I am working on botany leaves lab and I want the answers to the questionsThe Leaf
Leaves are the main appendages of the stem, and in most vascular plants, the principal
structure for photosynthesis. Although leaves vary tremendously in form and internal structure,
most consist of a petiole and a blade. Some of the variation in leaf structure is related to habitat.
Aquatic leaves and leaves of dry habitats have special modifications to permit survival in those
different habitats. Leaf shapes, margins, tips, and venation patterns are characteristics used to
identify different species of flowering plants.
A. Dicot Leaf Structure
Examine a prepared slide of a lilac, Syringa, or similar leaf. Note the large midvein. As
you scan your section locate the many branching veins, some of which will be in longitudinal
section while others are in cross section.
Observe a portion of the blade to one side of the mid vein. Identify the
• Upper epidermis, which has a relatively thin but discernible cuticle
• Palisade mesophyll
• The veins, with their bundle sheaths of sclerenchyma
• The spongy mesophyll
• Lower epidermis, which also has a cuticle.
The palisade and spongy mesophyll are composed of parenchyma cells, which contain
many chloroplasts for photosynthesis. Note the presence of intercellular air spaces among the
spongy mesophyll cells and the relative distribution of stomata and guard cells in the lower
epidermis. Most stomata open into an air space within the spongy mesophyll. The mushroomshaped structures of the epidermis are trichomes, or epidermal hairs.
Dicot Leaf Cross Section
B. A Monocot Leaf
Observe a corn (Zea mays) leaf section. Note the distribution of veins.
Monocot leaves generally have parallel veins rather than the branching network of veins
common to dicot leaves. . Note too that the corn leaf has a uniform mesophyll region rather than
distinctive palisade and mesophyll areas.
In the corn leaf the veins are surrounded by a sheath composed of large parenchyma cells.
These cells are involved with C-4 photosynthesis. The larger vascular bundles contain extensions
of sclerenchyma which connect to the epidermis for support.
Identify the xylem and the phloem regions of the veins.
Where are the stomata and guard cells located in the corn leaf? View the Monocot Leaf Cross
C. Environmental Adaptations of Leaves
Plants which live in arid environments are subject to drought, and often, intense sunlight.
Such plants are called xerophytes. These plants are subjected to intense evaporation of water, a
resource which is often in short supply. Many such plants have a number of modifications which
minimize water loss through transpiration, the evaporation of water from the plant surfaces.
Some plants drop their leaves during periods of drought; cactus plants photosynthesize with
modified stem tissue, and lack leaves entirely. Those plants which do produce and retain leaves
often have special features which we associate with the xeromorphic leaf. Nerium oleander is a
good example of a plant with xeromorphic leaves.
Examine the prepared slide of Nerium oleander leaf, xs. Note the very thick cuticle as
you focus on the upper epidermis. The epidermis is several layers thick, too. The palisade
parenchyma, beneath the epidermis layers, is in two layers. The spongy mesophyll is
loosely packed and quite wide. The unusual structures seen in the spongy mesophyll are a type
of crystal, called druses.
Veins may have bundle sheath extensions in additional to the bundle sheath layer. Look
for the mid vein. It has phloem on both sides of the xylem, which is unusual.
As you turn to the lower epidermis, note that it, like the upper epidermis, has several layers
and a thickened cuticle. As you move your slide along the lower epidermis, note the deep
invaginations of the epidermis layer into the lower leaf. These invaginations are called stomatal
crypts. There are a number of epidermal hairs in the crypts, along with the stomata. All of the
stomata are located in the crypts.
Why do you think this is?
Nerium oleander leaf, xs.
The leaves of the water lily float on the surface of ponds and lakes, although the water lily
is rooted in the lake bottom. Examine a prepared slide of Nymphaea leaf, xs, to observe
modifications water lilies have for flotation. Look first at both epidermis layers. Where do you
find stomata? Why? Look for small hairs in the lower epidermis layer. Now refocus on the upper
epidermis layer. Can you find the cuticle? It is very thin. Below the epidermis cells the
palisade mesophyll consists of three or four overlapping layers of cells, which are fairly
loosely packed, allowing for gases to enter from the upper epidermis. Note the huge
intracellular spaces in the spongy mesophyll layer.
The buoyancy of the water lily comes from these large air spaces The spongy mesophyll
also contains large, branching, thick-walled sclerids for support. There are crystals within the
sclerids, too. Note the reduced size of the veins in Nymphaea, compared to most leaves. The
vascular tissue, especially the xylem, is minimal in most hydromorphic leaves. You should find
more phloem than xylem in the vascular tissue as you observe the scattered veins. Nymphaea
Compare the adaptations of the hydromorphic and xeromorphic leaves with the typical
mesomorphic dicot leaf, such as Syringa.
Ecological Leaf Type Adaptive Structures Environment
D. Stomata Structure in Zebrina leaves
The epidermal surfaces of plants are covered with a protective cuticle.
However, CO2 must enter the leaf for photosynthesis and the O2 produced during
photosynthesis must be released from the plant. To solve this dilemma plants have specialized
cells in the epidermis, called guard cells, which form stomata (pores) in the epidermis. Stomata
can be open or closed, depending on the turgor of the guard cells. When stomata are open, gas
exchange can occur. Unfortunately, large amounts of water are lost from the plant through the
open stomata as well. (For example, as much as 90% of the water absorbed by the roots of a corn
plant growing in Kansas may be lost through the stomata of its leaves.) To avoid excessive water
loss, the guard cells have a mechanism to open the stomata during photosynthetic periods (i.e.,
daylight hours) and close the stomata when photosynthesis is not occurring.
You will observe guard cells and stomata in the lower epidermis of leaves of Zebrina.
Since the regular epidermal cells of Zebrina contain anthocyanin (purple) pigments, the guard
cells, which contain chloroplasts, are particularly conspicuous.
Leaf epidermis showing stomata and guard cells
Zebrina Epidermal Peel
• Cut a portion of a leaf from a Zebrina plant.
• With your fingernail or a sharp razor blade, peel a portion of the lower epidermis from the
leaf, starting at the cut edge. Note: The lower epidermis is purple pigmented.
The upper epidermis is silver and green striped.
• Make a wet mount of the epidermal peel. Try to have the peel flat on the microscope slide;
wrinkled portions have too many layers of cells and trap air bubbles.
• Observe your slide with your microscope. After locating guard cells with the lower power
magnification, use the 45x objective to observe one of the stomata closely.
Can you see the chloroplasts in the guard cells?
• What is the shape of the guard cells? Note the thickness of the inner walls of the guard cells.
Are any of the stomata open?
Recall from your observation of the prepared slide of a leaf that a stoma opens into an air space
of the spongy mesophyll. Of what advantage is this arrangement to the plant for photosynthesis?
E. C4 Photosynthesis and Leaf Structure
Most higher plants use a photosynthetic pathway known as the C3 photosynthetic
pathway, where the Calvin cycle of the “dark reactrions” begins with CO2 (carbon dioxide)
combining with ribulose biphosphate (RuBP) to form the 3- carbon compounds, PGA
(phosphoglyceric acid) and PGAl, (phosphoglyceraldehyde). Both the light reactions of
photosynthesis and the Calvin cycle occur within the same chloroplasts in all of the mesophyll
cells. The Ligustrum or Syringa dicot leaf cross section you observed shows the typical leaf
structure of a C3 plant. Some plants, known as C4 plants, use a different pathway for carbon
fixation, in which CO2 first combines with PEP (phosphoenolpyruvate) to produce 4-carbon
acids, such as oxaloacetic acid or malic acid. The reaction serves as a CO2 trap, since the CO2
taken into the leaf can now be stored in the form of the 4—carbon acids. This is especially
beneficial for plants in hot dry areas, which lose lots of water through their open stomata when
CO2 is absorbed. Many C4 plants can “stockpile” CO2 this way, freeing CO2 from the acids for
the Calvin cycle as needed. Some monocot C4 plants also separate the reactions of
photosynthesis into different chloroplasts within different types of cells, another energy
conserving measure. When plants do a lot of photosynthesis, the oxygen produced during the
light reactions competes with CO2 for the ribulose biphosphate (RuBP) enzyme. The light
reactions of C4 plants occur in mesophyll cells which surround the veins’ enlarged and modified
bundle sheath cells. The Calvin cycle occurs in chloroplasts of the enlarged bundle sheath cells.
This separation of reactions keeps oxygen away from the cells performing Calvin cycle steps. C4
photosynthesis has several benefits for the plant, resulting in a more efficient rate of
photosynthesis. It also results in an interesting modification of the typical leaf anatomy.
Observing a C4 leaf
Corn (Zea mays) is a C4 plant. Observe again the prepared slide of a corn leaf to see the
differences in C3 and C4 leaf structure.
• Note especially the layer of round cells which surround the veins in the corn leaf.
This layer is formed by the bundle sheath cells, which contain the chloroplasts in which the
Calvin cycle occurs.
• Note, too, that the mesophyll cells are not separated into well-defined palisade and spongy
mesophyll layers, such as you observed in the Ligustrum leaf. In the corn leaf, the mesophyll
cells surround the bundle sheath cells. Only the light reactions of photosynthesis occur in the
chloroplasts of the mesophyll cells. This C4 leaf structure is known as Kranz anatomy.
Corn leaf, xs Chloroplasts from bundle sheath cell (left) and mesophyll cell (right) of corn leaf.
• Observe the electron micrographs of the C4 mesophyll and bundle sheath cell chloroplasts
shown above. Note the different chloroplast structures in the two cells.
Why does the mesophyll cell have chloroplasts containing lots of grana composed of many
thylakoid layers? Why are well-developed grana absent in the chloroplasts of the bundle sheath
• Note the many plasmodesmata which connect the two cells. Why would you expect to see so
many plasmodesmata between the mesophyll cells and the bundle sheath cells in the C4 plant?
The (Main) PHOTOSYNTHESIS CENTER
Leaf Observations: Top, Bottom and Cross Section Observation
Objective: To carefully observe several leaves for defining characteristics and match these
characteristics (as much as possible) to the traditional labeling identified and shared by
Materials: Various leaves, tray, dissection kit, compound and dissecting microscope,
Procedure: Draw and describe the leaf of 1) a typical maple or oak, 2) a leaf of your choice
from three perspectives
a) Top view (the side most facing the sun)
b) Bottom view (the side facing away from the sun)
c) *Cross Section (a cut section of the leaf)
*In order to create a cross section of your leaf to view record the steps in the space below.
Draw the cells and veins you see of a cross section of a leaf. Label them.
Observations and Results
Leaf of Choice
How were the various leaves you observed similar?
How were the various leaves you observed different? What are the purpose of the veins in leaves?
Do plant cells go through cellular respiration? Explain (including an explanation of where the food used in this
process comes from).
Conclusion: What did you learn?
Stomata Part 2: Observation of leaf Stomata
1. Choose a leaf from one of the plants. DO NOT REMOVE THE LEAF FROM THE PLANT!!
2. Paint a thick patch (at least one square centimeter) of clear nail polish on the underside of the leaf surface being
3. Allow the nail polish to dry completely for several minutes.
4. Tape a piece of clear cellophane tape to the dried nail polish patch.
5. Gently peel the nail polish patch from the leaf by pulling on a corner of the tape and “peeling” the fingernail
polish off the leaf. This is the leaf impression you will examine.
6. Tape your peeled impression to a very clean microscope slide. Use scissors to trim away any excess tape. Label
the slide with plant name.
7. Examine the leaf impression under a light microscope at 400X.
8. Search for areas where there are numerous stomata, and where there is no dirt, thumb prints, damaged areas,
or large leaf veins. Draw the leaf surface as it appears in your field of view.
Then make a detailed drawing of ONE open stomata and one closed.
9. Count all the stomata in one microscopic field. Record the number on your data table.
10. Repeat counts for at least three other distinct microscopic fields. Record all the counts. Determine an average
number per microscopic field.
Number of Stomata per Leaf
Stomata in field 1
Stomata in field 2
Stomata in field 3
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