mays). Refer to figure 6. Scan this slide on 10X. Locate the epidermis. The collenchyma tissue is located just underneath the single-cell-layer-thick epidermis. Look at this area under 40X and examine the collenchyma. Note the extra thickness of the primary walls of these cells as compared to parenchyma.

Question 16: What is the function of collenchyma?

Question 17: What characteristics of collenchyma cells allows them to perform their function?

C. Sclerenchyma:
The cells composing this tissue are dead at maturity and have thick, hard secondary walls. The cells found in this type of cells vary in appearance. You should observe two types of cells found in sclerenchyma tissue.

Question 18: What is the function of sclerenchyma tissue?

1. Sclerenchyma Fibers: Obtain the slide labeled "Flax". Refer to figure 7. This slide has mounted on it some sclerenchyma fibers from a flax plant. Flax is grown for these fibers which are extracted and woven to make linen. Observe these fibers. Note the cells are long and tapered. On 40X try to distinguish a thin line running down the center of each fiber. This is the hollow space inside the fiber where the protoplasm was once located.

2. Sclereids (Stone Cells): Obtain the slide labeled "Pyrus Fruit Stone Cells". This slide was made by squashing and staining some of the tissue from a pear. The stone cells are the part of a pear that gives it its gritty texture. Refer to figure 8. Scan the slide using the 10X objective and locate small purple staining objects. View these on 40X. Compare these to figure 8.

Question 19: Name and describe two types of sclerenchyma cells:

D. Vascular Tissue:
These are complex plant tissues that function in transporting food, water and minerals. Xylem is the tissue for transporting water and minerals, and phloem is the tissue for transporting food.

1. Xylem: There are two types of water conducting elements found in xylem. (Note: xylem cells are dead at maturity, and usually stain red on the prepared slides). The modern conducting element is the vessel which resembles a long, hollow, thick-walled straw. You have seen this element on previous slides and will see them on future slides. The less modern xylem element from which vessels are though to have evolved is the tracheid. Obtain the slide labeled "Pinus Wood; Cross, Radial, and Tangential". Pine wood is composed of tracheids. Locate the radial section of the pine wood slide. Observe under 40X and refer to figure 9. Note the hollow tracheids, and on some the bordered pits. The bordered pits allow water passage from one tracheid to another.

Question 20: What is the function of the xylem?

Question 21: What cell is found in pine wood that executes this function?

Question 22: What is the function of bordered pits?

2. Phloem: There is one major conducting element found in phloem; the sieve tube. Sieve tubes are living structures composed of sieve tube elements (cells) placed end to end with a perforated sieve plate between the ends of the cells. Sieve tube elements have no nucleus and are thought to depend for maintenance on small adjoining nucleated cells called companion cells. Obtain the slide labeled "Cucurbita (cucumber) Stem", l. s.". Examine this slide and focus on an area toward the left side of the section. Switch to 40X and refer to figure 10. Note the sieve tubes and sieve plates. Many of the tubes show a build up of debris on the sieve plate called the slime plug which is stained red in this slide. Try to locate a companion cell, and note that it contains a nucleus.

Question 23: What is the function of phloem?

Question 24: What cells are instrumental in performing this function?

Question 25: What are companion cells and slime plugs?

Figure 1: Coleus stem tip