Title: Lecture 7 – Leaf Structure and

Function

Speaker: Teresa Koenig

Created by: Teresa Koenig, Kim Kidwell

online.wsu.edu

Leaf Structure and Function

Leaves: Flat appendages attached to the stems

Primary function is to conduct photosynthesis (Ps), which allows the plant to grow

Leaf Structure External:

a. Leaves are attached to the stem at nodes.

b. Blade: flattened portion that intercepts light and conducts photosynthesis.

Typical dicot leaf

Axil

Petiole

Leaf blade

Midrib

Vein

Node

Some leaves are pubescent:

Epidermis is covered with hairs

May be associated with insect resistance

Some leaves are glabrous:

Epidermis has no hairs

Grass Leaf Structure

Sheath: base of the leaf blade that surrounds

the stem

Ligule: collar-like extension at the top of the

sheath

Auricles: surround the stem at the junction of

the blade and sheaths

Collar: thin band of tissue at the union of the

leaf blade and the sheath

Culm (stem)

Grass Leaf Structure

Ligule

Sheath Collar

Auricles

Compound leaf = leaf blades divided

into two or more leaflets

Simple leaf = single leaf

attached to the petiole

Look for the

axillary bud

Leaf shapes and margins (leaf edge) Useful for identifying and keying out plants

Leaf Venation: Pattern of the vascular system (veins) in the leaf.

a. Monocots: Have parallel venation. Veins run the length of the leaf and are not branched.

b. Dicots: Have netted venation. Veins are highly branched.

Leaf Arrangement

a. Opposite leaf arrangement: Two leaves at each node on opposite sides of the stem.

b. Alternate leaf arrangement: One leaf at each node on alternating sides of the stem

c. Whorled leaf arrangement: 3 or more leaves at each node

- Grasses are always alternate - Dicots can have any of the three leaf arrangements

Palmate: pattern of your fingers arising

from the palm of your hand

Pinnate: a feather pattern

Palmately veined leaf: veins begin from

a common point at the base of the leaf.

Palmately compound leaf: leaflets begin

from a common point at the base of the

leaflets.

Pinnately veined leaf: veins attached in a featherlike fashion. Pinnately compound leaf: leaflets attached in a featherlike arrangement.

Palmately veined vs. Palmately compound

Pinnately veined vs. Pinnately compound

Internal Structure of Leaves:

Please see the Lesson 4 Overview page for an animated view of

the leaf structures and their functions.

Guard Cells: 1. Function in the exchange of gases into and out of the leaf. 2. Plant controls the opening and closing of these through water pressure regulation.

Results: a. Controls the movement of CO2

into the leaf for photosynthesis. Also allows oxygen to exit the leaf. b. Transpiration: regulates the loss of water vapor from the leaf.

Transpiration:

1. Is an evaporation process

2. It cools the plant

3. 95-97% occurs through stomata; 3-5% occurs through the cuticle

4. Stomata serve as the exit for water vapor

a. Opening and closing of stomata is controlled by turgidity of the guard cell, which form the stomata

1. Fully turgid: Holding as much water as possible; stomata open 2. Decrease turgor: Water pressure decreases and stomata close

Image courtesy of Ms. Long's Living Environment Class at Herricks High School

Decreased turgidity Fully turgid

Mechanism of Transpiration:

Water diffuses out of the plant when:

1. Air within the leaf is water saturated (=100% humidity).

2. Air outside the leaf is not saturated

3. Water diffuses from an area of higher to an area of lower concentration

Factors Affecting Transpiration Rate (TR):

a. Temperature:

Lower relative humidity outside the leaf increases TR

b. Wind: Air movement disperses water vapor more quickly; Wind increases TR c. Amount of water in the soil: Affects the amount of water transported through the xylem

Origin and Growth of Leaves

1. First leaves originate in the embryo

2. All subsequent leaves originate in the meristematic region (growing point) of the stem

a. Growing point produces a leaf primordium at each node

Contains an active meristem that turns into a leaf

b. Leaf begins to photosynthesize as soon as it is exposed to light It manufactures much of the energy needed to build itself.

Functions of leaves:

1. Photosynthesis:

Basis for growth and maintenance of all higher plants.

2. Photosynthate storage:

Although most photosynthate is moved to other plant parts, some is stored here for the leaf’s own maintenance.

3. Photosynthate translocation: The movement of organic and inorganic solutes from one part of the plant to another a. Moved through the phloem b. Must move rapidly because the rate of photosynthesis will decline if sugars accumulate in the leaf

4. Water translocation: Most water absorbed by the plant moves through the leaves due to transpiration

5. Gaseous Exchange:

Occurs through the stomata

Please follow the link on the Lesson 4 Overview Page

to view a short video showing the transpiration process