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Plant Physiology – Plant Tissues
•
Flowering plants consist of two major regions:
the root system and the shoot system
• How do plants grow?
 plants grow throughout their lives – have two
major categories of cells:
 meristem cells – embryonic, undifferentiated
cells capable of cell division
 apical meristem – (tip) located at the ends of
roots and shoots – results in primary growth
 lateral meristem – (side) also called cambium
– growth in width – results in secondary
growth
 differentiated cells – specialized in structure
and function
•
Major structures of plants (roots, stems,
leaves) consist of three tissue systems:
1.Dermal tissue – covers outer
surfaces of the plant body
–epidermal tissue (epidermis) –
outermost cell layer
–periderm – replaces epidermal
tissue on the roots and stems of
woody plants as they age
2. Ground tissue – all nondermal and
nonvascular tissues
– parenchyma – most abundant, carry out
most of metabolic activities
(photosynthesis), also function in storage of
sugars or starches, and secretion of
hormones
– collenchyma – elongated, many-sided cells,
source of support (especially in young or
herbaceous plants)
– sclerenchyma – cells with thick, hardened
secondary cell walls reinforced with lignin –
also used to support and strengthen plant
(found in xylem, phloem, nut shells, outer
covering of peach pits)
3.
Vascular tissue
A. Xylem – conducts water and minerals from roots to
shoots – two types:
• Tracheids – thin cells with slanted ends that
overlap
– overlapping walls contain pits – allow water and
minerals to pass from one tracheid to next
• Vessel elements – meet end to end, larger in
diameter than tracheids – ends either flat or
overlapping
– form wide-diameter “pipelines” called vessels
from roots to leaves
• xylem develops thick walls to help support plant
• when functional, xylem cells die leaving hollow
tube of cell wall
B.
Phloem – conducts sugars, amino acids, and hormones
– sieve tubes – constructed of a single strand of cells
called sieve-tube elements
• adjacent cells meet at the sieve plates which have
holes – interiors of adjacent sieve-tube elements
are connected through these holes forming
continuous conducting system
– each sieve-tube element is nourished by a smaller
adjacent companion cell – connected to sieve-tube
element by cytoplasm-filled channels called
plasmodesmata
• companion cells regulate movements of sugars into
and out of sieve tubes
Plant Physiology/Anatomy – Roots
• Root function: anchorage, absorption, and storage
• Root systems
– taproot systems – consist of a primary root which
becomes longer and stouter with time and many
smaller roots that grow from primary root
– fibrous root systems – many slender roots of equal
size
• Primary growth of roots
– longitudinal section
• root cap – protective
cover at tip covering
apical meristem –
protects against
abrasive soil
• meristematic region –
region of cell division
• elongation region – cells
begin to elongate
•
•
cell differentiation
– cells mature and
differentiate into
various types of
tissue (i.e. xylem,
phloem, etc)
root hairs –
microscopic
extensions of
epidermis –
increase surface
area for water
absorption
Root Cross Section
•
•
epidermis – outer
protective layer –
layer from which
root hairs grow
cortex – loosely
packed
parenchyma cells
used for storage
(starch)
•
vascular cylinder (stele) consists of:
1. endodermis – regulates movement of
materials into center of root (xylem)
•
each cell has special bandlike region – Casparian
strip – casparian strips contain suberin (fatty,
waterproof material)
2. pericycle – gives rise to lateral (secondary roots)
3. xylem – forms “x” in center
4. phloem – located in patches between xylem
“arms”
5. vascular cambium – “sandwiched” between
xylem and phloem – gives rise to secondary
vascular tissues
Plant Physiology/Anatomy – Stems
•
•
Main functions: support and conduction
All stems have:
1.
buds – undeveloped, embryonic shoots
•
terminal bud – located at tip of stem – covered
with bud scales when dormant
– leaves bud scars on stem between growing
seasons
•
axillary buds (lateral buds) – form stems bearing
leaves or flowers
2.
nodes – area on a stem where each leaf is attached
•
internodes – region between successive nodes
Dicot stem cross section
1. epidermis – outer
covering for protection
• in woody stems,
contains cork
cambium which
produces periderm
• eventually replaces
epidermis with cork
cells that are
waterproof and form
bark
2.cortex – photosynthesis
(herbaceous stems),
storage, and support
3.Vascular bundles – arranged
in a ring
• xylem and phloem
separated by vascular
cambium – results in
secondary growth
4. Pith - center of herbaceous
stems used for storage
•
woody stems – secondary xylem is
made to interior, secondary phloem
made to exterior of vascular
cambium
• younger, functional secondary
xylem is called sapwood
(closest to bark)
• older
wood in center of stem is
heartwood – no longer
functioning – plugged with
pigments, tannins, gums, resins,
and other materials – dense,
provides support
Monocot stem cross section
•
•
vascular bundles are scattered
no vascular cambium – no secondary
growth
Dicot Stem
Monocot Stem
Water (and mineral) transport in stems
•
•
First, water moves by osmosis from the soil
through the root into the xylem in the vascular
cylinder
• cells in root have more dissolved materials in
them compared to the soil
Water follows two
routes into the
center of the root:
1. Symplastic
route – through
the cells
2. Apoplastic
route – around
the cell walls
Cohesion-Tension Theory
• also called Transpiration–Cohesion
• water is pulled up the xylem powered by the
evaporation of water from the leaves –transpiration
 plants lose water (as water vapor) through
microscopic pores on the underside of leaves –
stomata

tension created
by this process
pulls water
upward from root
xylem into stem
xylem (like
sucking on a
straw) – as water
is pulled up,
additional water
from the soil is
drawn into the
roots
•
•
only possible with an unbroken column of water
in xylem throughout plant
– water forms unbroken column because
water molecules are cohesive (tend to
cling to each other because of hydrogen
bonding) AND water molecules tend to
cling to the walls of xylem (adhesion)
Water potential gradient plays an important role
in this theory
Water Potential
•
•
•
free energy of water, a measure of a cell’s ability
to absorb water by osmosis
water potential of pure water is 0 – when solutes
are dissolved in water, its free energy decreases
(negative number) – water moves from a region
of higher (less negative) water potential to a
region of lower (more negative) water potential
a water potential gradient exists in a plant from
the least negative (the soil) up through the plant
to the most negative (the atmosphere – has
much less water in it than the soil) – this literally
pulls water from the soil up through the plant
Root pressure
•
water that moves into a plant’s roots from the soil is
pushed up through xylem to the top - less important
mechanism for water transport
• occurs because nutrient mineral ions are actively
pumped into xylem, decreasing water potential
• water moves into root from soil because of
difference in water potential btw soil and root cells
• may result in guttation – liquid water is forced out
through special openings in leaves
Translocation of sugar in phloem
•
•
•
Sugar produced by leaves is converted into
sucrose and then loaded into phloem and
translocated in solution to rest of plant
Moves both upward and downward
Pressure-flow hypothesis – dissolved sugar
moves in phloem by means of a pressure
gradient (difference in pressure) which
results when sugar is translocated from a
source (area of excess sugar supply,
usually a leaf) to a sink (area of storage in
the form of starch)
• sugar is moved from leaf into sieve tube
members by active transport (increases
concentration)
• water moves in after the sugar because of
osmosis – this increases hydrostatic
pressure
• at the destination (sink) sugar is unloaded
from sieve tube members – reduces
concentration and water tends to also flow
out into surrounding tissues reducing
hydrostatic pressure
Plant Physiology/Anatomy – Leaves
•
•
Leaves are the primary site of photosynthesis
Leaf cross section

covered by upper and lower epidermis –
epidermis covered by waxy cuticle to reduce
water loss

lower epidermis has tiny pores that allow gas
exchange – stomata
• each stoma is flanked by guard cells – changes in
shape open and close stomata
• usually open during the day when CO2 is needed
and closed at night when photosynthesis is not
occurring
• light and concentration of CO2 trigger opening and
closing – works by triggering movement of H+ and
K+
• K+ is actively pumped into
guard cell vacuoles
increasing solute
concentration - causes
water to enter (increase
turgor pressure) and open
stoma
• In the afternoon/evening
the reverse occurs, closing
stomata (decreased turgor
pressure) – abscisic acid
assists in this process by
causing K+ to rapidly
diffuse out of guard cells –
made by roots during time
of water deficiency
• mesophyll – “middle leaf” – made up of parenchyma tissue
packed with chloroplasts - main site of photosynthesis
 palisade layer – upper layer of column-shaped cells –
main site of photosynthesis
 spongy layer – layer just below palisade cells, irregularly
shaped with lots of air spaces between to facilitate gas
exchange – also photosynthesize but primary function is
to allow for diffusion of gases
 Veins (vascular bundles) – contains xylem and phloem,
extend through mesophyll
Flowering Plants – Angiosperms
•
Largest, most successful group – flowers function in
sexual reproduction
•
•
flowers are
reproductive
shoots
• receptacle –
tip of stalk on
which some or
all flower parts
are borne
sepals (calyx) –
cover and protect
bud
• petals (corolla)–
vary in shape,
color, and
fragrance – attract
pollinators
• stamens – male
reproductive
organs
 filament – thin
stalk on which
anther sits
 anther – saclike
structure in which
pollen grains form
•pistil – female part of
flowers
stigma – sticky tip to
catch pollen
style – necklike
structure through which
pollen tube grows down
to ovary
ovary – juglike structure
that contains one or
more ovules (develops
into seeds)
each ovule contains
an embryo sac that
forms an egg and
two polar nuclei
Flower Fertilization
• starts with pollination – transfer of pollen from
anther to stigma
 pollen grain contains a tube cell and two sperm
cells
 tube cell forms a pollen tube and digests its way
down to the ovary – sperm cells follow
•
one sperm cells fertilizes egg (becomes embryo)
the other fertilizes the two polar nuclei forming
the endosperm (food source for embryo in seed)
•process is called double
fertilization
•ovule develops into seed
•surrounding ovary
develops into fruit
•seed – embryonic plant,
endosperm (food source),
wrapped in a seed coat
Comparison of Monocots and Dicots
Control of Flowering in Angiosperms
• Photoperiodism – plant’s response to light involving
the relative lengths of day and night
• Important factor in control of flowering
Plant Type
Flowering and light
Examples
Long – day
Bloom when days are
longest and nights
shortest (midsummer)
Radishes, spinach,
and lettuce
Short – day
Bloom in spring, late
summer, and autumn
when days are shorter
Poinsettias,
chrysanthemums,
and asters
Day – neutral Flower without regard to
day length
Roses, dandelions,
tomatoes
• Flowering is actually controlled by length of the night
• Short-day plants will only flower if exposed to a long
continuous period of darkness (must have a critical period
of darkness or more)
• Long-day plants will only flower is exposed to a shorter
continuous period of darkness (must have a critical period
of darkness or less)
Plant Hormones
• Auxin
– Found in the embryos of seeds, meristems of apical
buds and young leaves, and young developing shoots
– Responsible for phototropism – plant growth in
response to light (plant stems exhibit positive
phototropism and roots demonstrate negative
phototropism)
– Auxin causes positive phototropism of shoots and
seedlings
– Auxin causes elongation of cells on the side of the
shoot necessary to cause growth towards the light
– Auxin accumulates on the side of the stem away from
the light source
Phototropism