ppt on leaves college level

1. Leaves:
Form and structure
Chapter 6

2. The Plant Body: Leaves
• FUNCTION OF LEAVES
– Leaves are the solar
energy and CO2 collectors
of plants.
– In some plants, leaves
have become adapted for
specialized functions.

3. And so, on to leaves
• Leaves are the principle
structure, produced on
stems, where photosynthesis
takes place.
• Cacti are an exception. The
leaves are reduced to spines,
and the thick green, fleshy
stems are where
photosynthesis takes place.

4. General leaf form
• Leaves are the main
photosynthetic organs of most
plants
– but green stems are also
photosynthetic.
– While leaves vary extensively
in form, they generally consist
of a flattened blade and a
stalk, the petiole, which joins
the leaf to a stem node.
– In the absence of petioles in
grasses and many other
monocots, the base of the leaf
forms a sheath that envelops
the stem.
• Most monocots have parallel major
veins that run the length of the
blade, while dicot leaves have a
multi branched network of major
veins. Copyright © 2002 Pearson Education, Inc., publishing as
Benjamin Cummings
Blade
Petiole

5. Leaf Arrangement on the Stem
Opposite: 2 leaves at a node,
on opposite sides of the stem
Spiral: 1 leaf per node, with the
second leaf being above the first
but attached on the opposite
side of the stem
Whorled: 3 or more leaves at a
node

6. • Plant taxonomists use leaf shape, spatial arrangement of leaves,
and the pattern of veins to help identify and classify plants.
– A Simple leaves have a single, undivided blade, while compound
leaves have several leaflets attached to the petiole.
– A Compound leaf has a bud where its petiole attaches to the
stem, not at the base of the leaflets.
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
Leaf Arrangement on the Stem

7. Venation = arrangement
of veins in a leaf
• Netted-venation = one or a
few prominent midveins
from which smaller minor
veins branch into a
meshed network;
• common to dicots and
some nonflowering plants.
– Pinnately-veined leaves =
main vein called midrib with
secondary veins branching
from it (e.g., elm).
– Palmately-veined leaves =
veins radiate out of base of
blade (e.g., maple).

8. Venation = arrangement
of veins in a leaf
• Parallel venation =
characteristics of many
monocots (e.g., grasses,
cereal grains); veins are
parallel to one another.
• Dichotomous venation = no
midrib or large veins;
rather individual veins have
a tendency to fork evenly
from the base of the blade
to the opposite margin,
creating a fan-shaped leaf

9. Most dicots have
branch-like veins and
palmate leaf shape
Monocots have parallel
leaf veins and longer,
slender blades
Leaves - Comparisons
Monocots and dicots differ in the arrangement of
veins, the vascular tissue of leaves

10. INTERNAL STRUCTURE OF LEAVES
chloroplasts
• Each part of the leaf has an important job.
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

11. Structures of the Leaf
Photosynthesis
Conserves water
Transports water
and sugar to stem
and roots
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

12. Structures of the
Leaf
Cuticle – the outermost layer of both
the upper and lower surfaces of the
leaf. It is clear and waxy to prevent
against water loss.
Epidermis – a layer of cells one cell
thick that provides protection for the
inner tissues. These cells are clear to
allow light to reach the photosynthetic
tissues.
Mesophyll – between the epidermal
layers. It contains palisade cells that
are tall, tightly packed, and filled
with chloroplasts for photosynthesis.
It also has spongy cells which are
irregularly shaped, have large air
spaces between them, and fewer
chloroplasts.

13. Structures of the
Leaf
Stomates – openings in the surface of
the leaf and stems for gas exchange.
The lower surface of a leaf usually has
more. Water vapor also passes out
through these holes.
Guard cells – two of these special cells
surround each stomate and regulate
the opening and closing of the stomate.
Veins – contain the vascular tissue that
is continuous with that in the stem.
Xylem carries water and minerals
upward. Phloem carries dissolved food
throughout the plant.

14. Typical Dicot Leaf Cross-Section
Palisade
Parenchyma
Spongy
Parenchyma
Vascular
bundles
Epidermis
Cuticle
Stoma
Guard
Cells

15. Typical Monocot Leaf Cross-Section
Xylem
Phloem
Bulliform
Cells
Stoma
Epidermis
Midvein Vein
Bundle
sheath cell

16. Function of the Leaf
• Photosynthesis
• Gaseous exchange
– take in O2 and release CO2 during
respiration
– take in CO2 and release O2 during
photosynthesis
Water Vapour can be lost from the
surface of the leaf in a process known
as Transpiration.

17. TRANSPIRATION
• Plants must supply water to all
their tissues. It moves from the
roots up the stem to the leaves by
capillary action.
• Most of the water plants take up is
lost to the atmosphere by
evaporation.
• The evaporation of water vapor
from plant surfaces is called
transpiration.
• Most takes place through
stomates.

18. • The rate of transpiration is regulated
by the size of the opening of the
stomates.
• They are usually closed when there is
too little water available, temperature
is low, or there is little light.
• Most plants open their stomates during
the day and close them at night.
• This is controlled by
the guard cells.

19. Stomatal control
• Almost all leaf transpiration
results from diffusion of water
vapor through the stomatal pore
– waxy cuticle
• Provide a low resistance pathway
for diffusion of gasses across
the epidermis and cuticle
• Regulates water loss in plants
and the rate of CO2 uptake
– Needed for sustained CO2
fixation during
photosynthesis

20. Stomatal control
• When water is abundant:
• Temporal regulation of
stomata is used:
– OPEN during the day
– CLOSED at night
• At night there is no
photosynthesis, so no demand
for CO2 inside the leaf
• Stomata closed to prevent
water loss
• Sunny day - demand for CO2 in
leaf is high – stomata wide
open
• As there is plenty of water,
plant trades water loss for
photosynthesis products

21. Stomatal control
• When water is limited:
– Stomata will open less
or even remain closed
even on a sunny morning
• Plant can avoid
dehydration
• Stomatal resistance can
be controlled by opening
and closing the stomatal
pores.
• Specialized cells – The
Guard cells

22. GUARD CELLS AND PLANT HOMEOSTASIS
• Guard cells are kidney-shaped with
thick inner walls and thin outer walls.
• When they become full of water
(turgid) the unevenness of the walls
causes them to bow outward and the
stomate opens.
• When they lose water they become less
turgid and the stomate closes.
• Guard cells gain
and lose water by
osmosis.

23. Stomatal guard cells
• Guard cells act as hydraulic valves
• Environmental factors are sensed by guard
cells
– Light intensity, temperature, relative
humidity, intercellular CO2 concentration
• Integrated into well defined responses
– Ion uptake in guard cell
– Biosynthesis of organic molecules in guard cells
• This alters the water potential in the guard cells
• Water enders them
• Swell up 40-100%

24. Relationship between water loss and
CO2 gain
• Effectiveness of controlling water loss and allowing CO2
uptake for photosynthesis is called the transpiration ratio.
• There is a large ratio of water efflux and CO2 influx
– Concentration ratio driving water loss is 50 larger than
that driving CO2 influx
– CO2 diffuses 1.6 times slower than water
• Due to CO2 being a larger molecule than water
– CO2 uptake must cross the plasma membrane, cytoplasm,
and chloroplast membrane. All add resistance

25. water status of plants
• Cell division slows down
• Reduction of synthesis of:
– Cell wall
– Proteins
• Closure of stomata
• Due to accumulation of the
plant hormone Abscisic acid
– This hormone induces
closure of stomata during
water stress
• Naturally more of this
hormone in desert plants

26. Plants and water
• Water is the essential medium of life.
• Land plants faced with dehydration by water loss to the
atmosphere
• There is a conflict between the need for water conservation
and the need for CO2 assimilation
– This determines much of the structure of land plants
– 1: extensive root system – to get water from soil
– 2: low resistance path way to get water to leaves – xylem
– 3: leaf cuticle – reduces evaporation
– 4: stomata – controls water loss and CO2 uptake
– 5: guard cells – control stomata.

27. Photosynthesis
• One of the most important biochemical
process in plants.
– Let’s not forget cell wall biosynthesis and
adaptation during plant development, growth,
interaction with the environment, and disease
defense.
• Among the most expensive biochemical
processes in plant in terms of investment
• The biochemical process that has driven
plant form and function

28. General overall reaction
6 CO2 + 6 H2O C6H12O6 + 6 O2
Carbon dioxide Water Carbohydrate Oxygen
Photosynthetic organisms use solar energy to synthesize
carbon compounds that cannot be formed without the input
of energy.
More specifically, light energy drives the synthesis of
carbohydrates from carbon dioxide and water with the
generation of oxygen.

29. C3 and C4 Leaf structure

30. The C4 carbon Leaf
• This is a biochemical pathway that
prevents photorespiration
• C4 leaves have TWO chloroplast
containing cells
– Mesophyll cells
– Bundle sheath (deep in the leaf so
atmospheric oxygen cannot diffuse
easily to them)
• C3 plants only have Mesophyll
cells
• Operation of the C4 cycle requires the
coordinated effort of both cell types
– No mesophyll cells is more than
three cells away from a bundle
sheath cells
• Many plasmodesmata for
communication

31. Specialized or Modified
Leaves
• Drought-resistant leaves = thick,
sunken stomata, often reduced in size
• In American cacti and African
euphorbs, leaves are often reduced
such that they serve as spine to
discourage herbivory and reduce water
loss
• The stems serve as the primary
organ of photosynthesis.

32. Specialized or Modified
Leaves
• In pine trees, the leaves are adapted to
living in a dry environment too.
• Water is locked up as ice during
significant portions of the year and
therefore not available to the plant;
pine leaves possess
– sunken stomata,
– thick cuticles
– needle-like leaves
– hypodermis, which is an extra
cells just underneath the
epidermis –

33. Cotyledons or “seed leaves”
First leaves produced by a germinating seed
Often contain a store of food (obtained from the endosperm)
to help the seedling become established.

34. Tendrils
Garden Pea
Tendrils - blade of leaves or leaflets are
reduced in size, allows plant to cling to
other objects (e.g., sweet pea and
garden peas.

35. Figure 11.8 (1)
Specialized Leaves
• Some plants obtain nitrogen from
digesting animals (mostly insects).
• The Pitcher plant has digestive
enzymes at the bottom of the trap
• This is a “passive trap” Insects fall
in and can not get out
• Pitcher plants have specialized
vascular network to tame the amino
acids from the digested insects to
the rest of the plant

36. Figure 11.12 (2)
Specialized Leaves
• The Venus fly trap has an “active
trap”
• Good control over turgor pressure
in each plant cell.
• When the trap is sprung, ion
channels open and water moves
rapidly out of the cells.
• Turgor drops and the leaves slam
shut
• Digestive enzymes take over

37. The End.
Any Questions?