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The LEAF
Competencies:
Identify the parts of the Leaf and
its functions and classifications
Pre Activity:
1. Get a sample of a leaf (any leaf
or leaves with stalk)
2. Recall the parts of the leaf
THE LEAF
LEAF – expanded lateral extension of the stem
arising at the node with a bud on its axil.
2 types:
a. microphyll – lycopods with one vascular strand.
b. megaphyll – complex venation
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.
External Parts:
Petiole
– stalk which connects the lamina to the stem.
Function: act as passage of substances between
leaf and stem.
leaf without petiole
– sessile
with petiole
– petiolate
peltate – where petiole is attached some distance
from the leaf.
2. Leaf Blade or Lamina
– expanded portion of the leaf that varies on the tips, bases and
margins.
Function:
where photosynthesis takes place.
3 parts: a. tip – apex
b. side – margin
c. posterior – base
3. Stipules
– leafy appendages arising from the base of the petiole.
with stipule
– stipulate
without stipule
– exstipulate
function: for photosynthesis/
protection
4. Midrib –
large vessel serve as an extension of
petiole.
Function: conduction and support of leaf
tissues.
5. Netwrok of Vein and Veinlets –
also called Nervules- smaller branches of vessels
running on leaf blade.
for conduction and transport of materials.
Classification of Leaves:
1. Venation – arrangement of veins
*parallel – one sided
*netted/reticulate – close network
*obsolete/wanting – too obscured
because they are too thick.
• 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).
• 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
2. Number of Blades
Simple Leaf – blade of single piece.
Compound Leaf – blade is composed of two or more separate
part on a common petiole.
the division is called – Leaflets/Pinnae
stalk – petiolules
rachis – extension of petiole above the lowest leaflet to upper
parts.
Stipule-like – stipels
Peltate leaves = petioles that are attached to the
middle of the blade; examples include mayapple
Perfoliate leaves = sessile leaves that surround and
are pierced by stems; examples include yellowwort and thoroughwort
Mayapple
Yellow Wort
3. Compound Leaf classification:
Number of leaflets:
Unifoliate – 1 leaflet
Bifoliate – 2
Trifoliate – 3
multifoliate – more than 3
4. Position of leaflets on rachis
Palmately Compound – diverse leaflets from
a common point at tip of petiole.
Pinnately Compound – petiole becomes into
rachis along several leaflets arise at interval.
5. Phyllotaxy or leaf arrangement:
Alternate – one leaf to a node of opposite
side of stem.
Opposite – two leaves to one node of
opposite side of stem.
Verticulate or Whorld – more than two
leaves in one node arranged regularly
around the stem.
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.
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.
INTERNAL STRUCTURE OF LEAVES
chloroplasts
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
Conserves water
Photosynthesis
Transports water and
sugar to stem and roots
Typical Dicot Leaf Cross-Section
Cuticle
Palisade
Parenchyma
Epidermis
Vascular
bundles
Guard
Cells
Spongy
Parenchyma
Stoma
Typical Monocot Leaf Cross-Section
Midvein
Vein
Bundle
sheath cell
Epidermis
Phloem
Xylem
Bulliform
Cells
Stoma
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.
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 –
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.
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.
INSECT Trapping Leaves
Figure 11.8 (1)
• 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
Specialized
Leaves
Figure 11.12
(2)
• 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
Prickles and thorns: epidermal
outgrowths on stems and leaves
(e.g., holly, rose, and raspberries;
Hypodermic trichomes on stinging
nettles.
Storage leaves or succulent
leaves retain water in large
vacuoles
Bracts: petal-like leaves.
Window Leaves: plant is buried in
soil with transparent part exposed
to light. Being buried reduces loss
of war in arid environments.
Reproductive leaves, (e.g.,
Kalanchöe plantlets arise on
margins of leaves.
Application
1. Identify the parts of the leaf (Petiole, Lamina, Stipules, Midrib,
Veinlets)
2. Venation (Parallel, Netted, Obsolete) (pinnately or palmately parallel,
dichotomous) (peltate, perfoliate)
3. Compound leaf (leaflets, rachis, petiolules, stipels)
4. Number of leaflets
5. Position of leaflets for compound leaf(pinnately, palmately)
6. Phyllotaxy ( alternate, opposite, whorld)
7. Specialized function (if it has)
8. Blade forms (Linear Lanceolate Oblong Elliptic Ovate Cordate
Reniform Spatulate Orbicular
9. Margin ( entire, toothed, dentate, serrate, crenate, incised, lobed,
sinnate, undulate)