LEAVES - PPT

The
Leaf

The Plant Body: Leaves
• FUNCTIONS:
– Leaves are the solar
energy and CO 2
collectors of plants.
– In some plants,
leaves have special
functions.

External Anatomy of a Leaf

Types of Leaves
• Leaves can be simple or compound.
– Simple leaves - With a single blade
– Compound leaves - Blade divided into leaflets
• Pinnate compound leaves - Leaflets in pairs
along petiole (pinna: latin for feather)
• Palmate compound leaves - All leaflets
attached at same point at end of petiole.

Pinnately & Palmately Compound Leaves

Venation
• Monocots - parallel venation
• Dicots - netted or reticulate venation.
Parallel
venation
Reticulate venation

Venation
Reticulate
Venation

Phyllotaxy
• Leaves are attached to stems at nodes.
– Phyllotaxy - Arrangement of leaves on stem
• Alternate - One leaf per node
• Opposite - Two leaves per node
• Whorled - Three of more leaves at a node
Alternate
Opposite
Whorled

Phyllotaxy - Arrangement of leaves on a stem

The Epidermis of Leaves
• Stomata – pores
– allow gas exchange
– bordered by two
guard cells
• Trichomes – leaf
hairs
– functions: deter small herbivores,
reflect sunlight, reduce air flow.

Representative scanning electron microscopy images of trichomes on plants.
Dai X et al. Plant Physiol. 2010;152:44-54
©2010 by American Society of Plant Biologists

Guard Cells

Leaf – Internal Anatomy

Mesophyll and Veins
• Photosynthesis takes place in the mesophyll
between the two epidermal layers.
• Palisade Mesophyll
• Contain most of
the chloroplasts
• Spongy Mesophyll
– Loosely arranged
cells with air
spaces

Sun v. Shade Leaves
• Compared to sun
leaves, shade leaves:
– Tend to be larger
– Tend to be thinner
– Have fewer layers of
palisade mesophyll
– Have fewer
chloroplasts
Sun
leaf
Shade
leaf

Leaves in Arid Regions
• Arid regions have limited availability of water
– Leaves reduce water loss:
• Thick, leathery leaves
• Fewer stomata or sunken stomata
• Succulent, water-retaining leaves, or no leaves
• Abundant trichomes

Leaves of Aquatic Plants
• Leaves of Aquatic Areas
– Less xylem and phloem
– No palisade and spongy, just mesophyll.
– Large air spaces

Needles and Spines
Tendrils
Colorful
Bracts

Insect Trapping Leaves
• Carnivorous Plants
– Why are they
green
– Why do they trap
insects
Sundew
Pitcher plant
Venus’s
Flytraps

Deciduous Plants & Abscission
• Deciduous plants drop
their leaves in fall
– build new leaves each
year.
– Why not tropical plants

Autumnal Change in Leaf Color
• Leaves contain several types of pigments:
– Chlorophylls - Green
– Carotenoids – Yellow/Orange
– Anthocyanins (red or blue) may also be present
– In fall, chlorophyll is no longer produced and
the existing chlorophyll is broken down by the
plant
– Other pigment colors are revealed.

Abscission
Process by which leaves are shed
• Changes in the abscission zone (near the base
of the petiole)
– Protective layer
o
Cells coated with
suberin (wax)
o
stops water loss
– Separation layer
o
Pectin in the cell wall
is broken down by
enzymes