Name
LAB
Date
What is in a Leaf?
Block
Honors Biology, Newton North High
OBJECTIVES:
 Recognize each of the tissue types and structures found in leaves and explain what they do.
 Recognize the differences between monocot and dicot leaves.
BACKGROUND INFORMATION:
Leaves are what plants are all about; they are the sites of photosynthesis. They must perform a delicate
compromise between gas exchange and evaporative water loss, taking in enough carbon dioxide for
photosynthesis while limiting excessive water loss. Some evaporation is necessary for transpiration, but too
much will kill the plant. Leaves also require vascular tissue to transport water and inorganic nutrients into the
leaf and to transport the sugars produced through photosynthesis out to the rest of the plant. The leaves of
vascular plants contain several different specialized types of tissues, which interact to make the functions
organ we call a leaf. You should become familiar with each type of cell and what it does.
While leaves show a variety of shapes, they all do more or less the same job. Some of the differences in shape
can be understood in terms of the conflicting requirements that leaves face: absorbing light, exchanging gases,
avoiding dehydration, and avoiding predation. Within flowering plants (angiosperms), there are two large
groups with different styles of leaves. Dicots (roses, for example) have leaves with a netlike, branching system
of veins. Monocots (grasses, for example) have parallel veins. See the picture below for a comparison. Later in
this lab you will see more differences between dicots and monocots.
Monocot leaf
Dicot leaf
PROCEDURE:
Part One: Dicot Leaf
1. Examine the cross section of the leaf of a Syringa (Lilac) below. You should be able to recognize the guard
cells and their stomata (openings) as well as three major regions: the epidermis (dermal tissue) forming a
N. Berg, NNHS © 2015-6
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single layer over both top and bottom surfaces, the mesophyll (ground tissue), and the vascular bundle
(veins).
Epidermis: A single layer of cells on the top and bottom of the leaf. Each cell has a nucleus,
which may be visible as a dot in the cell. However, this is a thin slice of some large cells, and in
many cases the slice does not happen to include the nucleus. The epidermis secretes the cuticle,
a waxy layer that surrounds the outside of the leaf. The cuticle is not visible in these slides.
Stoma (plural: stomata): Openings in the leaf to allow for gas exchange. Stomata are created by
guard cells, which can expand or contract to open or close each stoma. Closing the stomata
reduces water loss, but can also slow photosynthesis by preventing CO2 uptake. The guard
cells are another example of dermal tissue.
Mesophyll (ground tissue): Mesophyll means “middle of the leaf,” and these cells fill most of
the leaf’s volume. They are the primary sites of photosynthesis, and they are filled with
chloroplasts, which are visible as small dots. In dicots, the mesophyll layer is divided into two
types: palisade mesophyll and spongy mesophyll. Palisade mesophyll is located directly
under the upper epidermis. It has the greatest concentration of chloroplasts. The spongy
mesophyll is located directly under the palisade mesophyll, toward the bottom of the leaf. The
spongy mesophyll cells are surrounded by air spaces that allow circulation for gas exchange.
Vascular bundles (vein): contain xylem and phloem. Xylem forms the system for transporting
water and inorganic nutrients from the roots all the way up to the leaves. This transport
sometimes involves powerful pressure gradients, so the xylem cells are heavily reinforced. You
can recognize them in this slide because they have very thick walls. The phloem cells help do
the job of transporting sugars produced in photosynthesis. Phloem transport typically involves
weaker pressure gradients than xylem transport. In contrast to xylem cells, the phloem cells
are smaller and thinner-walled. In a vascular bundle of the leaf, the xylem is usually on top and
the phloem on the bottom.
2. Label the cross section of the Syringa (Lilac) leaf below.
LAB: What’s in a Leaf?
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Part Two: Monocot Leaf
1. Obtain and observe a prepared slide of a Zea (corn) leaf cross-section under the high power using a light
microscope.
2. Sketch the Zea leaf cross-section below.
a. Label the following leaf structures: upper epidermis, lower epidermis, guard cells, stoma,
mesophyll, and vein.
3. Using complete sentences, answer the following questions based on your observations.
a. Varying position of stomata is one way that plants have adapted to increase water use efficiency.
Leaves can be described as hypostomatic, epistomatic, or amphistomatic. Like many scientific
words, these words contain Latin prefixes. In Latin, hypo- means “below”; epi- means “above”; and,
amphi- means “both”.
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i. What type of leaf is Syringa in regards to its stomata? Why?
ii. What type of leaf is Zea in regards to its stomata? Why?
b. What is different about the mesophyll in a Zea leaf compared to that of the Syringa leaf?
c. There are different types of plants in regards to their photosynthetic pathway. Most plants carry
out C-3 photosynthesis, in which both the light dependent reactions and the Calvin Cycle occur
within the same chloroplasts in all of the mesophyll cells. Other plants have adapted to extremely
hot, bright conditions by carrying out C-4 photosynthesis in an attempt to minimize water loss
while still allowing photosynthesis to take place in intense sunlight. During C-4 photosynthesis,
CO2 is stored in 4-carbon compounds so that the plant does not need to keep the stomata open at
all times. Some C-4 plants also separate the reactions of photosynthesis into different chloroplasts
within different types of cells, another energy conserving measure. The light independent reactions
occur in the mesophyll cells, while the Calvin cycle occurs in enlarged bundle sheath cells that
surround the vascular bundle.
i. Compare the cross-sections of the Zea and Syringa leaves. Which do you think is an
example of a C-4 plant?
LAB: What’s in a Leaf?
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Part Three: Pine Needle
The leaves of Pinus (pine) trees are called needles. Though their shape is different from the leaves of most
angiosperms, they contain more or less the same tissue types. Pines often live in harsh conditions: hot, dry
summers and freezing winters. They are good at withstanding environmental stress. Their needles, with a low
surface area-to-volume ratio, help reduce damage due to drying out or heavy snows.
Pine needles also have some features not seen in Syringa leaves. Transfusion tissue surrounds the vascular
bundle, and apparently helps transport materials into and out of the vascular tissue. This tissue is abundant in
pine needles, but not in most leaves of flowering plants. Resin ducts carry resin, which is a hydrocarboncontaining substance that may help protect the leaves. The cuticle is a waxy layer around the outside of the
pine needle. The stomata are sunk into small pits in the epidermis.
1. Label the following in the Pine needle cross-section below: epidermis, mesophyll, stoma
2. Using complete sentences, answer the following questions:
a. What type of macromolecule is produced by resin ducts? How would it aid in protecting the
Pinus?
b. Sunken stomata are often referred to as stomata crypts. How do they aid the Pinus in
surviving hot, dry summers?
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EXTENSION QUESTIONS: Use complete sentences for question #3.
Some leaves have adaptations for surviving in extreme environments:
(1) Mesophytes are plants whose leaves are not adapted to extreme environments. An example is the
Syringa leaf.
(2) Xerophytes are plants adapted to arid environments.
(3) Hydrophytes are plants adapted to grow in water.
(4) Sclerophytes are plants adapted to resist animals, freezing temperatures, and ultraviolet light.
1. Below is a cross-section of Nerium oleander.
The epidermis is several layers thick and is covered by a thick cuticle. On the bottom of the leaf
stomatal crypts can be seen. They contain hair-like structures called trichomes.
What type of plant would you classify Nerium oleander? ________________________________
2. Below are cross-sections of the Yucca plant.
The leaves are tough and fibrous with a thick cuticle. Thick mesophyll surrounds the vascular bundles.
The stomata occur in long narrow grooves between the fiber bundles. The vascular bundles have
fibrous sheets.
What type of plant would you classify the Yucca? _____________________________________
3. The Nymphaea (water lily) is a hydrophyte. It has broad, flat leaves that
float on top of the water.
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a. Identify two structural changes that you might see in any of the four regions of its leaf:
epidermis, stomata, mesophyll, and vascular bundles.
b. Explain why each structural change is an adaptation for the plant leaf.
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