LEAVES
2011
What is a leaf?
• The leaf is a lateral, generally flattened structure
borne on the stem.
• It develops at the node and bears a bud in its axil.
• The axillary bud later develops into a branch.
• Leaves originate from shoot apical meristems
are arranged in an acropetal order.
• They are the most important vegetative organs for
photosynthesis.
Parts of a leaf
Stipule
Lamina
Petiole
Leaf
base
Axillary
bud
Parts of a leaf
• A typical leaf consists of three main parts:
• leaf base, petiole and lamina
• The leaf is attached to the stem by the leaf base and
• may bear two lateral small leaf like structures called
stipules
Function of petiole
• The petiole help hold the blade to light.
• Long thin flexible petioles allow leaf blades to flutter
in wind,
• thereby cooling the leaf and bringing fresh air to leaf
surface.
Characteristics of a leaf
• . In monocotyledons, the leaf base expands into a sheath
covering the stem partially or wholly.
• In some leguminous plants the leafbase may become swollen,
which is called the pulvinus.
• The lamina or the leaf blade is the green expanded part of
the leaf with veins and veinlets.
• There is, usually, a middle prominent vein, which is known as
the midrib.
Function of Veins:
1. provide rigidity to the leaf blade
2. act as channels of transport for water,
minerals and food materials.
• The shape, margin, apex, surface and extent of
incision of lamina varies in different leaves.
venation
• Define venation
• Types of venation
Types of venation
Reticulate venation
Parallel venation
Venation
• The arrangement of veins and the veinlets in the
lamina of leaf is termed as venation.
1. When the veinlets form a network, the venation is
termed as reticulate
2. When the veins run parallel to each other within a
lamina, the venation is termed as parallel.
• Leaves of dicotyledonous plants generally possess
reticulate venation,
• while parallel venation is the characteristic of most
monocotyledons.
Types of Leaves
Simple leaf:
• A leaf when its lamina is entire or when incised, the
incisions do not touch the midrib.
Compound leaf:
• When the incisions of the lamina reach up to the
midrib breaking it into a number of leaflets
• A bud is present in the axil of petiole in both simple
and compound
• leaves, but not in the axil of leaflets of the compound
leaf.
Types of compound leaves
1. Pinnately compound leaf
• number of leaflets are present on a common axis,
• the rachis, which represents the midrib of the
leaf
E.g. neem.
2. Palmately compound leaves
• the leaflets are attached at a common point,
i.e., at the tip of petiole
E.g. silk cotton.
Phyllotaxy
• Phyllotaxy is the pattern of arrangement of leaves on
the stem or branch.
• Three types – alternate, opposite and Whorled.
Alternate:
a single leaf arises at each node in alternate manner
E.g. china rose, mustard and sun flower plants.
Opposite
• a pair of leaves arise at each node and
• lie opposite to each other
E.g. Calotropis and guava plants.
Whorled:
• If more than two leaves arise at a node
• form a whorl
E.g. Alstonia.
Anatomy of a leaf
a stomate appears on both the upper and lower
surfaces of the leaf.
A thin, waxy cuticle is also on the upper surface.
This cuticle tends to retard water loss through
epidermal cells.
The lower epidermis has a hair called a trichome.
The only epidermal cells bearing chloroplasts are the
guard cells.
Between the two epidermises are mesophyll cells
abundant in chloroplasts.
stomates.
The mesophyll cells are of two kinds.
1. palisade parenchyma
upper portion of the leaf, the mesophyll cells are elongated and
vertically arranged.
2.
spongy parenchyma, or spongy mesophyll.
lower portion of the leaf, the mesophyll cells are characterized by
conspicuous intercellular .
The intercellular spaces communicate with the substomatal spaces to
allow gas interchange with the atmosphere.
Carbon dioxide from the atmosphere enters the mesophyll cells
through the intercellular spaces,
while oxygen, a product of photosynthesis, is conveyed out of the
leaf through the stomata
Mesophyll
sub-stomatal
chamber
Modification of leaves
• Leaves of many plants which have to perform
specialized functions become modified or
metamorphosed into distinct forms.
These are as followed:
• Leaf tendrils
• Leaf spines
• Scale leaves
• Phyllode
• Pitcher
• Bladder
Tendril
• Leaves are modified into slender, wiry, often closely coiled
structures.
• Are always climbing organs and sensitive to contact with a
foreign body
• Whenever they come in contact with a neighboring object,
they coil round it and help the plant to climb.
• The leaf may be partially or wholly modified
Leaf tendrils
Leaf tendril
Spine
• Leaves of certain plants becomes modified for
defensive purposes into sharp pointed structure
• The spines are modifications are evident from
the fact that they occupy the same positions as
leaves and
• that they often bear a bud in their axil.
Leaf spines
Leaves modified into spines
Scale leaves
• Typically these are thin, dry stalkless, membranous structures,
usually brown in colour sometime colourless.
•
to protect the axillary but that they bear in their axil.
• Sometimes scale leaves are thick and fleshy as in onion their
function is to store up water and food.
Phyllode
• The petiole or any part of the rachis become flattened
or winged, taking the shape of the leaf
• turning green in colour.
• This flattened or winged petiole or rachis is known as
phyllode.
Pitcher
• Pitcher is the modification of leaf blade, the inner surface of
the pitcher corresponding to the upper surface of the leaf.
• Lid arise as an out growth of leaf apex
• The slender stalk which coils like a tendril, stiffening as it does
so, is the petiole.
• The laminated structure, which looks like and behaves as the
blade, develops from the leaf base.
• E.g Nepenthes
Bladder
• In bladderwort a rootless free floating or slightly
submerged weed common in many tanks, the leaf is
very much segmented.
• some of these segments are modified to form bladder
like structures with a trap-door entrance which allows
aquatic animal-cules to pass in but never to come out
What causes the leaves to fall in winter?
• While it might be supposed that diminishing temperature is the
cause
•
the true cause is diminishing number of daylight hours.
•
As the days grow shorter (and the number of hours
of light thus decreases) a zone of corky cells, called the abscission
layer, forms at the base of the petiole,
• This layer becomes differentiated into a protective layer on the side
toward the stem and as separation layer.
• At the latter region, the cells are weak, and the leaves break away
from the tree when the wind blows.
Leaf Functions
• Leaves are the primary site of photosynthesis in most plants.
• Mesophyll cells in leaf use ;light energy , carbon dioxide and
water to make sugars.
• Light energy is also used by mesophyll cells to synthesize
amino acids and variety of other organic molecules.
• Sugars made in leaf can be used by the leaf as an energy
source as building blocks for cells.
• They also may be transported to other parts of the
plant, where they are neither stored or used for energy
building blocks.
• A major limitations to plant photosynthesis is
insufficient water due to transpiration.
Task
• Differentiate between dicot and monocot on the basis
of the following:
1.
root development
2.
root morphology
3.
anatomy of root and stem
4.
leaves
5.
flower