Download Chapter 35: The Plant Body

Chapter 35: The Plant Body

structure reveals interaction with
environment on two scales:
– long-term = accumulate morphological
adaptations due to natural selection

Ex. Cactus leaves have become so reduced that the
stem is the primary photosynthetic surface
– short-term = structural responses to specific
environment

Ex. Cambomba
– leaves developed while underwater
• light & feathery (increase surface area)
– leaves above water are arrowhead-shaped
• aid in floatation


plant architecture is far more flexible than
animal
physiological adjustments of plant are even
faster than structural ones
– Ex. plant produces hormone that causes
stomata to close

morphology of terrestrial plants reflects
need to inhabit and draw resources from
two very different environments—soil and
air
– soil = source of H2O, minerals
– air = source of CO2, light
3 basic plant organs

ROOT SYSTEM (subterranean)
– monocots (i.e. grasses) usually have
fibrous root systems
– most absorption occurs at root tips; tiny
root hairs increase surface area
– some plants have adventitious roots
(grow above ground)
 adventitious
= plant part that grows in
atypical location
3 basic plant organs

SHOOT SYSTEM (stems & leaves)
– may be vegetative (leaf-bearing) or
reproductive (flower-bearing)
– stem = alternating system of nodes and
internodes
 node
= points at which leaves attach
 internode = stem segments between nodes
– in angle between node and internode is
axillary bud
 potential
dormant)
to form vegetative branch (most are
3 basic plant organs

SHOOT SYSTEM (stems & leaves)
– growth is usually concentrated at apex,
site of the terminal bud
 presence
is partially responsible for limiting
growth of axillary buds
 called apical dominance
– plant concentrates resources on growing taller 
increased exposure to light
 if
an animal eats the terminal bud or light is
more available at lower levels, axillary buds
break dormancy
– rationale for pruning shrubs and “pinching back”
plants (makes them bushy)
3 basic plant organs

SHOOT SYSTEM (stems & leaves)
– Modified shoots with diverse functions
have evolved
 stolons
= “runners” of strawberry plants
 rhizomes = horizontal stems that grow
underground
 tubers (I.e. potatos) = swollen ends of
rhizozomes specialized for storing food
 bulbs = vertical, underground shoots
consisting mostly of swollen bases of leaves
that store food (Ex. onion)
3 basic plant organs

LEAVES
– leaves = main photosynthetic organ of
most plants
 vary
in form, but usually have:
– (a) blade = flattened surface
– (b) petiole = stalk…joins leaf to stem
 shape,
spatial arrangement on stem, vein
pattern are used to identify leaves
plant organs are composed
of 3 tissue systems

DERMAL TISSUE (epidermis, “skin”)
– single layer of tightly packed cells that
covers and protects all young parts of the
plant
 general
function: protection
 other functions: depend on organ its covering
– Ex. root hairs  absorption
– Ex. leaf cuticle  water retention
plant organs are composed
of 3 tissue systems

VASCULAR TISSUE
– involved in transport of materials between
roots and shoots
– xylem = conveys water and dissolved
minerals (roots  shoots)
 water-conducting
elements
– tracheids and vessel elements = elongated cells
that are dead at functional maturity
– secondary walls interrupted by pits (only primary
walls present)
– secondary walls harden with lignin, which provides
structural support
plant organs are composed
of 3 tissue systems

VASCULAR TISSUE
– involved in transport of materials between roots
and shoots
– phloem = transports food from mature leaves to
roots and nonphotosynthetic parts (ex. fruits)

sucrose and other organic compounds are transported
through tubes formed by chains of cells called sievetube members
– alive at functional maturity, but lack many organelles
– end walls (sieve plates) facilitate flow of fluid
– companion cells have organelles…may supply sievetube members
plant organs are composed
of 3 tissue systems

GROUND TISSUE
– neither dermal nor vascular
– diverse functions include photosynthesis,
storage, support
root systems

TAPROOTS (dicots)
– one large, vertical roots with many
smaller lateral (branch) roots
– Ex. dandelion
– firmly anchors plant in soil
– store more food than fibrous roots
– plants consumes resources during time of
flowering or fruit production
root systems

FIBROUS ROOTS (monocots)
– mat of thin roots that spread out below
the surface
– increase exposure to soil water and
minerals
– tenacious anchor to ground
– Ex. grass roots concentrate a few cm
below the ground, holding topsoil in place
(prevents erosion)

the protoplasts of neighboring cells are
generally connected by
plasmodesmata, cytoplasmic
channels that pass through pores in
the walls
– the endoplasmic
reticulum is
continuous through
the plasmodesmata
in structures called
desmotubules

An adhesive layer, the middle lamella,
cements together the cells wall of
adjacent cells.
– The primary cell wall is secreted as the
cell grows.
– Some cells have
secondary walls
which develop
after a cell stops
growing.
plant tissues are composed of
three basic cell types:
parenchyma, collenchyma, and
sclerenchyma


parenchyma cells are often depicted as
“typical” plant cells because they generally
are the least specialized, but there are
exceptions
parenchyma cells perform most of the
metabolic functions of the plant,
synthesizing and storing various organic
products
– Ex. photosynthesis occurs within the
chloroplasts of parenchyma cells in the leaf
– Ex. the fleshy tissue of most fruit is composed
of parenchyma cells
parenchyma cells

developing plant cells of all types are
parenchyma cells before specializing further
in structure and function
– mature, unspecialized parenchyma cells do not
generally undergo cell division
– most retain the ability to divide and differentiate
into other cell types under special conditions during the repair and replacement of organs
after injury to the plant
– in the laboratory, it is possible to regenerate an
entire plant from a single parenchyma cell
collenchyma cells

have thicker primary walls than parenchyma
cells, though the walls are unevenly
thickened
– grouped into strands or cylinders, collenchyma
cells help support young parts of the plant shoot
– young cells and petioles often have a cylinder of
collenchyma just below their surface, providing
support without restraining growth
– functioning collenchyma cells are living and
flexible and elongate with the stems and leaves
they support
schlerenchyma cells

sclerenchyma cells also function as
supporting elements of the plant, with thick
secondary walls usually strengthened by
lignin
– they are much more rigid than collenchyma cells
– unlike parenchyma cells, they cannot elongate
and occur in plant regions that
have stopped lengthening
– many sclerenchyma cells are dead at functional
maturity, but they produce rigid secondary cells
walls before the protoplast dies
– vessel elements and tracheids in the xylem are
sclerenchyma cells that function for both support
and transport
growth
most plants demonstrate
indeterminate growth, growing as
long as the plant lives
 most animals (and certain plant organs
such as flowers and leaves) undergo
determinate growth, ceasing to grow
after they reach a certain size


annual plants complete their life cycle from germination through flowering and
seed production to death - in a single year
or less
– many wildflowers and important food crops, such
as cereals and legumes, are annuals

the life of a biennial plant spans two years
– often, there is an intervening cold period
between the vegetative growth season and the
flowering season

plants that live many years, including trees,
shrubs, and some grasses, are perennials
– these often die not from old age, but from an
infection or some environmental trauma

a plant is capable of indeterminate
growth because it has perpetually
embryonic tissues called meristems in
its regions of growth
– these cells divide to generate additional
cells, some of which remain in the
meristematic region while others become
specialized and incorporated into the
tissues and organs of the growing plant.

the pattern of plant growth depends on
the location of meristems

apical meristems, located at the tips of
roots and in the buds of shoots, supply cells
for the plant to grow in length
– this elongation, primary growth, enables roots
to ramify through the soil and shoots to extend
their exposure to light and carbon dioxide
– woody plants also show secondary growth,
progressive thickening of roots and shoots


secondary growth is the product of lateral meristems,
cylinders of dividing cells extending along the length of
roots and shoots
one lateral meristem replaces the epidermis with bark
and a second adds layers of vascular tissue
in woody plants, primary growth is
restricted to the youngest parts of the
plant - the tips of the roots and shoots
 each growing season, primary growth
produces young extensions of roots
and shoots, while secondary growth
thickens and strengthens the older part
of the plant


at the tip of a winter twig of a
deciduous tree is the dormant terminal
bud, enclosed by scales that protect its
apical meristem
– in the spring, the bud will shed its scales
and begin a new spurt of primary growth
– along each growth segment, nodes are
marked by scars left when leaves fell in
autumn
– above each leaf scar is either an axillary
bud or a branch twig
•farther down the twig
are whorls of scars left
by the scales that
enclosed the terminal
bud during the previous
winter
•each spring and
summer, as the primary
growth extends the
shoot, secondary growth
thickens the parts of the
shoot that formed in
previous years

the root tip is covered by a thimblelike
root cap, which protects the meristem
as the root pushes through the
abrasive soil during primary growth
– the cap also secretes a lubricating slime

growth in length is concentrated near
the root’s tip, where three zones of
cells at successive stages of primary
growth are located
– zones: zone of cell division, zone of
elongation, zone of maturation
zone of cell division

the zone of cell division includes the apical
meristem and its derivatives, primary
meristems
– the apical meristem produces the cells of the
primary meristems and also replaces cells of the
root cap that are sloughed off

near the middle is the quiescent center,
cells that divide more slowly than other
meristematic cells
– these cells are relatively resistant to damage
from radiation and toxic chemicals
– they may act as a reserve that can restore the
meristem if it becomes damaged
zone of cell division

just above the apical meristem, the
products of its cell division form three
concentric cylinders of cells that
continue to divide for some time
– primary meristems:
 protoderm
 procambium
 ground
meristem will produce the three
primary tissue systems of the root…dermal,
vascular, and ground tissues
zone of elongation

the zone of cell division blends into the
zone of elongation where cells
elongate, sometimes to more than ten
times their original length
– it is this elongation of cells that is mainly
responsible for pushing the root tip,
including the meristem, ahead
– the meristem sustains growth by
continuously adding cells to the youngest
end of the zone of elongation
zone of maturation

in the zone of maturation, cells begin
to specialize in structure and function.
– in this root region, the three tissue
systems produced by primary growth
complete their differentiation, their cells
becoming functionally mature

three primary meristems give rise to
the three primary tissues of roots.
– dermal tissue  epidermis
– ground tissue  endodermis and cortex.
– vascular tissue  stele, pericycle, pith,
xylem, and phloem.
meristems

the protoderm, the outermost primary
meristem, produces the single cell
layer of the epidermis
– water and minerals absorbed by the plant
must enter through the epidermis
– root hairs enhance absorption by greatly
increasing the surface area

the procambium gives rise to the stele,
which in roots is a central cylinder of
vascular tissue where both xylem and
phloem develop
meristems

the ground tissue between the
protoderm and procambium gives rise
to the ground tissue system
– these are mostly parenchyma cells
between the stele and epidermis
– they store food and are active in the
uptake of minerals that enter the root with
the soil solution

the innermost layer of the cortex, the
endodermis, is a cylinder one cell
thick that forms a boundary between
the cortex and stele
shoots



leaves arise as leaf primordia on flanks of
apical meristem
axillary buds develop from islands of
meristem cells left by apical meristems at
bases of leaf primordia
most of elongation occurs by growth in
length of slightly older internodes below
shoot apex
– due to cell division and elongation by internode

lateral branches originate from axillary buds
on surface (unlike roots where lateral roots
extend from deep pericycle)
shoots—primary tissues
vascular tissues run the length of the
stem in vascular bundles (perimeter)
 when shoot becomes root, vascular
system converges in vascular cylinder
in center of roots
 each vascular bundle is surrounded by
ground tissue
 dicot…inner core referred to as pith

ww.royaloakschools.com/portal/sites/default/files/apbio Genetics Review.ppt
shoots—primary tissues

leaf epidermis is composed of tightly
interlocked cells (like pieces of a
puzzle)
– 1st line of defense against physical
damage and pathogens
– cuticle is barrier against water loss
– cuticle is interrupted by stomata flanked
by guard cells (responsible for gas
exchange)
Chapter 37:
Plant Nutrition

the fact that plants are autotrophic
does not mean that they are
autonomous
– need sunlight to drive photosynthesis
– need CO2, H2O minerals to synthesize
organic compounds
mineral nutrients = essential
chemical elements absorbed from
the soil in the form of organic ions

80-85% of herbaceous plant mass is H2O
– considered a nutrient because it supplies the
majority of H and O for organic compounds
(most water is lost to transpiration)
– 90% of plant’s water is lost to transpiration
– bulk of plant’s organic material comes from CO2
– most of organic material is carbohydrate
– plants require micronutrients (small amounts)
and macronutrients (large amounts)
– essential nutrient = required for plant to grow
from seed and complete life cycle
symptoms of mineral deficiency
depend on function and mobility of
element





young, growing tissues have more “drawing
power”
Ex. deficiency of Mg (ingredient of
chlorophyll)  yellow leaves (chlorosis)
signs of deficiency are seen first in older
leaves
can confirm deficiency by appearance of
plant or by analyzing soil
hydroponic growth of plants allows precise
control of nutrients
Role of soil in plant nutrition


lichens, fungi, bacteria, mosses, roots of
plants secrete acids  breakdown of rock
 soil
topsoil = mixture of particles derived from
rock and living organisms
– Astonishing array of organisms


humus = residue of partially decayed
organic material (prevents clay from packing
together)
horizon = distinct soil layer
– texture of soil depends on size of particle

loam = moist, fertile soils
A
B
C
soil conservation
soil conservation is related to the idea
that agriculture is unnatural
 to grow a ton of wheat, the soil gives
up 18.2 kg of nitrogen, 3.6 kg of
phosphorus, and 4.1 kg of potassium

– each year, the fertility of the soil
diminishes unless fertilizers are applied to
replace lost minerals
soil conservation
concerns
fertilizer
 irrigation
 erosion

– no-till agriculture
– contour farming
– shelter belts
nitrogen as a plant nutrient

the metabolism of soil bacteria makes
nitrogen available to plants
– atmosphere is 80% nitrogen, but in
gaseous form
– plants require ionic form (NH4+ or NO3-)
– short-term source: decomposition of
humus by microbes
 however,
denitrifying bacteria convert NO3- to
N2
– longer-term source: nitrogen-fixing
bacteria
 convert
N2 to NH3
soil conservation
crop rotation
 role of clover and other legumes

mycorrhizae
symbiotic association of roots and
fungi that enhance plant nutrition
 symbiosis is mutualistic

– fungus benefits from a hospitable
environment and a steady supply of sugar
donated by the host plant
– in return, fungus increases surface area
for water uptake and selectively absorbs
minerals from the soil to supply the plant
nutritional adaptations

parasitic plants
– Ex. mistletoe = photosynthetic, but supplements
nutrition with haustoria to siphon xylem sap from
host tree
– epiphytes are often confused with parasitic
plants

carnivorous plants
– usually live in acid bogs where soil conditions
are poor
– fortify nutrition by feeding on animals
– modified leaves usually equipped with glands
that secrete digestive juices
Chapter 38:
Plant Reproduction

sexual reproduction
– characterized by alternation of
generations
= dominant generation  flower
 gametophyte = dramatically reduced (a few
cells)
 sporophyte
– flowers are specialized shoots bearing
the reproductive organs of the
angiosperm sporophyte
 flowers
are determinate (stop growing once
flower and fruit are formed)
sexual reproduction





stamens (filament + anther) = male
reproductive organs
carpels (stigma + style + ovary) = female
reproductive organs
after fertilizaition, ovule that contains
embryo develops into seed
entire ovary develops into fruit containing
one or more seeds
numerous floral variations have evolved
– complete flower = has all 4 organs
– incomplete flower = lacks one or more part(s)
sexual reproduction

bisexual (perfect) flower
– has stamens and carpels

unisexual (imperfect) flower
– has stamens or carpels
– monoecious (“one house”) vs. diecious
(same plant)

male and female gametophytes
develop within anthers and ovaries,
respectively (pollination brings them
together)
Development of the
male gametophyte

within sporangia (pollen sac) of anther:
Development of the
female gametophyte

within ovary:
Pollination is the first step in
chain of events leading to
fertilization
 plants have various mechanisms that
prevent self-fertilization
 self-incompatibility = ability of plant
to reject its own pollen and the pollen
of closely related individuals
– analogous to immune response
 however,
in this case, plant is rejecting “self”
– based on “self” genes; may be as many
as 50 genes at locus
– genes seem to have evolved separately
in various plant families
as many as 50 different alleles at S locus
Double fertilization gives rise to
zygote and endosperm



after landing on a receptive stigma, a pollen
grain absorbs moisture and germinates
germinated pollen grain is mature male
gametophyte
double fertilization
– sperm + egg  zygote
– sperm + 2 polar nuclei  endosperm


ensures that endosperm will develop only in
ovules where egg has been fertilized
like animals, block to polyspermy occurs
The ovule develops into a seed
containing an embryo and a
supply of nutrients.

after fertilization
– ovule  seed (major nutrient sinks)
 seed

stockpiles proteins, oils, and starch
– ovary  fruit
endosperm development
– usually precedes embryo development
– rich in nutrients to supply growing
embryo
The ovule develops into a seed
containing an embryo and a
supply of nutrients.

embryo development
– cotyledons = “seed leaves”
– development of embryo
(a) establishes root-shot axis
(b) establishes radial pattern of
protoderm, ground meristem,
procambium
seed structure
during the last stages of its maturation,
the seed dehydrates until its water
content is only 5-15% of its weight
 the embryo and its food supply are
enclosed by a protective seed coat
formed from the integuments of the
ovule

seed structure
ovary develops into fruit
adapted for seed dispersal

during fruit development, the wall of the ovary
becomes the pericarp, the thickened wall of
the fruit
– in apples, fleshy part of fruit is derived from
swollen recepticle; only the apple core develops
from the ovary

fruit usually ripens about the same time its
seeds are completing their development
– in fleshy fruits, the ripened fruit entices animals who
help spread the seeds
– with time, the inner parts of the fruit become softer as
enzymes digest components of cell walls
– organic acids are converted to sugars as fruit ripens
ovary develops into fruit
adapted for seed dispersal




fruit = protects enclosed seeds, aids in seed
dispersal by wind or animals
pollination triggers hormonal changes that
cause ovary to begin transformation to a fruit
usually ripens about same time seed is
completing its development
fruits in store are products of selective breeding
to produce exaggerated forms
Evolutionary adaptations of
seed germination contribute to
seed survival

as seed matures, it dehydrates and enters
dormancy
– low metabolic rate
– suspension of growth and development



conditions needed to break dormancy differ
between species
dormancy increases chances that germination
will occur at a time and place most
advantageous to the seedling
triggers: rainfall, heat, cold, light, chemicals
from seed to seedling





germination of seeds depends on the
physical process of imbibition (uptake of
water due to low water potential of dry seed)
imbibing water causes the seed to expand
and rupture its coat
also triggers metabolic changes to the
embryo that allow it to resume its growth
first organ to emerge is radicle
next, the shoot tip must break the soil
surface
Chapter 39: Plant
Responses to Hormones

signal transduction pathways link
internal and environmental signals to
cellular responses

hormone = chemical signals that coordinate
parts of the organism
– only minute amounts are required to induce
substantial change
– implies that signal must be amplified in some
way
– hormone actions:
(a) alter expression of genes
 (b) affect activity of existing enzymes
 (c) change properties of membranes

auxin

first plant hormone to be discovered;
stimulates cell elongation for primary
growth
– moves shoot  base at 10 mm/hour
– movement requires energy
– synthetic forms used as herbicides
(monocots like turfgrass can inactivate,
dicots cannot…die of hormonal overdose)
cytokinins

stimulate root growth and
differentiation; stimulate cell division
and cell growth
– produced in actively growing tissues
(especially roots, embryos, fruits)
– interact with auxin to control apical
dominance
gibberellins

promote seed germination; stimulate
flowering and development; promote
stem elongation
– Ex. treatment of seedless grapes
– increase size, growth of internodes
(more space,  air circulation, less
infection)
abscisic acid

slows down growth
(but no role in bud dormancy or leaf
abscission)
– effects seed dormancy
– enables plants to withstand drought
ethylene

promotes fruit ripening
– produced in response to stress such as
drought, flooding, mechanical pressure,
injury, and infection
– triple response to mechanical stress
 slow
stem elongation
 thickening of stem
 curvature of stem  horizontal
ethylene (cont.)

apoptosis
– associated with burst of ethylene
– dropping of leaf, shedding of flower, formation of
xylem vessel
– genetically, a very busy time

leaf abscission
– dropping of leaves keeps trees from desiccating
during winter when roots cannot absorb H2O
– before dropping leaves, many minerals are
salvaged and stored in stem parenchyma cells
– fruit ripening = example of positive feedback

CO2 inhibits synthesis of more ethylene
brassinosteroids

induce cell elongation at extremely low
concentrations (10-12 M)
Plant responses to light


plants detect not just the presence of light,
but also its intensity, direction, and
wavelength (color)
action spectrum relates physiological
response to wavelength of light
– red and blue light are most important

blue-light receptors are a heterogeneous
group of pigments
– blue light is most effective at initiating:
phototropism
 opening of stomata
 slowing of hypocotyl elongation

Phytochromes function as
photoreceptors in many plant
responses to light
regulate many of plant’s response to
light
 germination: red vs. far red
– seems to act as switch mechanism
for many events in plant life

Biological clocks control
circadian rhythms in plants (and
other eukaryotes)





not paced by any known environmental barrier
some oscillations due to changes in light levels,
temperature, relative humidity that are part of
normal 24 hour cycle
even under artificially constant conditions, many
physiologic processes (I.e. opening and closing of
stomata) continue to follow 24 hour periods
internal (without cues, periods vary from 21-27
hours)
critical night length = controls flowering, other
responses
critical night length

discovered in 1940s that night length
controls flowering and other responses
to photoperiod