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Monocots vs. Dicots: Leaves
LAB MANUAL
OBJECTIVES
1. Differentiate between dicots and monocots by:
a. Stem structure
b. Leaf structure
2. Understand the development and arrangement of angiosperm tissues
3. Identify leaf arrangements
LAB PREPARATION
1. Bring in completed assignments.
2. Read dichotomous key handout posted on Laulima. Google plant terms and characteristics that
you do not know.
3. Read and study this laboratory handout.
4. Bring your Photo Atlas for Biology book.
5. Bring your personal protective gear (closed-toed shoes, lab coat)
MONOCOTYLEDONS AND DICOTYLEDONS - A Review
In addition to the number of cotyledons, monocots and dicots normally have other morphological
differences that you can use to distinguish between them (Table 1 and Figure 1). Please note, that the
characteristics of leaf venation and flower parts are not absolutely reliable. Occasionally you may
encounter a monocot with net venation or a dicot with three flower petals. Be sure to use a combination
of characteristics to confirm your determination.
Monocots
1 cotyledon
Dicots
Roots
Stems
Stems
Leaves
Flowers
Usually fibrous root
system (may have
started with a taproot)
Vascular bundles
in complex
arrangement
Does not produce
vascular cambium
Veins
usually
parallel
Flower parts in
3’s, or multiples
of 3’s
Taproot usually
present
Vascular bundles
arranged in a ring
Can produce
vascular cambium,
secondary (woody)
growth common
Veins
usually
branched
or net-like
Flower parts
generally
arranged in
multiples of 4’s
and 5’s
2 cotyledons
Table 1. Comparison of monocot and dicot characteristics
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Figure 1. Comparison of monocot and dicot traits
Vascular Cambium and Secondary Growth
In plants, increase in girth is called secondary (or 2o) growth. Except for a few notable exceptions (ex.
Palm trees), monocots do not increase in girth. When they do, it is not referred to as secondary growth
since the growth process does not involve a vascular cambium.
In dicots, plants that undergo secondary growth are said to be woody, as opposed to those that do not,
which are called herbaceous. The process of becoming woody begins when the procambium between the
xylem and the phloem (in the vascular bundles) continues to divide (producing more xylem and phloem),
and merges with quiescent meristematic cortical tissue between the bundles. This forms a continuous
cylinder of meristematic tissue called the vascular cambium. As the cells of the vascular cambium
continue to divide, those cells that are pushed to the inside differentiate into xylem, while those pushed to
the outside differentiate into phloem. This new xylem and phloem is referred to as secondary xylem and
secondary phloem, distinguishing from the primary tissues in the original vascular bundles.
Figure 2. Secondary growth
in woody dicots
Eventually, the secondary
xylem and phloem become
the dominant tissues in the
plant. If the plant
experiences a variation in
temperature or rainfall over
the year (usually associated
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with seasonal change), the morphology of the secondary xylem can change slightly, producing the
familiar annual rings of tissue observed when a tree is cut down. These rings of xylem are collectively
called wood. In contrast, the layers of annual phloem tissue normally change very little in morphology.
As the girth of a plant continues to increase, the growing epidermis that originally covered the plant
normally can’t keep up. Eventually, it is stretched and broken. As a result, the plant needs to produce a
new skin. Cells in the underlying cortex then become active, dividing to produce a ring of tissue called
the periderm. The actively dividing cells of the periderm are called the phellogen or cork cambium
(Fig. 2). These cells produce 1-5 layers (usually 2) of
cells to the inside which are called phelloderm, and
many, many layers of cells to the outside called
phellem, or cork (Fig. 2). While the phelloderm
remains alive, the phellem, which is composed of cork
cells (containing suberin), dies and collapses, forming
a new water-impermeable skin to protect the growing
woody plant. All of the tissues outside the vascular
cambium are collectively referred to as bark.
Figure3.Commonleafarrangements
In the first lab, we learned to prepare wet mounts of cork.
Cork is the phellem layer of bark tissue harvested from an oak tree, Quercus suber, which has a thick
layer of bark that helps it to recover quickly after forest fires. The cork cambium layer of bark becomes
quiet thick, leading to its usefulness as a source of cork. Cork can be harvested every seven to ten years
from the tree without harming the tree, making it a sustainable resource. Cork is widely used in the wine
industry, but is also used in a multitude of ways, including insulation, floor materials, wall tiles, paper,
clothing, and many other uses. Workers shave off the cork without cutting into the phellogen layer.
LEAVES
Leaves, perhaps more than any other plant organ, vary greatly in external form and internal structure. As
the main photosynthetic organs of the plant, leaf morphology is influenced by the amount of sunlight they
receive. Additional environmental factors, such as water availability, wind, temperature, and herbivores,
also affect the morphology and arrangement of leaves on a plant.
Leaves are the primary location for photosynthesis. A large surface area is desired to maximize light
absorption and special openings, called stomata (singular: stoma), in the leaf surface function in gas
exchange with the atmosphere. Plants utilize carbon from carbon dioxide in the air to produce
carbohydrates. Water will also evaporate through the stoma in a process called transpiration. As water
vapor is lost by transpiration in the leaves, more water is pulled into the roots and up the plant body via
the xylem. Read about transpirational pull (Resource Acquisition and Transport in Vascular Plants) in
the Campbell textbook.
While transpiration is an integral component of the mechanism responsible for water movement
throughout the plant, excessive water loss can result in dehydration (wilting) and plant death. For this
reason, plants have adaptations that minimize or control water loss in most plants.
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Morphology of leaves
The blade, or lamina, of a leaf is the large flat part. The margin is the end of the leaf. The leaf can either
be directly connected to the stem or attached by the stalk-like petiole. In some leaves, the petiole has two
stipules, small leaf-like flaps, at its base. Vascular bundles go from the stem through the petiole into the
veins of the leaf. The mid-rib in the center of the leaf branches to produce veins formed with vascular
tissue. The veins are arranged in patterns, also called leaf venation.
Simple Leaves
Leaves are divided into simple leaves (Fig. 3) and compound leaves (Fig.
4 and 6). In simple leaves, there is a single blade. Simple leaves will have
very different shapes (see the leaf reference sheet on your lab bench for
examples) with varying margins.
Leaves are attached to the plant stem in patterns. Plants arrange their
leaves to maximize exposure to sunlight. Leaves are attached at nodes on
the stem.
Figure 4: Anatomy of a simple leaf
Common Leaf Arrangements:
Opposite: Two leaves attached at the node on
opposite sides
Alternate: one leaf per node with the second leaf
being above the first but attached on the opposite
side of the stem or where the successive leaves
attach above the first in a spiral – a variation of
alternate leaf arrangement.
Whorled: three or more leaves are attached at one
node
Figure5.Commonleafarrangements
Compound Leaves
In compound leaves, the leaf is divided into multiple leaflets. There
are two main types of compound leaves: pinnate and palmate. In
pinnate laves, each leaflet has its own stalk, called a petiolule,
attaching it to a rachis. In palmate leaves, the leaflets radiate out from
the tip of the petiole (like the fingers of your hand) and lack a rachis.
Figure 6. Compound leaf structure
Sometimes it may be unclear whether a leaf is a compound leaf or a
cluster of several simple leaves. When in doubt, remember that
simple or compound leaves have axillary buds (lateral buds) where
they attach to the stem and leaflets do not.
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Like simple leaves, each compound leaf is
then arranged around the plant stem. The
common leaf arrangements, opposite,
alternate, and whorled are described above.
Internal Anatomy of the Typical Leaf
Figure 7. Common types of compound leaves
The typical components of a leaf (Fig. 5) are
discussed sequentially (as they occur from top
to bottom) in the following sections. As you read, refer to the typical dicot leaf (Fig. 5 top). Keep in
mind, however, that there are many variations of this basic plan.
The cuticle is a hydrophobic layer secreted atop the epidermal cells. This waxy layer functions to reduce
water loss from the leaves. The epidermis is the outermost cell layer of the leaf and originates from the
protoderm. It provides function and regulates gas exchange through openings called stomata. The Greek
word stoma means mouth. The stoma, the opening, has specialized guard cells surrounding it that open or
close the stoma. Typically, stomata is located on the lower epidermis to reduce water loss and the number
of stomata may depend on the environment around the plant.
The bulk of leaf cells are chlorenchyma tissue called the mesophyll. In dicots, the mesophyll is divided
into two layers, the palisade layer of elongated cells containing chloroplasts, and the spongy layer that is
more loosely packed to allow oxygen and carbon dioxide to move through.
Vascular bundles run
through the veins of the
leaf bringing in water and
minerals from the roots
and transporting sap out
of the leaf.
Figure 8: Cross-sections of a dicot leaf (top) and monocot leaf (bottom)
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Figure9:Examplesofleafcharacteristics
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LABORATORY EXPERIMENT
[STEM SECONDARY GROWTH] Tilia, older stem, Vascular cambium and secondary growth
1. Examine the prepared slide labeled “Tilia, older stem” with your compound microscope at low
power.
a. At this stage in the growth of a dicot, the vascular bundles are no longer present. Instead,
we have one ring of meristematic cells, the vascular cambium, which now produces
secondary xylem and secondary phloem.
2. Switch to a higher magnification and identify the pith in the center of the section.
a. Directly outward from the pith is a thin layer of xylem tissue. This is what is left of the
primary xylem.
b. Proceeding further out you will encounter the first season’s growth of secondary xylem,
which you can identify by the radiating xylem rays composed of parenchyma cells, and
the larger vessel members present.
3. Switch back to a lower power to see the annual rings of wood.
a. How old was the plant before it was sectioned?
b. At the outer extreme of the secondary (2o) xylem is the vascular cambium (1-2 cells
thick).
c. The secondary phloem begins immediately outside the vascular cambium. Notice that it
is a complex tissue made up of several cell types.
d. There are sieve tube members (empty looking cells), fibers (extremely thick walled cells
stained pink-purple in this section), parenchyma cells, and companion cells.
e. Notice that the sieve tube members, fibers, and associated cells only make up part of the
2o phloem.
f.
In between are large wedge-shaped areas dominated by parenchyma cells, these are
phloem rays.
g. At the very outer extreme of the secondary phloem are some sieve tube members that
appear crushed and torn, this is what remains of the primary phloem.
h. Moving further away from the stem’s center is the much reduced in size cortex. It too
has been ripped and torn, unable to keep up with the expansion due to secondary growth.
i.
Examine the periderm from the outside in. There are several layers of flat reddish cells
exposed to the outside. These are cork cells that make up the phellem.
j.
If you continue inward, the phellogen (cork cambium) is the first layer of living, lessflattened cells. The phelloderm can be found in the layers (1-3) that follow immediately
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after the phellogen. The 2o phloem along with all the tissues outside it, make up the bark
of the woody dicot.
[LEAVES] Arrangements and VENATION
4. Examine the monocot plants on display.
a. Record the scientific and common name of the plant in your notes.
b. Identify the stem, leaves, and flower (if applicable).
c. Sketch and label the plant and a close-up of a leaf and its venation. Describe its shape,
venation pattern and margin.
5. Examine the dicot plant on display.
a. Record the scientific and common name of the plant in your notes.
b. Identify the stem, leaves, axillary buds (lateral buds) and flower (if applicable).
c. Sketch and label the plant and a close-up of a leaf and its venation. Describe its shape,
venation pattern and margin.
[LEAVES] Cross-section
DICOTS: Pre-prepared microscope slide of leaf cells
6. Examine the prepared slide of a typical dicot leaf under a compound microscope. Identify:
cuticle
upper epidermis
stomata
palisade mesophyll
spongy mesophyll
lower epidermis
air spaces
7. Identify their xylem, phloem, and the bundle sheath cells surrounding them. Notice that not all
bundles lie in the same plane. Some are in cross-section while others are longitudinal. How can
you explain this? What is the function of the xylem, phloem, and bundle sheath cells?
MONOCOTS: Pre-prepared microscope slide of leaf cells
8. Examine the prepared slide of a typical monocot leaf under a compound microscope. Identify:
cuticle
upper epidermis
stomata
mesophyll cells
vascular bundles
air spaces
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Leaf Epidermis Slide MUST SHOW SLIDE TO TA OR TI
9. Obtain a piece of leaf and peel off a small strip of the LOWER, purple epidermis. (If you have
difficulty obtaining only the epidermis, ask for help).
10. Quickly mount the epidermis in a drop of water on a microscope slide before it dries out, making
sure that the outside of the peel is facing up.
11. Cover the peel with a coverslip and examine it with your compound microscope.
12. Locate and identify a stoma. Is it open or closed?
13. Examine the guard cells surrounding a stoma and the normal epidermal cells that make up most
of the peel. How do these two cell types differ? Can you identify the subsidiary cells adjacent to
the guard cells?
[DISPLAY OF PLANT ADAPTATIONS]
Angiosperms have developed modifications in their stems, roots, and leaves during the course of
evolution to gain certain advantages in particular environments. Look through the examples on display
and record what you see in notes. You should make note of the different modifications on display, but
you can just use the common names. You do not need to memorize the scientific names.
Dichotomous Key and Campus Botanical Tour
Next week Your TA will be taking you for a short walk around campus. You will learn how to utilize a
dichotomous key to identify organisms and review how to distinguish monocots and dicots.
HOMEWORK:
Paperduenextlab:2.5-5.0pagesdouble-spaced,typed,usingaminimumoffourexternalreferences
(citedcorrectly)outsideoflabmanualand/ortextbook.Chooseaplantorfungusspeciessuggestedas
invasivetoHawaii.
“Howhasthisspeciesinfluenced(positivelyandornegatively)theHawaiianislandsandevaluateany
efforts(currentorproposedforthefuture)forcontroloreradication?”
•
•
•
•
Somebackgroundyoumaywanttoincludetoinclude:
Whatisaninvasivespecies?
Background(lifecycle,habitat,knownpredators)
Historyofintroduction/distribution
Environmentalimpacts(onnativespecies,environment,and/orhumans)
Evaluatecurrentmethods(control/eradication/ofinvasivespecies)orproposeyourown.
Determineanyrisksorconsequences
Attach outline and submit with paper.
Rubric for paper found on Laulima.
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GROUPTalk(10minutepresentationworth30“Assignment”Points)
Asagroup,youwillcompareandcontrastthespecieswithinyourgrouptoevaluateandranktheir
impactonHawaii
Asagroup,whichplantdoyouconsidertobethebiggestthreattoHawaii’secosystem.
Ranktheplantsinyourgroup.
Eachgroupmemberwillintroduceandgivebackgroundontheirinvasiveplant/fungus(1.5-2minutes
each)andthenyourgroupwillpresenttheiroverallrankingandtheirreasonswhy.You’vealreadydone
thelegworklearningaboutyourinvasivespecies.Whatwe’reinterestedishavingyousharewhatyou
learnedwiththerestoftheclassandseeinghowyouworkasagrouptocompareandcontrastyour
plantexamplesandcometoaconclusiontogether.There’snorightorwronganswer,butweare
interestedinwhatfactorsyouconsideredimportant.
Comepreparedbuthavefunteachingyourfellowclassmatesabouttheplantthatyou’vespentsomuch
timestudying.Youareallowedtobringupasmanynotecardsasyouwantbutyoumaynottakeyour
paperupwithyouoryouwilllosepoints.Yourtalkshouldbeconciseandinformative,timewillgofast
andyouwanttokeepyouraudienceattentive.
ThegroupshouldhaveasimplePowerPointorvisualaidready.Itshouldhavelargephotosofcanoe
plantsbeingdiscussedandminimaltext.
Eachpresentationwillendwithaquestionandanswersection.Everyoneshouldbepayingattention
andthinkingofthoughtfulquestionsbasedonwhatyou’rehearing.Presentationtimewillvary
dependingonthegroupsizebutpracticeandwatchyourtime.YourTAmaycutyouoffifyougotoo
long.
Printouttherubrics(nextpage)andbringwithyoutoclass:
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GroupMembers:
_____________________________________________________________________________
TA:_________________________________
CanoePlantGroupRubric
Total30pts
GroupPortion(16/20pts)x1.5
Groupx1
4
3
2
1
Readiness
Powerpointor
VisualAidUsed
Visual
Presentation
Presentationis
visuallyorganized
andcomplete
Presentationis
organizedand
complete
Presentationis
almostcomplete
Presentationis
disorganized
and/or
unfinished
Overallconclusion
Reasoningfor
groupconclusion
demonstrates
thoughtfulness
andunderstanding
ofplantsand
voyagingneeds,
provides
supportingpoints
Reasoningfor
group
conclusionis
logicaland
supported
Reasoningfor
groupconclusion
isnotfully
supportedor
explained
Reasoningfor
groupconclusion
isminimal
and/orshallow
DiscussionPortion
Groupasks
thoughtful
questionsduring
other
presentations
and/or
understandsand
answersquestions
posedtotheir
groupwith
competenceand
confidence
Asksquestions
duringother
presentations
andableto
answer
questions
withinreason
posedtotheir
group
Askquestions
thataretoo
simplisticor
unrelated(asking
aquestionjustto
askaquestion)
and/orgives
unclearor
incomplete
answersto
questionsposed
totheirgroup
Doesnotaskor
onlyasksafew
questionsand
doesnotgive
reasonable
answersto
questionsposed
totheirgroup
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IndividualSpeakerName:____________________________________________
CanoePlant:______________________________________________
TA:__________________________________________
IndividualPortion(4/20pts)x1.5=
Individual
4
3
2
1
Speakerisvery
informativeand
prepared,uses
notecards
minimallyornot
atall,andspeaks
foran
appropriate
lengthoftime
Speakeris
confidentabout
theircanoeplant
anduses
notecardsonly
forreference,
doesn’tspeakfor
tooshortortoo
long
Speakeris
confidentabout
canoeplant,but
readsdirectly
fromnotecards
orpaper,and/or
speaksfortoo
longortooshort
comparedto
othergroup
members
Speakerdoesnot
havenotecards,
readsdirectly
frompaperand
doesnotseem
prepared