Leaf Structure and
Function
Chapter 32
General Leaf Form
Blade – broad, flat portion
 Petiole – attaches the blade to the stem
 Stipules – leaf-like outgrowths at the base
of the petiole
 Simple leaves – have a single blade
 Compound leaves – blade is divided into
multiple leaflets
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General Leaf Form…

Node – the area of the stem where leaf is
attached
 Alternate leaf arrangement – every other
 Opposite (MAD dog) – two at each node
 Whorled – 3 or more at each node
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side
Venation – patter of veins in the leaf
 Parallel – side by side
 Pinnately netted – branching off long midvein
 Palmately netted – branching off a single point
fingers)
(like
Leaf Structure
Upper and lower epidermis – cells lack
chloroplasts – secrete a waxy cuticle
which contain cutin to reduce water loss;
also may have trichome cells which can
reduce water loss and deter herbivores
 Guard cells – on either side of stomata –
control opening and closing associated
with transpiration and gas exchange

Leaf Structure…

Mesophyll – middle tissues of the leaf;
photosynthesis occurs here
mesophyll – columnar cells stacked
closely together toward the upper epidermis;
the main area of photosynthesis
 Spongy mesophyll – irregularly shaped cells
loosely packed below the palisade mesophyll;
can do photosynthesis but main function is
gas exchange
 Palisade
Leaf Structure…
Vascular bundles (veins) – extend through
the mesophyll and carry both xylem and
phloem; use diffusion to move water to
cells and food away from cells
 Bundle sheath – surrounds the veins and
helps provide support; may have
extensions into the mesophyll cells

Stomata

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Usually open during the day and closed at
night… why? (think about photosynthesis)
Guard cells change shape to control this:
 Water
moves into guard cells from surrounding cells
 Guard cells swell and bend
 Pore is open
 Water leaves the guard cells
 Cells become flaccid and collapse
Transpiration

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Around 99% of water a plant absorbs is lost by
evaporation from the leaves
Transpiration – ‘plant perspiration’
Most occurs through the open stomata
Factors affecting transpiration:
 Temperature
 Light
 Wind
 humidity
Transpiration…

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Responsible for water movement in plants;
without it water would not reach the leaves from
the soil
Also serves to cool the plant, just like sweat
cools us
It brings dissolved nutrients from the soil into the
plant and distributes them
In periods of drought it can lead to wilting and
even death
Guttation
The loss of liquid water from the leaves of
plants
 Occurs when transpiration is low and soil
moisture is high

Leaf Abscission
The way in which plants shed leaves
 Controlled by hormones
 Chlorophyll, sugars, amino acids and
many essential nutrients are transported
from the leaves to other parts of the plant
 Abscission zone – the area where the
petiole detaches from the stem

Modified leaves
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Spines – hard and pointed; cactus; to deter
herbivores
Tendrils – help attach vines to other structures
(peas, squash)
Bulb – underground storage leaves (onions,
tulips)
Leaves of insectivorous plants – can be either
passive (pitcher plant) or active (Venus flytrap)