Plants

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Plants

• AP Biology

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Quick Facts About Plants

• Multicelled, eukaryotic, photosynthetic
  autotrophs
• Life cycle characterized by alternation of
  generations
• Ancestral plants lived in aquatic areas but are
  now mostly on land

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Bryophytes

• Non-vascular lack xylem and phloem
• Absorb water through diffusion
• Contain flagellated sperm that swim through water
  to fertilize an egg
• Lack lignin, tissue that supports tall plants
• restricted to moist habitats and are tiny

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Bryophytes

• Grow on rocks, soil and trees
• Can be used as fuel (sphagnum, peat moss)
• Examples mosses, liverworts, hornworts

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Tracheophytes

• Plants with vascular tissue
• Xylem and phloem for transport
• Lignified transport vessels to support plant
• Roots to absorb, anchor and support
• Leaves, increasing surface for photosynthesis
• Dominant sporophyte generation during developmen

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Two Types of Tracheophytes

• Seed plants
• More advanced and far more numerous
• Gymnosperms (bearing cones)
• Angiosperms (bearing flowers and fruits)
• Seedless plants
• Ferns

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Ferns

• Seedless tracheophytes
• Primitive plants reproducing with spores
• Homosporous, producing only one type of spore
  which develops into a bisexual gametophyte
• Restricted to moist habitats
• Sperm are flagellated and must swim to fertilize
  an egg

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Seed Plants

• Heterosporous, producing megaspores and
  microspores
• Megaspores female gametophytes
• Microspores male gametophytes
• Sperm are not flagellated, and therefore are not
  restricted to moist environments

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Gymnosperms (Conifers)

• First seed plants on earth
• Seeds are naked, as they are not enclosed
  inside a fruit, like angiosperms
• Seeds are exposed on modified leaves that form
  cones (dry environments)
• Modifications include needle-shaped eaves
• Depend on wind for pollination

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Gymnosperms (Conifers)

• Pines, firs, redwoods, junipers and sequoia

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Angiosperms (Flowering Plants)

• Seed plants whose reproductive structures are
  flowers and fruits
• 90 of all plants
• Color and scent of flowers attracts animals that
  will carry pollen from one plant to another
• After pollination and fertilization, ovary
  becomes fruit and ovule becomes seed

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Angiosperms (Flowering Plants)

• Fruit protects seeds and aids in their dispersal
• Examples
• Maple trees have seeds with wings helping them
  travel greater distances
• Some plants have burrs on their fruits that cling
  to fur or clothing
• Animals eat and digest fruit while tough seeds
  pass through digestive tract, eventually being
  deposited with feces as fertilizer

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Monocots vs. Dicots
Characteristic Monocot Dicot
Cotyledons (seed leaves) One Two
Vascular bundles in stem Scattered In a ring
Leaf venation Parallel Netlike
Floral parts Usually in 3s Usually in 4s or 5s
Roots Fibrous roots Taproots
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Plants Move Towards Land

• Plants moved to land as competition for resources
  increased
• Problems supporting plant body while absorbing
  and conserving water

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Modifications

• Cell wall made of cellulose to give support and
  maintain shape
• Roots and root hairs absorb water and nutrients
  from soil
• Stomates open to exchange gas and close to
  prevent water loss
• Cutin, waxy coating on leaf, prevents excess
  water loss from leaves

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Modifications

• Some plants have protective jacket of cells
  called gametangia, protecting gametes and zygotes
  as well as preventing drying out
• Sporopollenin, a tough polymer, is resistant to
  environmental damage and protects plants
• Found in walls of spores and pollen

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Modifications

• Seeds and pollen have a protective coat
  preventing desiccation
• Xylem and phloem vessels enable plants to grow
  tall
• Lignin embedded in xylem and other plant cells
  provide support

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Primary Growth

• Meristem
• Enables plants to grow as long as they live
• Continually divides and generates new cells
• Apical meristem
• In tips of roots and buds of shoots is the source
  of primary growth, the elongation of the plant
  down into the soil and up into the air

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Secondary Growth

• Lateral meristem
• Provides secondary growth, or increase in girth
• Herbaceous (nonwoody) plants
• Only primary growth
• Woody plants
• Secondary growth responsible for thickening of
  roots and shoots

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Plant Tissue

• Dermal tissue
• Covers and protects plant
• Includes epidermis, guard cells, root hairs,
  cuticle cells
• Vascular tissue
• Xylem and phloem, transporting water and
  nutrients
• Ground tissue
• Support, storage, photosynthesis

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Vascular Tissue Xylem

• Water and mineral-conducting tissue
• Consists of two types of elongated cells
• Tracheids and vessel elements
• Both dead at functional maturity
• Tracheids
• Long, thin cells that overlap and taper at the
  ends
• Water passes from one cell to another through
  pits, with no secondary cell wall
• Areas with secondary cell wall are hardened with
  lignin providing support and transport

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Vascular Tissue Xylem

• Vessel elements
• Wider, shorter, thinner walled, less tapered
• Align end to end
• Ends are perforated to allow free flow through
  vessel tubes
• Seedless vascular plants and gymnosperms have
  only tracheids most angiosperms have both
  tracheids and vessel elements
• Xylem makes up wood of plants

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Vascular Tissue Phloem

• Carries sugars from photosynthetic leaves to rest
  of plant by active transport
• Consist of chains of sieve tube members or
  elements whose end walls contain sieve plates
• Facilitate flow of fluid from one cell to the
  next
• Alive at maturity
• Lack nuclei, ribosomes and vacuoles

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Vascular Tissue Phloem

• Connected to each sieve tube member is at least
  one companion cell
• Contains a full set of organelles
• Nurtures sieve tube elements

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Vascular Tissue Ground Tissue

• Support, storage, photosynthesis
• Three cell types
• Parenchyma, scelernchma, collenchyma
• KEEP IN MIND THAT FORM RELATES TO FUNCTION

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Vascular Tissue Ground Tissue

• Parenchymal cells
• Primary cell walls that are thin and flexible
• Lack secondary cell walls
• Contain one large vacuole
• Some contain chloroplasts and carry out
  photosynthesis
• Parenchymal cells in roots contain plastids and
  store starch
• When turgid, give support and shape

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Vascular Tissue Ground Tissue

• Parenchymal cells
• After a plant is injured, parenchymal cells
  retain ability to divide and differentiate into
  other cell types
• An entire plant can be regenerated or cloned from
  one parenchymal cell (in a lab)

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Vascular Tissue Ground Tissue

• Collenchymal cells
• Unevenly thickened primary cell walls but lack
  secondary cell walls
• Mature cells are alive
• Support growing stem
• Strings of a stalk of celery are collenchymal
  cells

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Vascular Tissue Ground Tissue

• Sclerenchymal cells
• Thick primary and secondary cell walls fortified
  with lignin
• Support plant
• Two forms are fiber and sclereids
• Fibers are long thin and occur in bundles make
  rope and linen
• Sclereids are short and irregularly shaped make
  up tough seed coats and pits

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Roots

• Absorb nutrients from soil, anchor plant and
  store food
• Made of specialized tissues and structures

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Roots

• Epidermis covers surface of roots modified for
  absorption
• Root hairs extend out from each cell and increase
  surface area
• Cortex consists of parenchymal cells that contain
  plastids to store starch and organic substances

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Roots

• Vascular cylinder or stele of root consists of
  xylem and phloem surrounded by tissue called
  pericycle, from which lateral roots arise
• Endodermis surrounds vascular cylinder each
  endoderm cell is wrapped with the Casparian
  strip, a continuous band of suberin, waxy
  material impervious to water

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Roots

• Endoderm selects which minerals can enter
• Apical meristem which provides primary growth (up
  and down)
• Three zones of cells during primary growth
• Zone of cell division (bottom)
• Zone of elongation (middle)
• Zone of differentiation (top)

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Roots

• Root tip is protected by a root cap, which
  secretes a substance that digests the earth
• Zone of cell division
• Meristem cells actively divide
• Zone of elongation
• Cells elongate and push root cap deeper into soil
• Zone of differentiation
• Protoderm becomes epidermis, ground meristem
  becomes cortex, procambium becomes xylem and
  phloem

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Types of Roots

• Taproot
• Single large root that gives rise to branch roots
• Common in dicots
• Fibrous root system
• Holds plant firmly in place
• Common in monocots

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Types of Roots

• Adventitious roots
• Roots that arise above ground
• Aerial roots
• Found in marshes
• Stick out of water and give air to root cells
• Prop roots
• Grow above ground out from base of stem
• Support plant

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Stems

• Primary stem tissue
• Vascular tissue exists as vascular bundles
• Xylem inside, phloem outside, meristem tissue in
  between
• Monocots
• Vascular bundles scattered throughout stem
• Dicots
• Vascular bundles arranged in a ring

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Stems

• Ground tissue
• Consists of cortex and pith
• Used for storage

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Secondary Growth in Stems

• Produced by lateral meristem
• Replaces dermal tissue with bark
• Second lateral meristem adds vascular tissue
• Wood is secondary xylem accumulation

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The Leaf

• Epidermis upper and lower protection for leaf
• Cuticle waxy coating minimizing water loss
• Guard cells contain chloroplasts control
  opening of stomates
• Stomates tiny openings allowing for gas exchange

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The Leaf

• Mesophyll
• Palisade layer cells packed tightly contain
  many chloroplasts
• Spongy layer cells packed loosely, allowing for
  diffusion of gases less chloroplasts
• Vascular bundles (veins) found in mesophyll
  carry water and nutrients
• Specialized bundle sheath cells surround veins

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Stomates

• Accounts for majority of water loss
• Opened and closed by guard cells
• If guard cells absorb water by osmosis and become
  turgid, they curve and open stomates
• If guard cells lose water and become flaccid,
  stomate closes

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Stomates
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Why Stomates Open

• Loss of CO2 within air spaces of leaf
• Occurs when photosynthesis begins
• Increase in potassium ions
• Lowers water potential, causing water to diffuse
  into them
• Stimulation of blue light in a guard cell
• Stimulates proton pumps which promote uptake of
  potassium ions
• Active transport of H out of guard cells

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Why Stomates Close

• Lack of water
• Guard cells become flaccid and close
• High temperatures
• Increases cellular respiration which increases
  concentration of carbon dioxide
• Abscisic acid
• Produced in response to dehydration

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Transport in Plants

• Xylem rises against gravity without using energy
• Transpirational pull pulls water up
• Root pressure pushes xylem upward a few yards
• Water droplets on leaves in the morning result
  from root pressure, a process called guttation

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Root Pressure
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Transpirational Pull

• Transpiration is the evaporation of water from
  leaves
• This causes negative pressure to develop in xylem
• Cohesion of water helps pull a column of water
  upward
• Absorption of sunlight causes water to evaporate

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Transpirational Pull
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Transpirational Pull-Cohesion Tension Theory

• For each molecule of water that evaporates,
  another molecule of water is drawn from the root
  to replace it
• Factors affecting transpiration
• Humidity increased slows, decreased speeds
• Wind reduces humidity, speeding up transpiration
• Increased light increased photosynthesis,
  increased water vapor, increased transpiration
• Stomates if closed, stops transpiration

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Absorption of Nutrients and Water

• Apoplast and symplast
• Performs lateral movement of water and solutes
• Symplast
• continuous cytoplasmic system interconnected by
  plasmodesmata
• Long-distance transport
• Apoplast
• Network of cell walls and intercellular spaces
• Short-distance, extracellular transport

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Absorption of Water and Nutrients

• Mycorrhizae
• Symbiotic structures consisting of the plants
  roots intermingled with hyphae (filaments) of
  fungus
• Increase amount of nutrients to be absorbed

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Absorption of Water and Nutrients

• Rhizobium
• Symbiotic bacterium living in nodules on roots of
  specific legumes
• Fix nitrogen gas from air into a form that plants
  can use
• Nitrogen-fixing bacteria

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Absorption of Nutrients and Water

• Bulk flow
• Movement of fluids across great distances within
  a plant

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Translocation of Phloem Sap

• Phloem sap travels through plant from sugar
  source to sugar sink
• This is called translocation
• Source is where sugar is being produced (leaves)
• Sink is where sugar is stored or consumed (roots
  and fruit)

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Plant Reproduction

• Asexual
• Accomplished through vegetative propagation
• A piece of the vegetative part of the plant
  (root, stem or leaf) can produce a new plant
  genetically identical to the parent
• Grafting (plant parts are fused together)
• Cuttings (taking plant stem cells and placing
  them in soil)
• Bulbs (underground buds)
• Runners (stem grown underground)

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Sexual Reproduction in Flowering Plants

• Flower is the sexual organ
• Process begins with pollination
• One pollen grain contains three haploid nuclei
  (one tube nucleus, two sperm nuclei)
• Grain lands on the sticky stigma of the flower
• Pollen grain absorbs moisture and sprouts,
  forming a pollen tube that burrows down the style
  into the ovary

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Sexual Reproduction in Flowering Plants

• Two sperm nuclei travel down the pollen tube into
  the ovary
• Once inside, two remaining sperm nuclei enter the
  ovule through the micropyle
• One sperm nucleus (haploid) fertilizes the egg
  (haploid) and becomes the embryo (2n)
• The other sperm nucleus fertilizes the two polar
  bodies and becomes the triploid (3n) endosperm,
  food for the embryo

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Sexual Reproduction in Flowering Plants

• This process is double fertilization
• After fertilization, ovule becomes the seed and
  ripened ovary becomes the fruit
• Monocots food reserves remain in endosperm in
  coconuts, endosperm is the liquid
• Dicots mature seed lacks endosperm since they
  are transported

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The Seed

• Protective coat, embryo and cotyledon or
  endosperm
• Embryo
• Hypotcotyl becomes lower part of stem
• Epicotyl becomes upper part of stem
• Radicle (embryonic root) first organ to emerge
  from the germinating seed

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Alternation of Generations

• Haploid (n) and diploid (2n) generations
  alternate
• Gametophyte (n) produces gametes by mitosis
• Fuse during fertilization to form 2n zygotes
• Each zygote develops into a sporophyte (2n),
  which produces haploid spores (n) by meiosis
• Each haploid spore forms a new gametophyte

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Mosses and Bryophytes

• Alternation of generations
• Haploid generation is dominant diploid
  generation lives a short time and depends on the
  gametophyte for food

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Mosses and Bryophytes

• Overview
• Gametophyte dominates
• Archegonia and antheridia develop on tips of
  gametophyte
• Sporophyte grows out of the top of the
  gametophyte and obtains nutrients from it
• Haploid spores are formed in mature sporangia

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Ferns

• Sporophyte generation is larger than and
  independent from gametophyte
• Archegonia and antheridia both develop underneath
  heart-shaped haploid gametophyte
• Sperm swim from antheridia to archegonia (of
  different plants) to form a diploid zygote
• Zygote grows into a large diploid sporophyte
  plant
• Haploid spores emerge and land on the ground to
  sprout into gametophytes

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Seed Plants

• Flowering plants
• Gametophyte generation exists inside the
  sporophyte generation and depends totally on it
• Meiosis occurs inside anthers and pistils
• Anthers produce microspores, forming male
  gametophytes
• Ovules produce megaspores, forming female
  gametophytes

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Fertilization of Seed Plants

• Occurs in ovary, forming zygotes that develop
  into sporophyte embryos in the ovule
• Ovule becomes the seed, carrying embryo and food
  for it

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Seed Plants

• Gymnosperms (conifers)
• Sporophytes whose sporangia are packed inside
  cones
• Gametophyte generation develops from haploid
  spores
• Small pollen cones produce microspores, which
  become male gametophytes
• Larger ovulate cones produce megaspores, which
  become female gametophytes

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Plant Responses to Stimuli

• Hormones
• Coordinate growth, development and environmental
  responses
• Signal transduction pathways amplify hormone
  signals and connect them to specific cell
  responses

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Auxin

• Responsible for phototropisms
• Enhances apical dominance (upward growth)
• Stimulates stem elongation and growth
• Indoleacetic acid
• Spraying synthetic auxin on tomato plants induces
  fruit production without pollination (seedless
  tomatoes)

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Cytokinins

• Stimulate cytokinesis and cell division
• Delay senescence (aging) by inhibiting protein
  breakdown
• Produced in roots and travel upward

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Gibberellins

• Promote stem and leaf elongation
• Work with auxins to promote cell growth
• Induce bolting, the rapid growth of a floral stalk

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Abscisic Acid

• Inhibits growth
• Enables plants to withstand drought
• Closes stomates during stress
• Promotes seed dormancy, keeping seeds from
  sprouting until spring

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Ethylene

• Gas hormone
• Promotes fruit ripening (positive feedback)
• Produced during stressful times
• Facilitates apoptosis, or cell death
• Promotes leaf abcission
• If a leaf falls from the plant, a scar forms to
  prevent pathogens from entering

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Tropisms

• Growth of a plant toward or away from a stimulus
• Thigmotropism (touch)
• Geotropism (gravity)
• Phototropism (light)
• Growth toward a stimulus is positive tropism
  while away from it is negative tropism

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Signal Transduction Pathway

• Reception, transduction and response
• Receptor is stimulated and activates a second
  messenger
• Secondary messengers are cyclic nucleotides (AMP
  and GMP) that transfer and amplify signals
• This messenger leads to a response by altering
  factors and targeting specific cells

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Photoperiodism

• Physiological response to the photoperiod
• Relative lengths of day and night
• Biological clock set to 24 hour day (circadian
  rhythm)
• Long-day plants (short night plants)flower when
  light period is longer than a certain number of
  hours
• Short-day plants and day-neutral plants flower
  regardless of length of day

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Photoperiodism

• Phytochrome is the pigment responsible for
  keeping track of the length of days and nights
• Pr (red light absorbing)
• Phytochrome is synthesized in this manner
• When the plant is exposed to light, it convers
  to
• Pfr (infrared light absorbing)
• In the dark, Pfr reverts back to Pr
• Enables plant to keep track of time