Plant Structure,
Growth &
Development
Chapter 35
______________
Plant Responses
Chapter 39
Chapter 35
Hierarchical Organization in
Plants
 Three basic plant organs:
1. Roots
2. Stems
3. Leaves
Roots
 Anchors vascular plant in soil
 Absorbs minerals and water
 Stores carbohydrates
 Root Parts:
• Taproot – main vertical root of eudicots
and gymnosperms, develops from
embryonic root
• Lateral roots – branch from taproot
 In monocots, embryonic root dies and
small roots emerge from stem – fibrous
root system
Stems
 Raise or separate leaves, exposing them
to sunlight
 Also raise reproductive structures to
facilitate pollen dispersal
 Alternating system of:
• Nodes – point where leaves are attached
• Internodes – stem segments between
nodes
 Most of the growth of a young plant is in
the apical (terminal) bud
 Leaves emerge at the axillary bud
Leaves
 Main photosynthetic organ in vascular
plants
 Consist of:
• Blade – generally flat
• Petiole – stalk that join blade to stem at
node
 Monocots – parallel veins
 Eudicots – branching veins
 Simple vs Compound leaves:
• Simple- single, undivided blade
• Compound – blade consists of multiple
leaflets
• Doubly compound – each leaflet is divided
into smaller leaves
Evolutionary Adaptations of Leaves
 Tendrils – “lassoes” something to provide
more support for the stem
 Spines – protection, do not
photosynthesize
 Storage leaves – succulents
 Reproductive leaves – make plantlets
that fall off and take root in soil
 Bracts – often mistaken for petals;
surround a group of smaller flowers and
attract pollinators
Tendrils
Spines
Storage
Reproductive
Bracts
Plant Tissues
 Dermal – plant’s outer, protective
covering (epidermis and cuticle)
 Vascular – carries out long distance
transfer of materials between roots and
stems (xylem & phloem)
 Ground – neither dermal nor vascular;
performs specialized functions such as
storage, photosynthesis & support
Plant Cells
 Parenchyma – perform most of the
metabolic functions of plant
 Collenchyma – flexible, support young
parts of plant shoot
 Sclerenchyma – rigid support for mature
plants, especially trees
Plant Cells, cont.
 Water conducting cells in Xylem (dead
at functional maturity)
• Tracheids
• Vessel elements
 Sugar-conducting cells in Phloem (alive
at functional maturity)
• Sieve tube elements
• Sieve plates
• Companion cells
Growth in Plants
 Unlike animals, plant growth is not limited
to an embryonic and juvenile period
 Indeterminate growth – at any time the
plant as a whole has embryonic,
developing and maturing organs
 Leaves, thorns and flowers undergo
determinate growth
Growth in Plants, cont.
 Meristems - specific regions of growth
• Contain stem cells: perpetually embryonic tissue
• regenerate new cells
 Apical shoot & root meristem
o Provide growth in length
o primary growth
 Lateral meristem
o Provide growth in girth
o secondary growth
Apical meristems
shoot
root
Root structure & growth
protecting the meristem
Woody stem
How old is
this tree?
cork cambium
vascular cambium
late
early
3
2
1
xylem
phloem
bark
Chapter 39
Response to Stimuli
 At the organismal level, plants and animals
respond to environmental stimuli by different
means
 Animals, being mobile, respond mainly by
moving toward positive stimuli and away from
negative stimuli
 Plants are stationary and generally respond to
environmental cues by adjusting their individual
patterns of growth and development
• For this reason, plants of the same species
can have much more variation in body form
than animals of the same species
Tropisms
 Tropism – any growth response that results in
plant organs curving toward or away from a
stimulus
• Phototropism – growth of a shoot toward
(positive) or away (negative) from a light
source
• Photoperiodism – physiological response to a
photoperiod (like flowering)
• Gravitropism – growth of shoots and roots in
response to the pull of gravity
• Thigmotropism – directional growth in
response to touch
Growth in Plants
 Etiolation – morphological adaptations
for growing in darkness
• Pale stems
• Rapid stem elongation
• Unexpanded leaves
• Short, stubby roots
• Example – young potato plant in the
soil
 De-etiolation (greening)
• Stem elongation slows
• Leaves expand
• Roots elongate
• Shoot produces chlorophyll
• Receptor in cytoplasm called a
phytochrome is responsible
Plant Hormones
 Hormone – signaling molecule that is
produced in tiny amounts by one part of an
organism’s body and transported to other
parts, where it binds to a specific receptor
and triggers responses in target cells and
tissues (phew!!!!)
 Plant hormone = plant growth regulator
 One hormone can regulate a diverse array
of cellular and developmental processes
 Multiple hormones can influence a single
process
Plant hormones
 Auxin
 Gibberellins
 Abscisic acid
 Ethylene
 and more…
Auxin (IAA)
 Effects
 controls cell division
& differentiation
 phototropism
• growth towards light
• asymmetrical distribution of auxin
• cells on darker side elongate faster
than cells on brighter side
 apical dominance
Gibberellins
 Family of hormones
• over 100 different gibberellins identified
 Effects
• stem elongation
• fruit growth
• seed germination
plump grapes in grocery
stores have been treated
with gibberellin hormones
while on the vine
Abscisic acid (ABA)
 Effects
 slows growth
 seed dormancy
• high concentrations of abscisic acid
• germination only after ABA is inactivated or
leeched out
• survival value:
seed will germinate only
under optimal conditions
• light, temperature, moisture
Ethylene
 Hormone gas released by plant cells
 Effects
• fruit ripening
• leaf drop
o like in Autumn
o apoptosis
Fruit ripening
 Adaptation
• hard, tart fruit protects
developing seed from herbivores
• ripe, sweet, soft fruit attracts
animals to disperse seed
 Mechanism
• triggers ripening process
o breakdown of cell wall - softening
o conversion of starch to sugar - sweetening
• positive feedback system
o ethylene triggers ripening
o ripening stimulates more ethylene production
Apoptosis in plants
 Many events in plants involve
apoptosis
• response to hormones
o ethylene
o auxin
• death of annual plant after
flowering
o senescence
• differentiation of xylem
vessels
o loss of cytoplasm
• shedding of autumn leaves
What is the
evolutionary
advantage of
loss of leaves
in autumn?