Plant Science

Plan Diagram of Dicot Stem
• Cross-section shows:
– Epidermis – protective, often with waxy cuticle
– Cortex – often cells have thickenings which provide
secondary support of plant
– Vascular bundles: contain xylem, phloem and
– Cambium tissue: lateral meristem
where cell division produces secondary
xylem and phloem
- Pith – thin-walled cells (fill space and
sometimes degenerate if stem is hollow)
Actual Dicot Stem
Leaf Diagram
Functions of Leaf Tissues
• Absorption of light: mostly in
palisade layer
• Gas Exchange: mostly through
stomata – gases stored and
exchanged in spongy layer, usually
on bottom of leaf
• Support: xylem and phloem; veins
• Water conservation: waxy cuticle on
epidermis and stomata (closing)
• Transport of water: xylem
• Transport of Photosynthetic
Products: phloem
Does there seem to be a
relationship between the
placement of these
tissues and their
Monocots vs. Dicots
Monocots: Taproot with
adventitious extensions
Dicots: Tap root with lateral
Modifications of Roots
• Tubers: modified roots for storage of
photosynthetic products
• Eg. Potato stores starch (carbohydrate)
(eyes are axillary buds)
Modification of Roots
• Tap roots can also be
modified for other
• Eg. Carrot
• Grow in sandy soils –
anchor and store
water for plant
• Large surface area provided by root hairs and
- The extension of the cell wall increases the surface
area for the absorption of water and minerals at the
cellular level.
The root hair cell provides both an increase in the cell
wall (apoplastic pathway) and the cytoplasmic route
(symplastic pathway) for the movement of water
Movement of Minerals to the Root
3 Ways:
- Diffusion – often not sufficient if minerals are
in low concentration in soil
- Fungal hyphae – mycorrhizal relationships –
fungi increase surface area for absorption,
gain access to plant carbohydrates
- Mass flow…
Fungal hyphae on
Mass Flow Hypothesis
• Minerals dissolved in water that the plant is
absorbing form hydrogen bonds with water
and are thus ‘dragged’ near to plant with the
• This concentrates the minerals for absorption
Mineral Absorption in Roots By Active
• Clay particles are negatively charged – attract
positively charged ions like potassium,
sodium, calcium
1. proton pumps in roots pump
protons ( H+) outside of the cell.
This creates an electro-negative
2. When the root cells secrete
protons into the surrounding soil
water the hydrogen ions displace
the mineral ions from the clay
particle, freeing them into solution.
3. The mineral ions in the soil water
are free to be absorbed by various
Mineral Absorption in Roots By Active
• Plants can also directly actively transport
minerals into roots
• Experiments that metabolically poison the
root (stop ATP production) causes all mineral
absorption to stop.
Modifications of Stems
• Bulbs: short vertical underground stems
• Many fleshy highly modified leaves for the
storage of nutrient.
• Eg. Onion and Lily
• Not to be confused with
corms (daffodil, tulip, garlic) which are
More solid in tissue.
Stem Modification
• Runners: eg. Strawberry – allow plant to form
new plantlets (asexual reproduction)
• Tendrils: used by climbing plants for support
(eg. Peas)
• Tendril Animation
• Are areas of active cell
division in plant body
• Two main types:
– Apical
– Lateral (growth in girth)
Cambium forms
secondary growth
Apical Meristems
(a) Shoot apical meristem
(b) Leaf primordial
(c) Axillary bud primordium
(d) leaf
(e) Stem tissue
Apical Meristems
Root Apical Meristem
(a) Root cap.
(b) Root apical meristem.
(c) Ground meristem.
(d) Protoderm.
(e) Epidermal tissue of the root.
(f) Vascular tissue (central stele).
Primary Growth
• Added by apical
This tissue diagram is a cross
section of the stem of the
primary plant body.
This means that there has
been no additional secondary
thickening of the cell walls.
Secondary Growth
Secondary growth added by the Lateral meristem
(cambium) has two types:
1. Vascular cambium that produces secondary
xylem and phloem
2. Cork cambium produces some of the bark layer
of a stem.
• Bending of plant toward light
• Auxins are a group of plant hormones
• Unlike animal hormones, plant hormones
often have multiple target sites and actions
• Most important auxin may be IAA:
Darwin’s Experiments
Charles Darwin studies of auxin effects are
published a book called, 'The Power of movement'.
The cylindrical shoot is enclosed in a sheath of cells
called the coleoptile.
" when seedlings
are freely exposed
to a lateral light
some influence is
transmitted from
the upper to the
lower part, causing
the latter to bend".
• Stems may exhibit positive
phototropism, roots
negative phototropism
• Auxins cause cell elongation
and cell division on ‘dark’
side of plant
• (a type of auxin was used as
a herbicide “Agent Orange”
in Viet Nam – dioxins that
contaminated it caused
many health problems)
Plant Support
• Plants can support themselves in 3 ways:
– Thickening of cellulose walls
– Lignified xylem vessels
– Turgor pressure
Thickened Cellulose Walls
Cells deposit extra cellulose in areas that require
more support
Lignified Xylem Vessels
• A polymer that binds to cellulose to
strengthen cell walls
• Often deposited in rings
Rhubard with lignin rings
Turgor Pressure
• The pressure of a cell’s
contents (mostly caused
by the vacuole in plant
cells) on the interior cell
• Maintained by osmotic
• Low turgor pressure
causes ‘wilting’
• Regulate transpiration,
thus osmotic pressure in
• Regulated by abscisic
• Dehydrated mesophyll
cells release this
hormone which
stimulates stomata to
close, no matter how
much photosynthesis is
Factors Affecting Transpiration
• Humidity – high humidity reduces rate of
transpiration – low water gradient
• Wind – increases evap, increasing
• Temperature – increasing temp. increases
transpiration – more evap from leaf surface
• Light – more photosynthesis stimulates the
stomata to open, increasing transpiration
• (a) The guard cell absorbs light and produces ATP in
the light dependent reaction.
• (b) The ATP is used to drive proton pumps that
pump out H+ . The inside of the cell becomes more
• (c) Potassium ions enter the cell which increases the
solute concentration.
• (d) Water moves from the surrounding tissue by
• Plants adapted to low water conditions
• Adaptations include:
Reduced or rolled leaves
Deep roots
Thick cuticle
Stomata in pits surrounded by hairs
Water storage tissue
Low growth form
C4 physiology
• Tendency of Rubisco to fix
oxygen instead of carbon
dioxide when carbon
dioxide levels are low
• Inefficient, but may have a
safety function in
prevention of oxygen free
radical formation
• Prevented by C4 and CAM
Maize: a C4 plant
• Crassulacian Acid
Metabolism – plants “hold
breath” all day!
• At night carbon dioxide is
combined with phosphoenol
pyruvic acid (C3) to form
Oxoloacetic acid (C4).
• Oxoloacetic acid is changed
to malic acid or aspartic acid.
This stores the carbon
dioxide until required for
photosynthesis during the
• During the day, malic acid
converted back to carbon
dioxide for use in Ps.
CAM Plants
C4 Plants
• Enzyme used to ‘fix’
carbon into a four
carbon (instead of
three carbon)
• This is then is
shuttled as
oxaloacetate or
malate to bundle
sheath cells for
• Phloem actively translocates sugars
and amino acids from their source
(usually leaves or storage areas) to
the ‘sink’ (fruits, seeds, roots).
• The source is where food is
produced, this would be the leaves.
They produce glucose which is then
converted to sucrose which enter
the phloem. This makes the water
potential more negative making
water from the surrounding xylem
• All of this extra material increases
the pressure and forces the solution
down and through the sieve plate.
Then it gets to the sink where the
sucrose is moved by active transport
into the parenchyma; where it is
made into insoluble starch so the
water returns to the xylem.
• Sugars can move in the opposite direction if
no light (at night) when energy must come
from storage areas (eg. Tubers)
1. Source produces organic
2. Glucose from photosynthesis
3.Glucose converted to sucrose
for transport
4. Companion cell actively loads
the sucrose
5. Water follows from xylem by
6. Sap volume and pressure
increased to give Mass flow
7. Unload the organic molecules
by the companion cell
8. Sucrose stored as the
insoluble and unreactive starch
9. Water that is released is
picked up by the xylem
10. water recycles as part of
transpiration to re supply the
sucrose loading

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