Everything You’ve Ever Wanted to Know About Plants

Plant Structure• The three principal organs are roots, stems, and leaves.• Roots• 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, • 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: • 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 GibberellinsBunch 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”)

• Tropisms (2:37)

Tropism DescriptionPhototropism Response to lightGravitropism Response to gravityHydrotropism Response to H2OThigmotropism Response to touch

Tropisms

Plant Responses• Rapid Responses• 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 PlantsGiant 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

RafflesiaMistletoe

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