Transcript leaf primordia

THE LEAF
STRUCTURE & FUNCTIONS
FUNCTIONS
• Photosynthesis
• Transpiration
• Storage
Development
• Primary growth
– From the shot apical meristem
• Specifically from the leaf primordia – earliest stage
of leaf development.
Parts of a Leaf
• Blade
• Petiole – stalk that supports the blade.
Blade
Petiole
• Location of where the leaf attaches to the stem
is the node, the space between the two
successive nodes is known as the internode.
Stem and Leaf Structures
• Axil – upper angle where the leaf joins the
stem.
• Axillary bud can grow into a branch with
leaves or flowers.
• Pulvinus is the swelling in the joint like
structure of the petiole that allows the
plant to respond to stimuli.
Leaf Arrangement
• Alternate leaf arrangement have one leaf
per node, typically found alternating
around along the stem.
• Opposite leaf arrangements have two
leaves per node.
• Whorled leaf arrangements have three or
more leaves (up to 25).
Leaf Arrangement
Leaf Arrangement
• Simple leaves – flat, undivided blade that
is supported by the petiole.
• Compound leaves have a blade that is
divided into leaflets.
Leaf Arrangement
• Compound leaf arrangement
Compound
Simple
– Pinnately compound leaves – leaflets form in
pairs around a central, stalk-like rachis.
– Palmately compound leaves – Leaflets attach
at the same point and radiate.
– Peltate leaves are simple leaves with the stem
attached to the middle of the blade, (Fig 9.8 a;
Page 209)
– Perfoliate leaves are simple, sessile leaves that
surround the stem. (Fig 9.8 b; Page 209)
Leaf Arrangement
Pinnately
Compound
Palmately
Compound
Other leaf types
• Leaves of ferns are called fronds, usually
form in curled structures called a
fiddlehead.
Venation
• Vascular tissues (xylem and phloem) form in
strands which are called veins.
• Normally supported by fibers and a layer of the
parenchyma cells called the bundle sheath.
• Dicots – Netted venation, where larger veins and
smaller veins branch into a mesh-like network.
• Monocots – Parallel venation, or long strands of
vascular tissues run parallel to each other.
Leaf Tissues
• Epidermis with cuticle
– Upper epidermis
– Lower epidermis
• Guard cells
• Mesophyll (parenchyma tissue)
– Palisade
– Spongy
Tissues
• Epidermis
– Typically transparent and non-photosynthetic.
– Contains numerous stomata
• Monocots – stomata form in parallel rows
• Dicots – stomata form in a random,
scattered pattern.
• More numerous on the underside of
horizontally oriented leaves, and equally
distributed amongst vertical leaves.
Guard cells
Stoma
Chloroplasts
Regulation of Transpiration
• Opening and closing of the stomata
Organization of Mesophyll Tissues
• Photosynthetic, parenchyma cells.
• Store products produced by photosynthesis.
• Upper layer is the Palisade Mesophyll, which are
densely packed, and contain large numbers of
chloroplast – specialized for light absorption and
photosynthesis.
• Lower layer of mesophyll tissues are the Spongy
mesophyll, which are irregularly shaped, loosely
organized and contain chloroplast. Specialized
for gas exchange and promotes movement of
gases into and out of the leaf through the
stomata.
X. S of a Typical Dicot Leaf
Upper
epidermis
Palisade
mesophyll
Spongy
mesophyll
Lower
epidermis
Guard cells
Water and Water Loss
• Water accounts for ~85-95% of the weight of the
plant and 5-10% of the seeds weight. Water lost
from transpiration is replaced by water absorbed
from the roots – water can move up to leaves
ate the rate of 75 cm/hr.
• Used for
– Making organic molecules
– Solvent for chemical reactions
– Medium in which nutrients move
• 95% of the water a plant gather is lost to
transpiration.
Water and Water Loss
• Transpiration – loss of water from plants
depends on:
– Amt. of surface area available for evaporation
• Internal SA of the leaf can be more than 200X
greater than the outside of the leaf
• One corn plant can transpire more than 500L of
water during its four month growing season.
– Equal us drinking 40L of water a day
Water and Water Loss
• Other environmental factors include:
 Humidity – inc. in dry air
 Internal conc. of CO2 – lower CO2 concentrations
cause stomata to open, inc. transpiration.
 Wind – thin moist layer around the leaf called the
boundary layer, wind removes this layer replacing it
with dry air.
 Air temperature – inc. temperature results in faster
transpiration rates, temp. inside the leaf can exceed
the outside temp. by as much as 10°c.
 Soil - the amount of soil moisture affects the rate of
transpiration
 Light intensity – Light stimulates the stomata to
open and therefore increase transpiration.
Adaptations to prevent water loss
• Leaf position – plants re-orient leaves to avoid the
midday sun, decreasing temperatures.
• Abscic Acid – prevents desiccation by causing stomata
to close preventing transpiration.
• Cuticle – Waxy substance that prevents water loss from
the leaf, thicker in dry, arid regions.
• Sunken stomata – increase the thickness of the
boundary layer.
• Reduced leaf areas – reduce the size of the leaf, which
in turn reduces the amt. of water lost to transpiration.
• Leaf abscission – plants drop their leaves when
temperatures or water becoming limiting.
• Circadian rhythm – Daily, 24 hour cycle where plants
open stomata during the day and close them at night,
regulated by light and an internal biological clock.
Water Transport
• Plants rely on vascular tissues for transportation
of water to leaves for photosynthesis.
– Evolution of xylem and phloem is largely due size and
distances that water must travel in multicellular plants.
– Diffusion of water is slow and inadequate in
multicellular organisms.
• Molecule that my diffuse in a single celled organism in
seconds can take up wards of 8 years to diffuse in a
multicellular organism.
• Transport of molecules between cells is speeded
up by cytoplasmic streaming – continuous
movement of cytoplasm in the cell.
Water Transport
• Guttation or the loss of water from the
leaves of the plant, is a common
phenomenon. Caused by root pressure,
the influx of minerals into the root at night
causes the water pressure in the xylem to
increase, eventually pushing water out of
marginal openings in the leaves.
Guttation is Caused by Root
Pressure
Deciduous and Evergreens
• Deciduous – plants that drop all of their
leaves in the fall. (Ex. oak, pecan)
– Larger leaves can produce more sugars from
photosynthesis, but lose more water from
transpiration.
• Evergreen – leaves can live 3-5 years and
can be shed anytime of the year.
– Smaller leaves lose less water from
transpiration, smaller leaves produce less
sugars.
Deciduous and Evergreens
Specialized Leaf Structures
• Stipules – small, leaf-like projections at the base of the
petioles; these are photosynthetic and can form
protective spines.
• Tendrils – modified leaves for support, coiling around
objects and holding plants off of the ground, allow certain
plants to climb up structures.
• Spines – modified leaves that protect plants from
herbivores, and reduce water loss.
• Bud Scales – tough, overlapping leaves that protect
buds from low temperatures, drying out and pathogens.
• Storage leaves – fleshy leaves that concentrate and
store food.
• Succulent leaves – common asexual means of
reproduction, produce tiny plantlets when the fall off of
the parent plant.
Carnivorous Plants
• Live in nutrient poor soils, evolved to use other
organisms as a source of nitrogen and other
minerals they require.
• Many species have evolved to where they have
no roots and lack chlorophyll.
• Examples are;
–
–
–
–
Venus Fly Trap
Genlisea
Butterwort
Pitcher plant
Poisons
• Many plants have evolved to concentrate toxins
in their leaves to protect against herbivores.
• Many of these are very toxic
• Examples are;
– Hemlock
– Milkweed (some sp. Of grasshoppers only eat weed,
they then concentrate these poisons in their body
which they then spray at attacking predators)
Economically Important Plant
Leaves
• Spices (thyme, oregano, peppermint, spearmint and
sage)
• Fibers for producing textiles
• Foxglove – drugs produce from foxglove are used in
digoxin and digtalis which are used to treat low blood
pressure and strengthen the heart.
• Coffee and tea – caffeine
• Cannabis (marijuana) and coca (cocaine) – used to
produce illegal narcotics, oils produced by Cannabis
sativa produce tetrahydrocannabinols (THC).
• Carnuba wax is produced from the carnuba palms.