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The houseplant observation
• For years, people
houseplants tended to
lean toward a source of
• Charles Darwin and his
son Francis, wondered
why. How does a plant
“know” where to lean?
• The Darwins
studied the leaning
• Oat coleoptiles are
growth is fairly
The Oat Experiments
• In the next several slides, you’ll see
representations of experiments done by
the Darwins and other scientists.
• On your own paper, answer the
questions on each of the slides. After
writing your answers, discuss them with
a neighbor or in a small group. You will
hand these in at the end of class.
Darwin Experiment 1
Oat shoots tend to bend toward the light. When the tip of the shoot is
covered with a small cap, the shoot does not bend.
doesn’t the shoot
with the cap bend
toward the light? List
reasons that could
be tested with a
• The Darwins speculated that somehow the tip of
the plant perceives the light and communicates
chemically with the part of the shoot that bends.
• Question: How could they test these alternative
• The cap itself prevents bending.
• Light further down the shoot, rather than on
the tip, causes bending.
Darwin Experiment 2
Some shoots were covered with small caps of glass. Others were covered
with a sleeve that left the tip exposed but covered the lower shoot.
information does this
experiment give us
about the cause of
What new questions
does it raise?
• Several decades later, Peter Boysen-
Jensen read of the Darwins’ experiments,
and had further questions. He designed
a set of experiments to try to further
explain why plants bend toward the
• Boysen-Jensen cut the tips off of oat
coleoptiles and found that they did not
bend toward the light.
• Question: What further information does
this tell us about the role of the tip in this
phenomenon? What questions does it
• Boysen-Jensen then cut the tips off of
several oat coleoptiles and put the tips
back on. These coleoptiles bent toward
• Questions: Why did Boysen-Jensen do
this? What further information does this
experiment give us?
Boysen-Jensen then tried putting a porous barrier (agar gel) and an
impenetrable barrier (a flake of mica) between the shoot tip and the rest of
the shoot. The shoot with an agar barrier bent toward the light. The shoot
with the mica barrier did not.
experiment give us
new information or
only confirm the
results of other
In another experiment, Boysen-Jensen took a tiny, sharp sliver of mica and
pushed it into the coleoptile so that it cut off communication between the tip
and the rest of the plant on one side only. If the sliver was on the side that
was lit, it still leaned that toward the light, but if it was on the opposite side,
the plant did not lean toward the light.
information does this
tell us about why
plants lean toward
the light? What new
hypotheses could be
• In the early 20th century, F.W. Went
worked on identifying the factor that
was causing plants to bend toward the
• By building on the work of the Darwins
and Boysen-Jensen, Went was able to
isolate the factor and show how it
F.W. Went 1
Went first cut the tips off of oat coleoptiles and placed them on a block of
agar and allowed juices from the tip to diffuse into the agar.
F.W. Went 2
Went then cut blocks from the agar. If he cut a tip from an oat coleoptile
and placed an agar block on top, then put the coleoptile in the dark, it grew
just as it would if the tip were intact.
Why use the agar block
infused with plant juice instead
of just cutting and replacing the
Why place the plants in the
dark instead of shining light on
one side as in the other
F.W. Went 3
Went also compared what happened when he placed an agar block
squarely on top of a clipped coleoptile versus what happened when he set
the block on one side of the cut tip. In the first case, the coleoptile grew
straight up. In the second, it bent.
What does this tell
us about the role of
juice from the
coleoptile tip in plant
What effect do you
think the juice is
having at the cellular
The Mystery Factor
• Eventually, F.W. Went was able to
isolate a chemical from coleoptile juice:
Indole acetic acid (IAA), one chemical in
a class of plant hormones called auxins.
Auxins stimulate genes in cells associated with plant growth.
• Auxins carry out multiple roles having
to do with plant growth including:
• Apical dominance
• Growth of adventitious roots
• Fruit growth
• Tropisms are the growth of a plant
toward or away from a stimulus,
• Phototropism: in response to light
• Gravitropism: in response to gravity
• Thigmotropism: in response to touch
Tropisms: cell elongation
• In general, tropisms
suppression of cell
elongation on one
side of a plant,
causing the plant to
grow in a particular
• Look at the sprouts
in the bottom
picture and the
diagram at the top.
Explain why the
sprouts are all
leaning in the same
• In this Impatiens
plant, shoots grow
upwards and roots
grow downwards in
response to gravity.
• On which side of the
shoot and root do you
think auxins are more
Gravitropism in shoots
• In shoots, auxins
concentrated on the
lower side of the
stem, causing the
cells there to
• Why is this
Gravitropism in roots
• In roots, however,
on the lower side of
the root suppresses
• The upper side of
the root continues
to grow, causing the
roots to bend
Plastids and Gravitropism
How does a root “know” which way is down?
Plastids, particularly leucoplasts, in the root cap cell tend to settle on the
bottom side of the cell. This stimulates the release of auxins.
• In some plants,
vining stems or
tendrils will grow
in response to
• Which side of the
might the auxin be?
(Remember, this is
the shoot system.)
• Auxins are released
from the shoot tip.
These stimulate cell
elongation in the stem,
but suppress the
• Cytokinins, produced
in the roots, can
stimulate lateral buds
if the shoot tip is
• How does
pinching back a
plant, such as this
cause it to become
• Adventitious roots are
those growing out of
places where roots don’t
• Auxins stimulate root
growth on the end of a
• When people grow new plants from
cuttings, they often dip the end of the
cutting in rooting compound to
stimulate root growth.
• What hormone is in the compound?
• How does it work?
• Developing seeds
produce auxins that
stimulate growth of the
plant ovary into a fruit.
• Removal of seeds from a
strawberry prevents the
fruit from growing, but
add auxin and will grow.
• How could this be used
Foolish rice seedlings
• Gibberellins were
bakanae, or “foolish
rice seedling” disease,
that caused seedlings
to grow excessively
tall, then fall over.
Discovery of Gibberellins
• In 1898, Shotaro Hori suggested that the
disease was caused by a fungus that
infected the rice.
• Eiichi Kurosawa in 1926 was able isolate
secretions from the fungus. The
secretions caused the same symptoms
when applied to other rice plants.
• In 1934, Teijiro Yabuta isolated the active
substance and named it gibberellin.
Functions of Gibberellins
• Promotes cell elongation in the
internodes of plants.
• Stimulates growth of the ovary wall into
• Stimulates seed germination and release
of food reserves in seeds.
• On the left are
grapes with seeds.
On the right is a
grapes. These both
came from the same
grapevine. How can
• You’re at the State Fair, looking at the
giant vegetable competition, and you
notice a super-tall sunflower entered by
your favorite biology professor. You
notice that it has the same number of
leaves as its competitors, just extra-long
internodes. Should you alert the judges?
In search of a growth factor
• In the early 1950’s, Dr. Folke Skoog and Dr. Carlos
Miller were in search of a better medium in which to
grow plant tissues and to manipulate cells to grow
roots and shoots.
• After experimenting with coconut milk and yeast
extract, they found evidence that a derivative of a
nucleotide (DNA component) might be the factor in
these substances that stimulated cell growth.
• Miller, looking for a source of nucleotides, found an old
bottle of herring sperm DNA in the storeroom. When
he used it on plant tissue, he found terrific growth.
Isolating the factor
• Miller ordered a new bottle of herring sperm DNA, but
the new sample didn’t cause cell division as the old one
• After much work, Skoog and Miller isolated the one
factor that coconut milk, yeast extract, and old DNA had
in common, and that stimulated cell division. The
substance, which they named “kinetin,” was structurally
similar to the DNA base, adenine and appeared to be a
chemical derivative of adenine.
• Since the time of Miller and Skoog’s work, similar
molecules have been found, and grouped together under
the name of “cytokinins.”
Functions of Cytokinins
• Promote growth of lateral buds when
auxin concentrations are low.
• Promote cell division in meristems.
• Stimulate fruit and seed development.
• Delays senescence of plant parts.
• You’re a plant scientist preparing a plant
tissue callus in a petri dish. You want to
have lots of rapidly dividing,
undifferentiated cells in the dish. What
hormone will you use and why?
• In ancient China, people placed pears or
oranges in rooms with burning incense
to make them ripen faster.
• For centuries, people assumed heat or
light was responsible for fruit ripening.
In the 19th century, fruit ripening sheds
were built using gas or kerosene heaters.
When these were replaced with electric
heaters, fruit didn’t ripen as fast.
• In the 1800’s, gas lighting was first
installed in cities. People noticed that
houseplants growing near gas light
fixtures grew abnormally. Cut flowers
aged and wilted quickly.
• Physiologist Dimitry Neljubow analyzed
natural gas and found that one
component, ethylene gas, was responsible
for the effects.
Functions of Ethylene
• Released by fruits and causes the fruits
to ripen faster.
• Causes plant parts to age and die
• Inhibits stem elongation.
• Which of these
faster and why?
• Putting them on
• Putting them in
a paper bag.
In search of an inhibitor
• In separate studies in 1963, F.T. Addicott found a
substance that stimulated abscission of fruits in cotton,
and named it “abscisin.” P.F. Wareing found a
substance that promoted dormancy in sycamore tree
leaves and called it “dormin.”
• By 1967, both teams realized they were studying the
same substance. At a conference they decided to call
the substance abscisic acid.
Functions of Abscisic Acid
• Controls seed and bud dormancy.
• Inhibits gibberellins.
• Promotes senescence in plants.
• You’ve bought a big bouquet of flowers
for your mother for Mother’s Day. You
want the flowers to last for a long time.
Would it be a good or bad idea to expose
the bouquet to:
• abscisic acid?
• ethylene gas?
Hinge control in Venus Fly Trap - Nastic movement
How it works
• Nastic movements are rapid, reversible
movements in a plant.
• Electrical potentials across cell
membranes, similar to those in our nerve
cells, signal plant cells at the base of the
Mimosa leaf to rapidly lose water. This
causes the leaf to droop.
• Sunflowers follow the sun during the
• Leaves of many plants turn to follow the
• Some plants flower in response to the
length of periods of darkness.
• Spring-blooming flowers are long night
(short day) plants, while summer-
blooming flowers are short night (long
• Some plants are day-neutral.
Action of phytochrome on flowering time.
Pfr to Pr switch is how plants “tell time.”
• You’re trying to grow tomatoes and
strawberries in a greenhouse. Tomatoes
blossom and ripen in the summer.
Strawberries blossom and ripen in the spring.
You set each plant in a chamber with lights set
to the right day length. Your assistant comes in
to clean up in the middle of the night and
switches the lights on briefly. Which plants
will now fail to bloom: strawberries or
• Plants communicate chemically.
• Injured plants send out chemical signals
• signal other plants to prepare for an
• attract other insects that eat the insects
that are attacking the plant.