Transpiration
• Transpiration is the major mechanism that
drives the movement of water through a
plant. In the first section of this laboratory
you will investigate factors that influence
the rate of transpiration. In the second
section you will study plant anatomy as it
relates to transport.
• To do this laboratory, you should
understand the basic concepts of water
potential. You can review these by looking
back at Lab 1. Diffusion and Osmosis.
Movement of Water Up Xylem
Vessels
• When water enters the roots, hydrogen bonds link each
water molecule to the next so the molecules of water are
pulled up the thin xylem vessels like beads on a string.
The water moves up the plant, enters the leaves, moves
into air spaces in the leaf, and then evaporates
(transpires) through the stomata (singular, stoma).
Hydrogen Bonding
• Recall that a hydrogen bond is a
weak interaction between a
hydrogen atom of one molecule
and, in this case, the oxygen of
another molecule.
• Water is a polar molecule, with
the region around the oxygen
atom having a slight negative
charge and the regions around
the hydrogen atoms having a
slight positive charge. In water,
the negative regions on one
molecule are attracted to the
positive regions on another, and
the molecules form hydrogen
bonds.
The Process of Transpiration
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There are hundreds of stomata in
the epidermis of a leaf. Most are
located in the lower epidermis.
This reduces water loss because
the lower surface receives less
solar radiation than the upper
surface. Each stoma allows the
carbon dioxide necessary for
photosynthesis to enter, while
water evaporates through each
one in transpiration.
Now that you have been
introduced to the concept of
transpiration, the questions on the
following page will help you
understand how the properties of
water and water potential are
important to this process.
Test Yourself
• 1. What property of water accounts for the fact that
molecules of water "grab" the walls of the thin xylem
vessels?
• 2. Water molecules pull each other like beads on a
string; as one molecule is evaporated through a stoma,
another is pulled up. What property of water is
demonstrated here?
• 3. Place the following in order (list the letters), from
highest water potential to lowest water potential, under
normal conditions. a. Mesophyll of leaves
b. Outside air around leaf
c. Spaces around roots
d. Inside the xylem vessels
How Do Guard Cells Function?
• Guard cells are cells surrounding each stoma. They help to regulate
the rate of transpiration by opening and closing the stomata. To
understand how they function, study the following figures. As you
look at the figures, keep in mind that an increase in solute
concentration lowers the water potential of the solution, and that
water moves from a region with higher water potential to a region of
lower water potential.
Guard Cell Function
• Stomatal closing
1.Potassium ions move
out of the vacuole and out
of the cells.
• 2.Water moves out of the
vacuoles, following
potassium ions.
• 3.The guard cells shrink
in size.
• 4.The stoma closes.
• Stomatal opening
1.Potassium ions move
into the vacuoles.
• 2.Water moves into the
vacuoles, following
potassium ions.
• 3.The guard cells expand.
• 4.The stoma opens.
Equation for photosynthesis
• By now you should have the basic
equation for photosynthesis fixed in your
memory!
• Write it Here:
Balance
• A leaf needs carbon dioxide and water for
photosynthesis. For carbon dioxide to enter, the
stomata on the surface of the leaf must be open.
As you have seen, transpiration draws water
from the roots into the leaf mesophyll. However,
the plant must not lose so much water during
transpiration that it wilts. The plant must strike a
balance between conserving water and bringing
in sufficient amounts of CO2 for photosynthesis.
Factors That Affect the Rate of
Transpiration
• Transpiration is affected by various conditions in the
plant's environment. The following exercises help you
think about some of these effects before you test them in
your laboratory experiment.
• 1. Which condition would result in the higher rate of
transpiration light or dark? Explain
• 2. Which condition would result in the higher rate of
transpiration humid environment or dry environment?
• 3. Which condition would result in the higher rate of
transpiration breezy conditions or still air?
• 4. Which condition would result in the higher rate of
transpiration hot environment or warm environment?
Design of the Experiment
• A potometer is a device that
measures the rate at which a
plant draws up water. Since
the plant draws up water as it
loses it by transpiration, you
are able to measure the rate of
transpiration. The basic
elements of a potometer are:
• •A plant cutting•A calibrated
pipette to measure water
loss•A length of clear plastic
tubing•An air-tight seal
between the plant and the
water-filled tubingYour
instructor will show you how to
assemble your potometer.
An Overview of the Experiment
How to Calculate Leaf Surface Area
• The rate of transpiration is measured as the
amount of water lost/ square meter/ minute.
Because water evaporates through the many
stomata on the leaf surface, the rate of
transpiration is directly related to the surface area.
To arrive at the rate of transpiration, therefore, you
must calculate the leaf surface area of each plant:
Because most stomata are found in the lower
epidermis, you will determine that surface area.
• •Lay the leaves to be measured on a 1-cm grid and
trace their outlines.•Count the number of square
centimeters. Estimate the area of the partial
squares. (Here's a simple method for this estimate:
Count a partial square if it is at least half covered
by the leaf; do not count partial squares that are
less than half covered.)•Do not include the area of
the stem (petiole) in your calculations.
What is the surface
area of this leaf in
cm2?
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1. If guard cells in a plant were deficient in K+, which of the following would be most likely to
occur?
a.
Wilting would become more likely.
b.
Photosynthesis would decrease.
c.
Transpiration would increase.
d.
Food transport would decrease.
Match the letter and number
• a.phloem
b.mesophyll
c.epidermis
d.xylem
3. Which line
indicates a
guard cell?
4. Which type of cells
are indicated by
label line 2?
• a.
parenchyma
b.xylem
c.phloem
d.epidermis
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5. Several factors account for the movement of water up xylem vessels. Which single
factor is most important in pulling water toward the top of a tall tree?
a.Evaporation of water through stomata
b.Osmosis in the root
c.Capillary action
d.Atmospheric pressure
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6. All of the following enhance water transport in terrestrial plants EXCEPT:
a.Hydrogen bonds linking water molecules
b.Capillary action due to adhesion of water molecules to the walls of xylem
c.Evaporation of water from the leaves
d.K+ being transported out of the guard cells
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7. Under conditions of bright light, in which part of a transpiring plant would water
potential be lowest?
a.Xylem vessels in the leaves
b.Xylem vessels in the roots
c.Root hairs
d. Spongy mesophyll of the leaves
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8. What type of environment would result in the greatest rate of transpiration?
A. Cloudy, humid conditions
b.Warm, humid conditions
c.Warm, light-breezy conditions
d.Cool, humid conditions
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In your own laboratory, you will examine slices of a plant stem under the microscope and identify the different types of tissue and their
functions. You've already seen a dicot. The following exercise will give you practice in identifying plant tissues in a monocot.
• Identify each of the structures in the micrograph of a monocot stem
above by answering the following questions using the appropriate
choices from the list. When you are finished, check your answers.
• Choices for Tissue Type: xylem, phloem, parenchyma, epidermis
• Choices for Function: food transport, water transport, food storage,
protection
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Help Wanted!
Different parts of plants do different jobs. Read the classified advertisements and help each
of the plants parts listed below to find the right job.
Cambium, guard cells, phloem, root cap, sepals,cuticle,petals,stamen,pistil,chloroplasts, tap
root, xylem
Advertizing Executive Colorful personality needed to advertise availability of pollen and
nectar. Must have experience working with bees. Door Attendant Full-time positions
available at entrance to stomates Waiters/Waitresses Deliver food to hungry plant cells.
Work in busy roots, stems and leaves. Anchorperson Interested in holding a plant in place?
If you have experience digging deep in search of water, we're looking for you. No branching
necessary. Egg Farmer Female needed to manage egg production and receive pollen
Chemist Person needed who can convert carbon dioxide and water into glucose. Must wear
a green uniform to work and enjoy working in the sun. Bodyguard Help needed to protect
buds. Apply before spring Hardhats Construction workers. Drill for water. Protect other
members of water-search team will drilling. Apply at root tip Pollen Production Assistant
Help needed to produce pollen. Seasonal work only in the spring. Conductor Individual
needed to carry water. Rapid advancement. Start at the roots and work up to the top!
Outdoor Work Protect and cover upper and lower leaf surfaces. Must be able to prevent
water loss and seepage. Apply at the leaf. Plumber Vascular specialist needed to lay new
plumbing each growing season. Experience making new xylem and phloem necessary.
Potential for Advertising Executive_______________________
Anchorperson ______________________________
Egg Farmer_______________________Chemist ______________________________
Bodyguard_______________________
Hardhats______________________________Pollen Production Assistant
_______________________
Conductor______________________________Outdoor Work_______________________
Plumber ______________________________DoorAttendant________________________
Waiter/Waitress _______________________________
Flower Anatomy
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Flower Structure and Reproduction Flowers are the plant's reproductive structures. Angiosperms are types of plants that
bear fruits and flowers. Flowers are usually both male and female, and are brightly colored to attract insects to help them
carry pollen used for sexual reproduction. Not all flowers are colorful, though. These flowers usually use the wind for
pollination.
Parts of the Flower
The receptacle is the part of the branch on which a flower forms. Color the receptacle (B) brown. Sepals are leaf like
structures that surround and protect the flower before it blooms. Color the sepals (C) green. Petals are the colorful part of
the flower that attracts insects and even other small animals, such as mice, birds, and bats. Color the petals (D) a bright
color of your choice. All flowering plants have flowers, but some are not brightly colored. The petals of these flowers are
reduced or absent and the plant relies on the wind or water for pollination.
The flower has both male and female reproductive parts. The female reproductive structures are called carpels. In most
flowers, the carpels are fused together to form a pistil. Color the pistil (P) pink. The pistil has three parts, which can be
seen, in the box labeled "pistil". The stigma at the top is often sticky and is where the pollen attaches. Color the stigma (J)
purple. The style is the long tube that attaches the stigma to the ovary. Sperm from the pollen will travel down this tube to
the ovules. The ovules, or eggs, are stored in the ovary until they are fertilized. Plants can only fertilize eggs of the same
species. Special chemicals prevent sperm from fertilizing the eggs of flowers that are not the same kind. Color the style (K)
red, and the ovary (L) pink. Color the ovules (O) black.
The male reproductive structures are called the stamens. Color the stamens (H) blue. Each stamen consists of an anther
(A), which produces pollen, and a filament (F), which supports the anther. In the box labeled "stamen" color the anther dark
blue, and the filament light blue. Pollen produced by the anther is carried by insects or other animals to the pistil of another
flower where it may fertilize the eggs.
The other flowers in the picture follow the same plan, although they come in many different colors and styles. Color each of
the flowers according to the colors above (blue for stamen, pink for pistil, bright colors for the petals. etc.). Note that in
some of the flowers, not all the structures are visible.
Plant Reproduction
Sexual reproduction in plants occurs when the pollen from an anther is transferred to the stigma. Plants can fertilize
themselves: called self-fertilization. Self-fertilization occurs when the pollen from an anther fertilizes the eggs on the same
flower. Cross-fertilization occurs when the pollen is transferred to the stigma of an entirely different plant.
When the ovules are fertilized, they will develop into seeds. The petals of the flower fall off leaving only the ovary behind,
which will develop into a fruit. There are many different kinds of fruits, including apples and oranges and peaches. A fruit is
any structure that encloses and protects a seed, so fruits are also "helicopters" and acorns, and bean pods. When you eat
a fruit, you are actually eating the ovary of the flower.
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1. What is an angiosperm?
2. The flower attaches to what part of the plant?
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3. Why are flowers brightly colored?
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4. Name two mammals that might pollinate a plant.
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5. If the petals of a flower are reduced or absent, how is the plant pollinated?
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6. The female reproductive structures are called the:
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7. Name the three parts of the pistil:
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8. Where are the ovules stored?
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9. Name the two parts of the stamen:
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10. Describe sexual reproduction in plants.
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11. The ovary develops into what structure?
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12. Define fruit.
13. Some flowers are not brightly colored at all, but have a very pungent odor that smells like
rotting meat. How do you think these flowers are pollinated?
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14. In many flowers, the pistils and stamens reach maturity at different times. Considering what
you know about pollination, why would this be an advantage to the plant?
Label The Parts of a Flower
LEAF ANATOMY
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The leaf is the primary photosynthetic organ of the plant. It consists of a flattened portion,
called the blade, that is attached to the plant by a structure called the petiole. Sometimes
leaves are divided into two or more sections called leaflets. Leaves with a single
undivided blade are called simple, those with two or more leaflets are called compound.
The outer surface of the leaf has a thin waxy covering called the cuticle (A), this layer's
primary function is to prevent water loss within the leaf. (Plants that leave entirely within
water do not have a cuticle). Directly underneath the cuticle is a layer of cells called the
epidermis (B). The vascular tissue, xylem and phloem are found within the veins of the
leaf. Veins are actually extensions that run from to tips of the roots all the way up to the
edges of the leaves. The outer layer of the vein is made of cells called bundle sheath
cells (C), and they create a circle around the xylem and the phloem. One the picture,
xylem is the upper layer of cells (D) and is shaded a little lighter than the lower layer of
cells - phloem (E). Recall that xylem transports water and phloem transports sugar
(food).
Within the leaf, there is a layer of cells called the mesophyll. The word mesophyll is greek
and means "middle" (meso) "leaf" (phyllon). Mesophyll can then be divided into two
layers, the palisade layer (F) and the spongy layer (G). Palisade cells are more columnlike, and lie just under the epidermis, the spongy cells are more loosely packed and lie
between the palisade layer and the lower epidermis. The air spaces between the spongy
cells allow for gas exchange. Mesophyll cells (both palisade and spongy) are packed with
chloroplasts, and this is where photosynthesis actually occurs.
Epidermis also lines the lower area of the leaf (as does the cuticle). The leaf also has tiny
holes within the epidermis called stomata (H). Specialized cells, called guard cells (I)
surround the stomata and are shaped like two cupped hands. Changes within water
pressure cause the stoma (singular of stomata) to open or close. If the guard cells are full
of water, they swell up and bend away from each other which opens the stoma. During
dry times, the guard cells close.
Cuticle (light blue)
Epidermis (yellow)
Guard cells (pink)
Palisade Mesophyll (dark green)
Phloem (purple)
Xylem (orange)
Spongy Mesophyll (light green)
Bundle Sheath (dark blue)
Leaf questions
• 1. What two tissues are found within a vein?
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2. What does the word "mesophyll" mean?
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3. What two layers of the plant contain chloroplasts?
• 4. The outermost layer of cells: _________________________
• 5. The waxy covering of the leaf.: _______________________
• 6. These cells function to open and close stomata.
_____________________
• 7. Outer layer of the vein: ________________________
• 8. Column like cells that lie just under the epidermis. ___________________
• 9. Openings that allow for gas exchange. _________________________
• 10. The stalk that connects the leaf to the stem. ______________________