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Leaves

Leaves. Chapter 7. Outline. Introduction Leaf Arrangements and Types Internal Structure of Leaves Stomata Mesophyll and Veins Specialized Leaves Autumnal Changes in Leaf Color Abscission Human and Ecological Relevance of Leaves. Introduction. Leaves originate as primordia in buds

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Leaves

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  1. Leaves Chapter 7

  2. Outline • Introduction • Leaf Arrangements and Types • Internal Structure of Leaves • Stomata • Mesophyll and Veins • Specialized Leaves • Autumnal Changes in Leaf Color • Abscission • Human and Ecological Relevance of Leaves

  3. Introduction • Leaves originate as primordia in buds • At maturity, most leaves have: • Stalk = petiole • Lacking petiole = sessile • Flattened blade = lamina • Network of veins = vascular bundles • Stipulesat base of petiole • Leaves of flowering plants associated with leaf gapsand have axillary buds at base

  4. Introduction • Leaf Complexity • Simple Leaves -single blade • Compound Leaves -blade divided into leaflets • Pinnately Compound - leaflets in pairs along rachis (petiole) • Bipinnately Compound -leaflets subdivided • Palmately Compound -leaflets attached at same point at end of petiole Bipinnately compound leaf Palmately compound leaf

  5. Introduction • Green leaves capture light energy by means of photosynthesis • Photosynthesis -trapping and storing of energy in C6H12O6molecules constructed from H2O and CO2 • Stomata -tiny pores on lower surfaces of leaves • Allow CO2 to enter and O2to diffuse out • H2O vapor escapes via stomata • Guard Cells - control H2O loss by opening or closing pore of stomatal apparatus

  6. Introduction • Other functions of leaves: • Wastes from metabolic processes accumulate in leaves and disposed of when leaves are shed • Play major role in movement of H2O absorbed by roots • Transpiration - occurs when H2O evaporates from leaf surface • Guttation -root pressure forces H2O out hydathodes at tips of leaf veins in some plants

  7. Leaf Arrangements and Types • Leaves attached to stems at nodes, with stem regions between known as internodes • Phyllotaxy -arrangement of leaves on stem • Alternate -1 leaf per node • Opposite -2 leaves per node • Whorled -3 of more leaves at node Alternate Opposite Whorled

  8. Leaf Arrangements and Types • Venation -arrangement of veins in leaf or leaflet blade • Pinnately Veined Leaves - main midvein included within enlarged midrib • 2° veins branch from midvein • Palmately Veined Leaves -several 1° veins fan out from base of blade Pinnate venation Palmate venation

  9. Leaf Arrangements and Types • Monocots - 1° veins parallel= Parallel Venation • Dicots - 1° veins divergent in various ways =Netted or Reticulate Venation • Dichotomous Venation -veins fork evenly and progressively from base of blade Parallel venation Dichotomous venation Reticulate venation

  10. Internal Structure of Leaves • 3 regions: Epidermis, mesophyll, veins (vascular bundles) • Epidermis -single layer of cells covering entire surface of leaf • Devoid of chloroplasts • Coated with cuticle (with cutin) • Protect tissues inside leaves • May accumulate waste materials • Glands may be present

  11. Stomata • Lower epidermis typically with thinner layer of cutin and numerous stomata • Stomata bordered by 2 guard cells • Guard cells originate from same parent cell, and contain chloroplasts • Regulate gas exchange between leaf interior and atmosphere • Regulate evaporation of H2O • Changes in amount of H2O in guard cells cause them to inflate or deflate • Inflate - Stomata open • Deflate - Stomata close

  12. Mesophyll and Veins • Most photosynthesis takes place inmesophyllbetween 2 epidermal layers • Palisade Mesophyll • Compactly stacked, barrel-shaped parenchyma cells, commonly in 2 rows • Contains most leaf’s chloroplasts • Spongy Mesophyll • Loosely arranged parenchyma cells with abundant air spaces

  13. Mesophyll and Veins • Veins (vascular bundles)scattered throughout mesophyll • Consist of xylem and phloem tissues surrounded by bundle sheath of thicker-walled parenchyma Dicot leaf cross section

  14. Mesophyll and Veins • Monocots have some differences: • Usually do not have mesophyll differentiated into palisade and spongy layers • Bulliformcells on either side of main central vein • Partly collapse under dry conditions • Cause leaf to fold or roll, reducing transpiration Monocot leaf cross section

  15. Specialized Leaves • Shade Leaves • Compared to sun leaves, shade leaves: • Tend to be larger • Tend to be thinner • Have fewer well-defined mesophyll layers and fewer chloroplasts • Have fewer hairs Sun leaf Shade leaf

  16. Specialized Leaves • Leaves of Arid Regions • Arid regions = limited availability of H2O, wide temperature ranges, and high light intensities • Leaves reduce loss of H2Oby: • Thick, leathery leaves • Fewer stomata or sunken stomata • Succulent, H2O-retaining leaves, or no leaves • Dense, hairy coverings • Leaves of Aquatic Areas • Less xylem and phloem • Mesophyll not differentiated • Aerenchyma!

  17. Specialized Leaves • Tendrils • Curl around more rigid objects, helping plant to climb or to support weak stems • Garden peas Tendrils • Spines • Reduce leaf surface and H2O loss, and protect from herbivory • Cacti • Leaf tissue replaced with sclerenchyma • Photosynthesis occurs in stems Spine

  18. Specialized Leaves • Thorns -modified stems arising in axils of leaves of woody plants Thorn • Prickles -outgrowths from epidermis or cortex Prickle

  19. Specialized Leaves • Storage leaves • Succulent leaves modified for H2O storage • Have parenchyma cells with large vacuoles • Many desert plants • Fleshy leaves store carbohydrates • Onions, lily

  20. Specialized Leaves • Flower-Pot Leaves • Leaves develop into urn-like pouches that become home of ant colonies • Ants carry in soil and add nitrogenous wastes, providing good growing medium for plant’s own roots • Dischidia, an epiphyte of Australia Flower-pot leaf sliced lengthwise

  21. Specialized Leaves • Window leaves • Succulent desert plants of Africa • Leaves buried in ground, except for exposed end • End has transparent, thick epidermis and transparent H2O storage cells underneath • Allows light into leaf, while buried leaves keep plant from drying out

  22. Specialized Leaves • Reproductive Leaves • Walking fern - new plants at leaf tips • Air plant - tiny plantlets along leaf margins Air Plant

  23. Specialized Leaves • Floral Leaves (= Bracts) • At bases of flowers or flower stalks • Poinsettia – no petals, instead brightly colored bracts surround flowers • Clary’s sage – colorful bracts at top of flowering stalks above flowers Poinsettia Clary’s sage

  24. Specialized Leaves • Insect-Trapping Leaves • Grow in swampy areas and bogs • N and other elements deficient in soil • Specialized leaves trap and digest insects • Pitcher Plants • Insects trapped and digested inside cone-shaped leaves Pitcher plant

  25. Specialized Leaves • Insect-Trapping Leaves • Sundews • Round to oval leaves covered with glandular hairs with sticky fluid of digestive enzymes at tip • Venus’s Flytraps • Only in North Carolina and South Carolina • Blade halves trap insects Sundew Venus’s Flytraps

  26. Specialized Leaves • Insect-Trapping Leaves • Bladderworts • Submerged or floating in shallow water • Tiny bladders on leaves have trap doors that trap insects inside bladders Bladder of bladderwort

  27. Autumnal Changes in Leaf Color • Chloroplasts of mature leaves contain several groups of pigments: • Chlorophylls– green • Carotenoids – yellows • In fall, chlorophylls break down and other colorsrevealed • H2O soluble anthocyanins (red or blue) and betacyanins(red) may be present in vacuole

  28. Abscission • Deciduous plants drop leaves seasonally • Abscission - process by which leaves are shed • Result of changes in abscission zone near base of petiole • ProtectiveLayer - cells coated and impregnated with suberin • SeparationLayer- pectinsin middle lamella of cells broken down by enzymes

  29. Human and Ecological Relevance of Leaves • Landscaping - shade trees • Food - cabbage, lettuce, celery petioles, spices • Dyes • Perfumes - oils of orange tree, lavender • Ropes and Twine - Agave, hemp fibers • Drugs - narcotics, tobacco, marijuana • Beverages - tea, tequila (agave leaves) • Insecticides - rotenone • Waxes - carnauba and caussu waxes • Aesthetics - floral arrangements, gardens

  30. Review • Introduction • Leaf Arrangements and Types • Internal Structure of Leaves • Stomata • Mesophyll and Veins • Specialized Leaves • Autumnal Changes in Leaf Color • Abscission • Human and Ecological Relevance of Leaves

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