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

Petiole Axillarybud

Leafbase

Lamina

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-stomatalchamber

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