Stuyvesant High School
Department of Biology & Geo-Science
LABORATORY EXERCISE #16
HOW ARE TISSUES OF PLANTS ORGANIZED?
INTRODUCTION
Just as in animals, the tissues of the leaf, stem, and roots of plants are organized to carry out life
functions. You will examine histological sections of these organs from several sources. The
organs come from the branch of the Plant Kingdom called TRACHEOPHYTES. As the name
implies, these plants have tubes like your trachea but instead are used to transport materials
such as water and dissolved minerals. A subgroup of the Tracheophytes are the FERNS, the
cone bearing plants called GYMNOSPERMS (“naked seeds”), and the fruit bearing plants called
ANGIOSPERMS (“covered seeds”). We will be looking at structures from the two subgroups of
the angiosperms called MONOCOTS and DICOTS. Monocots have non-branching stems and
parallel leaf venation (examples: corn and other members of the grass family). Dicots have
branching herbaceous (soft) or woody stems and branching leaf venation (examples: lilacs and
other woody flowering trees).
An important and unique life function that plants perform is to derive their energy from the sun’s
rays, capturing it in the BONDS of GLUCOSE molecules manufactured in the process of
PHOTOSYNTHESIS. The plant must be able to regulate the uptake and release of both water
vapor and carbon dioxide, which are needed as raw materials. In order to accomplish this, root
hairs and pores called LENTICELS along the stems of the plants absorb and release gases
through the process of GASEOUS DIFFUSION. In addition, the lower epidermis of leaves contain
openings called STOMATES. The size of the stoma (stoma is singular, stomates is plural)
opening is regulated by the chloroplast containing GUARD CELLS which surround it. Gas
exchange through the stomates is advantageous because the amount of exchange can be
controlled by the opening and closing of the guard cells. This can prevent the plant from losing
too much water in dry conditions.
The guard cells are specialized epithelial cells. These cells are bean shaped. The inner edge has
a thicker cell wall than the outer edge. During the day, as photosynthesis occurs, these cells fill
up with water and become TURGID, causing them to change shape [See diagram below].
Because of the difference in the thickness of the cell wall, the cells separate and form the stoma
opening. During the night, the cells lose water and become FLACCID. The stoma opening grows
smaller. Thus, when the plant needs carbon dioxide during the day, the stomates are open.
Many scientists have been interested in how the guard cells regulate the size of stoma. There is
evidence that LIGHT-DEPENDENT ACTIVE TRANSPORT is used to bring potassium ions into
the guard cells during the day. This causes a change in osmotic balance and water will rush in,
causing the cells to become turgid. At night, the active transport no longer occurs and the
potassium ions diffuse out. The water will then also diffuse out, causing the cells to become
flaccid.
Fig. 18.1 GUARD CELLS control
stomata openings
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Laboratory Manual
Stuyvesant High School
Department of Biology & Geo-Science
STUDENT OBJECTIVES
1. Observe and describe the structure of the leaf, stem, and roots of various types of
angiosperms.
2. Contrast the structure of stems in monocots and dicots.
3. Observe the leaves of a member of the onion family, the scallion.
4. Examine the stomates on the underside of a lettuce and scallion leaf.
5. Improve microscope and sketching techniques.
PRE-LAB QUESTIONS
1. What structures regulate water loss and CO2 absorption by a leaf?
2. Describe the daily opening and closing of guard cells.
3. Stomates of the lettuce leaf are located primarily on the lower epidermis. How is this
advantageous to the plant?
4. Why could removing the true bark kill a tree?
PART A: ANGIOSPERM TISSUES
MATERIALS
USE THE DRAWING SHEET FORM FROM THIS LAB MANUAL FOR ALL DRAWINGS.
Microscope, slides of plant tissues: lilac leaf, corn stem, basswood stem, and onion root tip; lens
paper, texts, charts and atlases of plant structures
PROCEDURE
Work in pairs. Observe all the slides in any order. Review the parts of the leaf, stem, and roots of
angiosperms using charts and texts.
I. Lilac Leaf Cross-Section (Syringa)
1 Observe the slide with your naked eye. Determine the orientation of the leaf by examining it
for the position of the main vein.
2 Observe the leaf under LOW POWER. Note the presence of various stains in the section.
They will help your resolution of the different cell parts and different tissues.
3 Pick a portion of the leaf AWAY FROM THE MAIN VEIN, center it, and then switch to high
power.
4 Draw the section of the leaf and label the following tissue types
a. UPPER EPITHELIUM – single layer of cells on upper side of leaf.
b. MESOPHYLL – PALISADE LAYER – narrow, upright cells; note the presence of many
chloroplasts.
c. MESOPHYLL – SPONGY LAYER – open, porous layer of photosynthetic cells, similar to
a sponge; cross-sections of VEINS may be present.
d. LOWER EPITHELIUM – single layer of cells on lower side of leaf; contains special
structures called STOMATES, openings which are surrounded by kidney-bean shaped
cells called GUARD CELLS.
II. Monocot Stem Cross Section (corn)
1. Observe the corn stem slide with your naked eye. (A longitudinal section might also be on the
same slide. The cross-section is round.)
2. Observe the section under low power. Draw a section of the stem and label the following
tissues.
a. VASCULAR BUNDLES. These are groups of tubes whose walls stain more
darkly than
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Laboratory Manual
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b. the open, large.PARENCHYMA CELLS, which make up the tissue called PITH.
Pith makes up most of the corn stem.
c. The outer stem is covered by a single layer of cells called the EPIDERMIS.
3. Turn to high power and observe the structures of the stem. Draw and label the following
tissue types
a. The vascular bundles are made up of large, thick-walled XYLEM cells, the smaller, thin
walled TRACHEIDS,
b. and PHLOEM cells with their accompanying tiny COMPANION cells
III. Dicot Herbaceous Stem (Helianthus/ Sunflower)
1. Observe the cross-section of the sunflower under low power. Note the distribution of
vascular bundles along the periphery of the stem and the presence of the large expanse of
pith. The outer edge will be stained darker than the rest of the tissue. Center a vascular
bundle in the field of vision.
2. Switch to HIGH POWER. Draw and label a section of the stem
a. Each vascular bundle will be divided into an inner and outer section by a strip of cells called
CAMBIUM. The cambium contains cells which are rapidly dividing and give rise to the other cells
of the stem.
b. The mass of cells outside the cambium are the PHLOEM and the small COMPANION
CELLS.
c. Large, open cells which are located just interior to the cambium are the XYLEM CELLS.
Smaller, thin-walled cells are called TRACHEIDS. Thick-walled, smaller cells are the
SCHLERENCHYMA CELLS.
d. The thick, outer layer of cells is called the CORTEX. The cortex consists of several layers of
tough COLLENCHYMA CELLS
e. and an outer later called the EPIDERMIS.
IV. Dicot Woody Stem (Tilia/Basswood)
1. Look at the slide with your naked eye. The cross section is very striking. Observe the slide
under LOW POWER. Move the slide around so that you see all the different areas; see how
the inner tissue appears to make concentric circles. These circles are the ANNUAL RINGS.
They are made up of XYLEM and TRACHEID CELLS. Each ring equals one year. In the wet
spring weather, the xylem grows larger. In the summer, the xylem does not grow as large.
The alteration of the diameter of the xylem created the illusion of “rings”. This xylem tissue
becomes the wood of the tree. The annual rings are connected by spoke-like RAYS.
2. Observe the pith in the center of the stem. In a mature tree, the pith becomes crushed.
3. At the end of the ring area, there is a ring of VASCULAR CAMBIUM, the layer of cells which
give rise to vascular tissue. As the cambial cells divide, those on the inner surface change
into xylem cells. Those on the outside become the PHLOEM CELL layer. In a young tree, the
groups of phloem cells take on the appearance of a ring of pyramids arranged along the
periphery of the stem. Later on, the pyramids become flattened and are obliterated.
4. On the outside of the phloem layer, there is a layer of large CORTEX CELLS, made up of
PARENCHYMA (storage) and COLLENCHYMA (strengthening) cells. There is then a
second layer of cambial tissue called CORK CAMBIUM. The outside cells formed by this
layer become CORK, the material which the layman calls “bark.” TRUE BARK consists of
everything from the vascular cambium out.
PART B: STOMATES
MATERIALS
Scallion, fresh lettuce, forceps, microscope, slides, and coverslips, Lugol’s solution, water,
dropper, and paper towels.
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Laboratory Manual
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Department of Biology & Geo-Science
PROCEDURE
I. Examination of a Scallion Leaf
1. Place 2 drops of water onto your slide.
2. Scallions are a common vegetable that are similar to onions. The leaves that grow above
the ground are wrapped around a central stem. Using your forceps, carefully remove a
section of one leaf. Using the scalpel, gently scrape the tissue of the outer epidermis.
Place the epidermis in the water on your slide. Cover it with a coverslip.
3. Observe the leaf under low power. Note the arrangement of the cells. Look for the kidney
bean shaped guard cells. Switch to high power and observe.
4. In order to make the cells appear more distinct, you may use Lugol’s Solution to stain the
leaf. Place two drops of Lugol’s Solution next to the coverslip. Place a small piece of
paper towel next to the opposite side and draw the stain under the coverslip. Observe
again under low, then high power.
5. DRAW and LABEL a low-power and a high-power sketch of the leaf cells of the
scallion. Indicate the magnification used. Please do all drawings in pencil.
A.
B.
Fig. 18.2 Lower epidermis of a plant leaf with stomates. A. Low power, B. high power
II. Examination of the Lettuce Leaf
1. Pick up and observe the whole lettuce leaf, if available. There is a distinct top and
bottom. Note the differences between the two sides. The top is shinier and greener while
the bottom is paler. The main rib of the leaf bulges out on the lower side. You will observe
the lower epidermis of the leaf and of the rib itself.
2. Place two drops of Lugol’s Solution on your slide.
3. Hold the lettuce leaf in your hand, bottom side up.
4. Break a side vein so that part of the lower epidermis is separated from the rest of the leaf.
Use your forceps to pull the tissue AWAY from the main rib. MAKE SURE NO GREEN
MESOPHYLL IS CONNECTED TO THE LOWER EPIDERMIS. Pull off an 0.5 cm piece
and place it in the Lugol’s Solution.
5. Cover the tissue with a coverslip.
6. Observe the leaf under low power. Note the appearance of the epidermal cells.
7. Look for the darkly stained, bean-shaped guard cells. Switch to high power and observe.
8. DRAW and LABEL a low-power and a high-power sketch of the cells and stomates of
the lower epidermis of the lettuce leaf. Indicate the magnification used.
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Laboratory Manual
Stuyvesant High School
Department of Biology & Geo-Science
EXTRA CREDIT
Find a root tip slide that has been stained with HEMOTOXYLIN stain. Try to find several stages of
mitosis and make a drawing of what you see.
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