title: lecture 7 leaf structure and function …...leaf structure and function leaves: flat...
TRANSCRIPT
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Title: Lecture 7 – Leaf Structure and
Function
Speaker: Teresa Koenig
Created by: Teresa Koenig, Kim Kidwell
online.wsu.edu
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Leaf Structure and Function
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Leaves: Flat appendages attached to the stems
Primary function is to conduct photosynthesis (Ps), which allows the plant to grow
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Leaf Structure External:
a. Leaves are attached to the stem at nodes.
b. Blade: flattened portion that intercepts light and conducts photosynthesis.
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Typical dicot leaf
Axil
Petiole
Leaf blade
Midrib
Vein
Node
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Some leaves are pubescent:
Epidermis is covered with hairs
May be associated with insect resistance
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Some leaves are glabrous:
Epidermis has no hairs
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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
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Culm (stem)
Grass Leaf Structure
Ligule
Sheath Collar
Auricles
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Compound leaf = leaf blades divided
into two or more leaflets
Simple leaf = single leaf
attached to the petiole
Look for the
axillary bud
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Leaf shapes and margins (leaf edge) Useful for identifying and keying out plants
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Leaf Venation: Pattern of the vascular system (veins) in the leaf.
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a. Monocots: Have parallel venation. Veins run the length of the leaf and are not branched.
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b. Dicots: Have netted venation. Veins are highly branched.
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Leaf Arrangement
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a. Opposite leaf arrangement: Two leaves at each node on opposite sides of the stem.
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b. Alternate leaf arrangement: One leaf at each node on alternating sides of the stem
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c. Whorled leaf arrangement: 3 or more leaves at each node
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- Grasses are always alternate - Dicots can have any of the three leaf arrangements
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Palmate: pattern of your fingers arising
from the palm of your hand
Pinnate: a feather pattern
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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.
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Pinnately veined leaf: veins attached in a featherlike fashion. Pinnately compound leaf: leaflets attached in a featherlike arrangement.
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Palmately veined vs. Palmately compound
Pinnately veined vs. Pinnately compound
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Internal Structure of Leaves:
Please see the Lesson 4 Overview page for an animated view of
the leaf structures and their functions.
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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.
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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.
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Transpiration:
1. Is an evaporation process
2. It cools the plant
3. 95-97% occurs through stomata; 3-5% occurs through the cuticle
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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
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1. Fully turgid: Holding as much water as possible; stomata open 2. Decrease turgor: Water pressure decreases and stomata close
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Image courtesy of Ms. Long's Living Environment Class at Herricks High School
Decreased turgidity Fully turgid
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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
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Factors Affecting Transpiration Rate (TR):
a. Temperature:
Lower relative humidity outside the leaf increases TR
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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
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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
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a. Growing point produces a leaf primordium at each node
Contains an active meristem that turns into a leaf
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b. Leaf begins to photosynthesize as soon as it is exposed to light It manufactures much of the energy needed to build itself.
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Functions of leaves:
1. Photosynthesis:
Basis for growth and maintenance of all higher plants.
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2. Photosynthate storage:
Although most photosynthate is moved to other plant parts, some is stored here for the leaf’s own maintenance.
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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
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4. Water translocation: Most water absorbed by the plant moves through the leaves due to transpiration
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5. Gaseous Exchange:
Occurs through the stomata
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Please follow the link on the Lesson 4 Overview Page
to view a short video showing the transpiration process