Everything You’ve
Ever Wanted to
Know About Plants
Plant Structure
• The three principal organs are roots, stems, and leaves.
• Roots
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Absorb and transport H2O and nutrients
Anchor plants to ground, hold soil in place, and prevent erosion
Protect plants from soil pathogens
Hold plants upright against environmental forces
• Stems
• Support plant, transport materials, protect plant from predators
and disease
• Range from less than an inch to hundreds of feet
• Leaves
• Primary sites of photosynthesis
Plant Structure
• The three principal tissues are dermal, vascular, and ground.
• Dermal Tissue
• Outer layer; consists of a single layer of epidermal cells
• Often covered with a thick waxy layer called a cuticle to prevent
H2O loss and protect plant
• Vascular Tissue
• Transport system that moves H2O and nutrients throughout plant
• Consists of xylem and phloem
• Xylem transports H2O, while phloem transports solids (mostly carbs)
• Two main cells of xylem are tracheids (dead, skinny, transport and
hold H2O) and vessel elements (dead, wide, transport H2O)
• Two main cells of phloem are companion cells (living, helper cells)
and sieve tube elements (living, transport tubes)
Plant Structure
Plant Structure
• Ground Tissue
• Cells found between dermal and vascular tissues
• Three types of cells
• Parenchyma –photosynthesis or store H2O/carbs; soft tissue
• Collenchyma – strong, flexible cells walls; support cells
• Sclerenchyma – thick, rigid cell walls; main support cells
• Meristematic Tissue
• Tissue where mitosis occurs
• The apical meristem is found at the tips of roots and stems
• New cells produced here are undifferentiated, but gradually they
become specialized into dermal, ground, and vascular tissue cells
Plant Structure
Meristematic Tissue
Transport in Plants
• Two types of transport: H2O in the xylem and nutrient (carbs)
in the phloem
• Water Transport: Root pressure, capillary action, transpiration
• Root pressure
• Causes H2O to move from soil to roots
• High amounts of root pressure can cause guttation.
Transport in Plants
• Water Transport: Root pressure, capillary action, transpiration
• Capillary pressure
• The tendency of H2O to rise in a thin tube such as a tracheid or vessel
element (xylem tubes)
• Involves adhesion (H2O attracted to xylem tissue) and cohesion (H2O
attracted to H2O)
• The diameter of the xylem tubes decreases as plant height increases
Transport in Plants
• Water Transport: Root pressure, capillary action, transpiration
• Transpiration
• As H2O evaporates from leaves, a drop in osmotic pressure occurs
within the plant.
• As a result, the movement of H2O out of the leaves “pulls” H2O upward
through the plant all the way from the roots.
• All of this water movement occurs without any expenditure of
energy by the plant.
• An acre of corn transpires about 3,000-4,000 gallons/day
• A mature oak transpires about 110 gallons/day
• Water Transport (3:01)
Transport in Plants
Transport in Plants
• Transpiration is controlled by specialized cells in the epidermis
called guard cells.
• Guard cells surround tiny openings in the epidermis called
stomata (stoma – singular).
• When H2O is abundant, it flows into the leaf and raises the
osmotic pressure within the guard cells, which then open
stomata and H2O transpires.
• When H2O is scarce, the opposite occurs.
• In dry conditions,
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H2O leaves vacuoles
Cells bend inward
Leaves wilt
Stomata close
H2O is conserved
Guard Cells and Stoma
Transport in Plants
• Unlike H2O which is pulled through xylem, sugars and
nutrients are pushed through phloem.
• Sugars and nutrients are pushed through phloem from leaves
or roots into stems, and then through stems into fruit.
• Also, during the cold season, sugars and nutrients are pumped
down into roots through phloem for storage over winter.
• In spring, the process is reversed.
• Sugars and nutrients move from source cells to sink cells.
Source/Sink
Hormones and Plant Growth
• A hormone is a chemical that’s produced in one part of an
organism and affects another part of that organism.
• Plant hormones are chemicals that control plant growth and
development and responses to environmental conditions.
• Hormones affect target cells or target tissues.
• These cells must possess membrane receptors that are specific to
a particular hormone, like a lock and key (enzyme/substrate).
• Hormones can induce cells to divide, change their metabolism
or growth rate, or activate certain genes.
Hormones and Plant Growth
• Auxins
• Hormones produced in the apical meristem that stimulate
cell elongation
• Auxins are responsible for a whole bunch of stuff:
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Phototropism (growth toward light)
Gravitropism (response of a plant to the force of gravity)
Hydrotropism (growth toward H2O)
Inhibiting lateral bud growth near the apical meristem
Wound healing (formation of new xylem and phloem)
Delay the aging of fruit
Effect of Auxins
Auxin Transport Inhibited
Hormones and Plant Growth
• Cytokinins
• Hormones produced in roots and developing fruits and seeds
• They are responsible for:
• Stimulating mitosis and the growth of lateral buds
• Delaying the aging of leaves
• Causing dormant seeds to sprout.
• They often produce effects opposite to those of auxins.
Cytokinins
Auxins
Inhibit cell elongation
Stimulate cell elongation
Stimulate growth of buds
Inhibit growth of buds
Stimulate mitosis
Cell growth, but not mitosis
Hormones and Plant Growth
• Gibberellins
• Hormones produced in seed tissue and responsible for early plant
growth
• Gibberellins are responsible for:
• Dramatic increases in size in stems, fruit, and flowers
• Initiating enzyme function
• Delaying the aging of leaves and fruit
Effects of Gibberellins
Bunch on left is untreated control; bunch on right
sprayed with gibberellins
Hormones and Plant Growth
• Ethylene
• Gaseous hormone produced in fruit tissue
• Because it’s gaseous, it can be released from a fruit and absorbed
by another stimulating it to ripen and release ethylene, which
stimulates nearby fruit to ripen and release ethylene … and so on
• Many fruits are harvested unripe and are then exposed to ethylene
when they reach their destination
• In response to auxins, fruit tissues release ethylene
• What does ethylene do? Hmmm…
• Stimulates fruit to ripen
• Stimulates the release of fruits and leaves (abscission)
• Stimulates flower opening
• Initiates aging of flowers and fruit
Effects of Ethylene
Plant Responses
• Tropisms (Greek for “turning”)
Tropism
Phototropism
Gravitropism
Hydrotropism
Thigmotropism
• Tropisms (2:37)
Description
Response to light
Response to gravity
Response to H2O
Response to touch
Tropisms
Plant Responses
• Rapid Responses
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Quick responses due to stimuli
Involve quick and dramatic changes in osmotic pressure
Mimosa pudica and Venus flytrap
Mimosa Reaction (:21)
Plant Responses
• Photoperiodism
• The flowering of plants in response to changing lengths of day
and night as the season progresses
• Short-day and long-day plants
• Short-day plants flower when days are short; long-day plants flower
when days are long
• Mediated by phytochrome, a pigment that absorbs red light
Photoperiodism (short-day)
Photoperiodism (long-day)
Plant Responses
• Winter Dormancy
• What happens? Ummm… Let me think about that…
• Photosynthesis shuts down
• Carbs and nutrients are transported from leaves to roots
• Auxins decrease; ethylene increases
• Leaves get sealed off from rest of plant and eventually fall off
• Buds form at base of leaves; these buds will survive the winter
and become the new leaves in spring
• Mediated by phytochrome, which means dormancy is ultimately
due to …
Plant Adaptations
• Aquatic Plants
• Adaptations:
• Can tolerate mud that is saturated with H2O and nearly devoid
of O2
• Possess tissues with many air-filled cavities through with O2
can diffuse and then make its way down to the roots
• Produce seeds that float
• Grow quickly after germination so plant can make its way up
to water’s surface
Aquatic Plants
Giant Amazon Water Lily (4:27)
Plant Adaptations
• Salt-Tolerant Plants
• Adaptations:
• Possess roots that are capable of tolerating high salt concentrations
(hypertonic environment)
• Have specialized cells with membrane proteins that pump excess salt
out of their cells onto surface of leaves (salt washed off by rain)
Plant Adaptations
• Desert Plants
• Adaptations:
• Extensive root systems that extend deep into the ground or
spread out for long distances just below surface
• Reduced leaves, e.g., needles that keep transpiration at a
minimum
• Thick flexible stems capable of storing lots of H2O and carrying
out photosynthesis
• Roots covered with unusually high number of hairs (increased
surface area)
• Parts of plants and their seeds can remain dormant for years in
between dry spells
• When rain does come, plants can mature, flower, and set seeds in
just a couple of weeks or even days
• Desert Tree (2:00)
• Desert Bloom (3:18)
Plant Adaptations
• Carnivorous Plants
• Carnivorous plants often live in environments, such as bogs,
where the soil is too wet and acidic for bacterial growth.
• Without the bacteria, organic matter is not decomposed, leaving
the soil nearly void of nutrients (esp. N and P)
• In order to obtain necessary nutrients, carnivorous plants
possess leaves designed to trap insects.
• They also have specialized glands that secrete digestive enzymes.
• Examples: Venus flytrap, pitcher plants, sundews, bladderworts
• Carnivorous Plants 1 (3:31)
• Carnivorous Plants 2 (3:32)
Carnivorous Plants
Plant Adaptations
• Parasitic Plants
• Because these plants have no or a limited ability to
photosynthesize, they rely on other plants for their nutrients.
• They have specialized organs that are able to penetrate the tissue
of other plants.
• They absorb their host plant’s water and nutrients, thereby
harming the host plant
• Examples: mistletoe, dodders, rafflesia
• Rafflesia (2:23)
Parasitic Plants
Rafflesia
Mistletoe
Plant Adaptations
• Epiphytes
• Plants that are not rooted in the soil but instead grow directly on
the bodies of other plants
• Most found in tropical rain forests
• Non-parasitic
• Obtain nutrients from the air, rain, and debris surrounding them
Plant Adaptations
• Chemical Defenses
• Many plants possess nasty chemicals and toxins designed to ward
off predators, mainly insects
• These chemicals are often poisons or imitator hormones that
disrupt insect growth and prevent them from reproducing
• They’re also proteins that disrupt insect nervous system