LEAVES
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Main Ideas:
Historical Significance of Tobacco:
Tobacco (Nicotiana tabacum) has had a significant impact on human history and health.
Discovered by Christopher Columbus and cultivated by native peoples of North and South America.
Initially used for medicinal purposes and religious rituals.
Gained popularity in Europe for its supposed medicinal values.
Physiologically active ingredients are nicotine and related alkaloids.
Smoking tobacco contributes to serious health problems.
If smoking stopped in the US, over 300,000 lives could be saved annually.
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Leaf Form and Structure:
Leaf Variability:
Leaves are highly variable in shape, size, and attachment to stems.
Plant biologists developed terminology to describe leaf characteristics.
Leaves can be round, needlelike, scalelike, heart-shaped, fan-shaped, etc.
Leaf Composition:
Most leaves have a blade and a petiole.
Some leaves have stipules at the base of the petiole.
Leaves can be simple or compound.
Leaf Arrangement:
Leaves can be arranged alternately, oppositely, or in a whorled pattern on a stem.
Leaf Venation:
Leaves can have parallel or netted venation.
Parallel veins are typical of monocots, while netted veins are typical of eudicots.
Leaf Tissues:
Epidermis, mesophyll, xylem, and phloem are major tissues in a leaf.
Leaf structure is optimized for photosynthesis with upper and lower epidermal layers.
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Leaf Structure and Function
Extra thickness provides protection against injury or water loss.
Epidermal cells secrete a waxy layer called cuticle to reduce water loss.
Different types of leaves: pinnately compound, palmately compound, simple, etc.
Leaf arrangement can be alternate, opposite, or whorled.
Venation patterns include parallel, pinnately netted, and palmately netted.
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Leaf Adaptations
Cuticle thickness varies in different plants based on environmental conditions.
Leaves adapted to hot, dry climates have thick cuticles.
Upper epidermis generally has a thicker cuticle than the lower epidermis.
Epidermis contains stomata for gas exchange, flanked by guard cells.
Guard cells open and close stomata, associated with subsidiary cells.
Edible Leaf Crops
Various vegetable crops like cabbage, lettuce, spinach, celery, and rhubarb are grown for their edible leaves.
Leaves are rich in vitamins A and C, iron, and calcium.
Examples of edible leaf crops and their characteristics.
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Leaf Adaptations Continued
Stomata are numerous on the lower epidermis of horizontally oriented leaves.
Some species have stomata only on the lower surface to reduce water loss.
Epidermis may have trichomes, hairlike structures for protection.
Leaf Tissues
Leaves contain dermal, ground, and vascular tissue systems.
Dermal tissue includes upper and lower epidermis with stomata and guard cells.
Ground tissue is represented by mesophyll layers.
Vascular tissue includes xylem and phloem in the veins.
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Plant Organs: Leaves
Trichomes on leaves serve multiple functions:
Reduce water loss by retaining moist air and reflecting sunlight.
Some secrete irritants to deter animals.
Others excrete excess salts from the soil.
Mesophyll, the photosynthetic tissue, is sandwiched between upper and lower epidermis.
Mesophyll cells are parenchyma cells with chloroplasts and facilitate gas exchange.
Mesophyll divided into palisade and spongy layers with different functions.
Palisade mesophyll is the main site of photosynthesis.
Spongy mesophyll allows diffusion of gases, especially CO2.
Veins in leaves contain xylem and phloem for water and nutrient transport.
Leaf structure differs in eudicots and monocots:
Eudicot leaves have netted venation and a petiole.
Monocot leaves lack a petiole, are narrow, and have parallel venation.
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Leaf Structure and Function
Bulliform cells in grass leaves help in rolling or folding the leaf during drought.
Guard cells in eudicots and certain monocots have different shapes affecting stoma opening.
Leaves' primary function is photosynthesis, converting light energy to chemical energy.
Leaf structure is optimized for maximum light absorption and efficient gas diffusion.
Mesophyll is the photosynthetic tissue in the leaf.
Bundle sheath surrounds vascular bundles in a leaf.
Photosynthesis is the process of converting light energy into chemical energy.
Figures
Figure 8-5: Bundle sheath extensions in a wheat midvein.
Figure 8-7: Bulliform cells in grass leaves.
Figure 8-8: Guard cells in eudicots and monocots.
Plant Organs: Leaves
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Leaf Structure
Epidermis of a leaf is transparent, allowing light penetration to mesophyll.
Mesophyll cells have air spaces for rapid diffusion of carbon dioxide.
Veins supply water, minerals, and sugars to the leaf.
Bundle sheaths provide support to prevent leaf collapse.
Stomata on leaf surfaces allow gas exchange.
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Stomatal Function
Stomata regulate gas exchange, including carbon dioxide and oxygen.
Guard cells control stomatal opening and closing based on water content.
Environmental factors like light, CO2 concentration, and dehydration affect stomatal behavior.
Circadian rhythms and hormonal control also influence stomatal activity.
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Stomatal Opening
Light triggers stomatal opening, a response to environmental signals.
Blue light is crucial for stomatal responses, involving yellow pigments in guard cell membranes.
Stomata open during the day for gas exchange and close at night.
Guard cells' shape changes based on water content, affecting stomatal aperture.
Stomata play a crucial role in gas exchange and water regulation in plant leaves, responding to environmental cues like light and water availability.
Plants and Air Pollution Effects
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Air Pollution Overview
Air pollution consists of harmful gases, liquids, or solids in the atmosphere.
Human activities, like motor vehicles and industry, are major sources of air pollution.
Effects on Plants
Leaves are vulnerable due to their structure and function.
Pollutants diffuse into leaves through stomatal pores, affecting photosynthesis.
Impact on Crop Plants
High levels of pollution reduce crop productivity.
Ozone is a significant pollutant, damaging mesophyll cells and inhibiting photosynthesis.
Forest Decline
Various stressors, including air pollutants like ozone and heavy metals, contribute to forest decline.
Interaction of stressors weakens trees, leading to symptoms like reduced growth and eventual death.
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Stomatal Opening Mechanism
Blue light triggers proton pumps in guard cells, leading to the movement of ions and water.
Potassium and chloride ions accumulate in guard cell vacuoles, increasing turgidity and opening stomata.
Stomatal Closing
Potassium ion concentration decreases during the day, while sucrose concentration increases to maintain open pores.
Sucrose, derived from starch splitting in guard-cell chloroplasts, helps regulate stomatal opening and closing.
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Mechanisms of Stomatal Regulation
Guard cells lose turgidity and close the pore when sucrose is converted back to starch.
Water leaves by osmosis causing the pore to close.
Stomatal opening is associated with the uptake of potassium and chloride ions.
Stomatal closing is associated with the declining concentration of sucrose.
Leaf structure adaptation reflects the environment to which a plant is adapted.
Water lilies have stomata on the upper epidermis and long petioles for floating.
Conifers have waxy needles with adaptations for surviving winter.
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Transpiration and Guttation
Transpiration is the loss of water vapor from aerial plant parts.
Most transpiration occurs through open stomata.
Environmental factors influence transpiration rate.
Higher air temperatures and light increase transpiration.
Wind and dry air increase transpiration, while humid air decreases it.
Transpiration is essential for water movement in plants and cooling leaves.
It helps in the movement of essential minerals from roots to stems and leaves.
Transpiration can be harmful to plants under certain circumstances.
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Transpiration and Wilting
Plants lose more water through transpiration than they absorb from the soil.
Results in loss of turgor in cells, causing the plant to wilt.
Temporary wilting occurs when plants recover overnight due to closed stomata and water absorption from the soil.
Prolonged drought can lead to permanent wilting and plant death.
Transpiration and Climate
Climate influenced by factors like temperature and precipitation, affected by transpiration.
Forests impact local climate by cooling the air through transpiration.
Transpiration is part of the hydrologic cycle, leading to cloud formation and precipitation.
Deforestation can lead to drier climates and temperature rise due to reduced transpiration.
Water Cycle
Transpiration and evaporation recycle 75% of water, while 25% seeps into the ground or runs off.
Forests play a crucial role in returning precipitation water to the atmosphere through transpiration.
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Guttation and Leaf Abscission
Guttation is the exudation of liquid water from plants when transpiration is low and soil moisture is high.
Leaf abscission is the shedding of leaves, influenced by factors like temperature and water requirements.
Abscission involves physiological changes and hormone levels in plants.
Protective bud scales, modified leaves, cover winter buds to protect them from damage and drying out.
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Leaf Abscission Process
Abscission zone near the base of the petiole is structurally different and weak, facilitating leaf detachment.
Protective layer of cork cells with suberin forms in the abscission zone.
Enzymes dissolve the middle lamella, allowing the leaf to detach with a slight breeze.
Modified leaves like bud scales protect winter buds from injury and drying out.
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Leaf Modifications
Overlapping bud scales protect buds on a maple twig.
Barrel cactus leaves are modified into spines for protection.
Poinsettia has showy red bracts around its inflorescence.
Sweet pea tendrils are modified leaves aiding in climbing.
Bulb leaves like those of onions are fleshy for food and water storage.
Stone plants have succulent leaves for water storage and photosynthesis.
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Specialized Leaves
Plants have leaves specialized for deterring plant-eating animals.
Spines on desert plants like cacti discourage animals from eating succulent stems.
Some leaves are modified as bracts around flower clusters.
Vines have tendrils for climbing support.
Bulbs like onions have fleshy leaves for storage.
Plants in arid conditions have succulent leaves for water storage.
Unusual environments lead to specialized foliar adaptations.
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Carnivorous Plants
Carnivorous plants grow in acidic bogs to meet mineral requirements.
Leaves of carnivorous plants attract, capture, and digest animal prey.
Pitcher plants have passive traps with acid-containing reservoirs.
Venus flytrap has active traps with trigger hairs and rapid closure.
Insects and microorganisms live inside pitcher plants, feeding on carcasses.
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Major Tissues of the Leaf
Leaf structure for photosynthesis
Epidermis allows light penetration
Mesophyll where photosynthesis occurs
Stomata for gas exchange
Waxy cuticle for survival in dry conditions
Leaf veins function
Xylem conducts water and minerals
Phloem conducts sugar
Contrast in eudicots and monocots
Monocots have narrow leaves with parallel venation
Eudicots have broad leaves with netted venation
Physiological Changes in Stomatal Opening and Closing
Stomatal opening process
Triggered by blue light activating proton pumps
Protons pumped out, creating a gradient for potassium and chloride ions
Water enters guard cells by osmosis, causing turgidity and stoma opening
Stomatal closing
Sucrose converted to starch, water leaves, guard cells lose turgidity, and pore closes
Transpiration and Its Effects
Loss of water vapor through stomata
Factors affecting transpiration: temperature, wind, humidity
Can be beneficial or harmful to the plant
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Leaf Abscission
Woody plants shed leaves in fall for winter survival
Physiological and anatomical changes in leaf abscission process
Sugar reabsorption, mineral transport, chlorophyll breakdown
Abscission zone development with cork cells and enzyme dissolution
Modified Leaves and Their Functions
Bud scales for meristematic tissue protection
Spines for defense
Bracts associated with flowers
Tendrils for support
Bulbs for storage
Carnivorous plant leaves for trapping insects
Review Questions
Diagram leaf structures
Photosynthesis equation and leaf organization
Eudicot vs. monocot leaf differences
Relationship between leaf structure and photosynthesis/transpiration
Water's role in stomatal opening/closing
Blue light effect on guard cells
Evolution of leaves to conserve water
Transpiration and environmental factors
Influence of environment on stomatal regulation
Leaf abscission and reasons for fall leaf loss
Differentiate spines, tendrils, bud scales, and bulbs
Features of carnivorous plant leaves
Causes of tree decline
Label a provided diagram.
Page 24: Thought Questions
Stomatal Opening and Chlorophyll
Stomata open in response to light.
Question: Could chlorophyll be the pigment involved in stomatal opening?
Reasoning needed to support the answer.
Arrangement of Vascular Tissues
Xylem and phloem positions in leaf veins.
Vascular tissue continuity between leaf and stem.
Suggest a possible arrangement of vascular tissues in the stem.
Identifying Epidermis Sides
Observing a leaf cross-section micrograph.
Determining upper and lower epidermis sides.
Characteristics to look for in making this determination.
Advantages of Leaf Size
Advantages and disadvantages of plants with few large leaves.
Advantages and disadvantages of plants with many small leaves.
Habitats where each type of plant might be found.
Additional Resources
Website link for further resources.
Flashcards, tutorial quizzes, readings,