Transcript Slide 1
The Shoot System I: The Stem Chapter 5 Organization of Shoot System • Shoot system of flowering plant consists of – Stem with attached leaves, buds, flowers, and fruits terminal bud contains SAM bud node module internode leaf Shoot system Root system primary root lateral root RAM Fig. 5-1, p. 71 Shoot System • Functions – Provide axis for attachment of leaves, buds, flowers – To produce new cells, tissues, leaves, and buds – Provide pathways for movement of water and dissolved minerals from roots to leaves – Provide pathways for food synthesized in leaves to move into roots – May be modified for different functions such as water storage Shoot System • Modules – Repeating units of the stem – Consists of internode plus the leaf and bud attached to the stem • Node – Point of attachment Groups of Flowering Plants Group Cotyledons Examples Descriptions Monocotledonous plants (monocots) Produce embryos with one cotyledon (seed leaf) Corn, onion Stem has scattered vascular bundles, primary phloem usually positioned toward the outside Dicotyledonous plants (dicots) Produce embryos with two cotyledons (seed leaves) Peas, oak Have pith surrounded by cylinder of vascular bundles, primary xylem toward inside, primary phloem toward outside SAM • SAM – Shoot apical meristem – Composed of dividing cells • Three primary meristems – Protoderm – Ground meristem – procambium young leaf SAM procambium protoderm ground meristem Fig. 5-3, p. 73 Protoderm • Outermost layer of cells in shoot tip • When cells stop dividing and mature called epidermis Ground Meristem • In center of shoot tip • Just inside protoderm • Cells slowly lose ability to divide Ground Meristem • Differentiate into parenchyma cells of cortex and pith – Parenchyma cells nearest outside of cortex may contain chloroplasts – Parenchyma cells of cortex or pith may store starch – Pith region may become hollow due to breakdown of parenchyma Procambium • Forms as small bundles of long, thin cells with dense cytoplasm – Bundles arranged in ring just inside outer cylinder of ground meristem and below SAM • Cells divide – At position down axis, cells stop dividing and differentiate into primary xylem and primary phloem Procambium • Each bundle of procambium becomes vascular bundle – Primary xylem toward inside of stem – Primary phloem toward outside of stem • Residual procambium – Occurs in plants with secondary growth – Procambium between primary xylem and phloem – Remains undifferentiated Distribution of Primary Vascular Bundles in Dicot Stem • In vascular cylinder • Leaf traces – Bundles that network into attached leaves • Organization of bundles in stems depends on – Number and distribution of leaves – Number of traces that branch into leaves and into buds Apical meristem Three primary meristems protoderm ground meristem procambium primary phloem residual procambium primary xylem epidermis cortex vascular bundle pith leaf trace stem of primary plant body Fig. 5-4, p. 73 leaf traces vascular bundle internode small vascular bundle petiole node Fig. 5-5, p. 73 Distribution of Primary Vascular Bundles in Dicot Stem • Number of vascular bundles in cylinder and number of leaf traces – Varies by species – Dependent on number and arrangement of leaves Leaf Arrangements Pattern Leaves/node Angle of divergence Alternate 1 leaf/node 180º Opposite 2 leaves/node 90º Whorled 3 or more leaves/node 60º Spiral 1 leaf/node 137.5º Fig. 5-6, p. 74 Monocot Stem Primary Growth Primary growth • Scattered vascular bundles – Terms pith and cortex usually not used when bundles are scattered • Stem same diameter at apex and base – Primary thickening meristem (PTM) • Absent in dicot stems • Contributes to both elongation and lateral growth epidermis vascular bundle cortex hollow center Fig. 5-7, p. 75 Secondary Growth • Most monocots show little or no secondary growth – Herbaceous (nonwoody) plants – Normally complete life cycle in one growing season • Dicots and gymnosperms – Display secondary growth starting first year of growth – Woody plants residual procambium residual procambium parenchyma primary xylem primary phloem vascular bundle primary phloem parenchyma Cells begin dividing vascular bundle primary xylem interfascicular cambium fascicular cambium vascular cambium secondary xylem vascular cambium secondary phloem secondary xylem secondary phloem Vascular cambium forms secondary phloem secondary xylem Secondary xylem and phloem form vascular cambium secondary phloem secondary xylem Fig. 5-9, p. 76 Formation of Secondary Xylem and Phloem Formation of vascular cambium • cell division occurs in residual procambium inside vascular bundles and parenchyma cells between bundles • Plant hormone probably provides signal • Dividing residual procambium within bundles called fascicular cambium fascicular cambium epidermis primary phloem interfascicular cambium primary xylem Fig. 5-10a, p. 77 vascular cambium Fig. 5-10b, p. 77 Formation of Secondary Xylem and Phloem • Dividing residual procambium between bundles called interfascicular cambium • Fascicular cambium + interfascicular cambium = vascular cambium Vascular Cambium • Only one or two cells thick • Divides in two directions • Cells formed to outside form secondary phloem • Cells formed to inside form secondary xylem • Typically produces more xylem than phloem cells initial surface of stem or root cell of division one cell division one cell vascular differentiates differentiates cambium into xylem, into phloem, divisions at start of one stays one stays and secondary meristematic meristematic differentiation growth continues DIRECTION OF GROWTH Fig. 5-11, p. 77 Vascular Cambium • Fusiform initials – Cambium cells – Form into cells of axial system • Ray initials – Form cells of ray system – Rays composed of ray parenchyma cells and ray tracheids – Ray system transports water and minerals laterally Wood • Composed of secondary xylem • Planes of view – Tangential section – end view of rays – Radial section – side view of rays – Transverse section – end view of cells of axial system Annual Rings • Concentric rings of cells of secondary xylem • In temperate zones – One ring/growing season – Determine age of tree by counting rings • In tropical rain forests – Irregular growth rings – Growth occurs year round primary growth, some secondary growth secondary growth year 1 2 3 Ray system Axial system bark vascular cambium Fig. 5-12a, p. 78 Annual Rings • Oldest known trees – Redwoods (Sequoia sempervirens) – Bristlecone pines (Pinus longaeva) Annual Ring Components • Springwood or earlywood – Cells in inner part of annual ring – Cells larger in diameter – Formed during first growth spurt of new season • Summerwood or latewood – Cells smaller in diameter – Formed later in growing season Annual Ring Components • Ring porous – Large diameter vessels mainly in springwood • Diffuse porous – Large diameter vessel members uniformly distributed throughout springwood and summerwood Heartwood • Heartwood – Darker wood in center – Cells blocked with resins and other materials – No longer functions in transport – Vessel members may be blocked by tyloses • Form when cell wall of parenchyma cell grows through pit and into vessel member periderm secondary phloem secondary xylem heartwood sapwood bark vascular cambium Fig. 5-16a, p. 80 Sapwood • Lighter wood near periphery • Secondary xylem – Has functional xylem cells • Where actual transport of water and dissolved minerals takes place sapwood heartwood branch (knot) Fig. 5-16b, p. 80 Gymnosperm Structure • Wood –simpler structure • Mostly tracheids in axial system and simple rays • May have resin ducts – Secretory structures that produce and transport resin Resin • Synthesized and secreted by lining of epithelial cells • Sap – Resin flowing through resin ducts to outside of stem • Rosin – Hardened resin • Amber – Fossilized rosin Bark • Protective covering over wood of tree • Everything between vascular cambium and outside of woody stem • Composition varies, depending on age of tree – Young tree • Secondary phloem, few cortex cells, 1 or 2 increments of periderm – Old tree • Layers of secondary phloem and several layers of periderm Secondary Phloem • Forms to outside of vascular cambium • Cell types – Sieve-tube members, companion cells, phloem, parenchyma, phloem fibers, sclereids in axial system, ray parenchyma in ray system • Cannot count phloem rings to determine age of tree • Phloem rays – Phloem ray parenchyma cells Periderm • Made up of – Phellem – Cork cambium – phelloderm • Functions – Inhibits water evaporation – Protects against insect and pathogen invasion Periderm • Cork cambium (phellogen) • New cork cambium usually produced each spring – Divides in two directions to produce • Phellem cells (cork cells) – Produced toward the outside • Phelloderm cells – Produced toward the inside Periderm • Phellem cells – Regular rows – Cell walls contain suberin – Usually dead by time periderm is functional • Phelloderm cells – Form regular rows – Cells live longer and resemble parenchyma cells Periderm • Lenticels – In bark of young, woody tree branches – Loosely packed parenchyma cells – Provide area for gas exchange • Girdling – Removal of continuous strip around tree circumference kills tree – Nutrient transporting secondary phloem severed in process Main Bark Patterns Pattern Description Example Ring bark Continuous rings Paper birch Scale bark Small, overlapping scales Pine trees Shag bark Long, overlapping, thin sheets Eucalyptus Buds • Short, compressed branches • Covered with hard, modified leaves called bud scales • Types of buds – Terminal bud • At end of branch – Lateral bud • At base of petioles of leaves on side of a branch – Flower bud • Produces flower parts Buds • • • • Bud scale scar Leaf scar Bundle scar Can identify plants in winter by – Structure of leaf scar – Number and distribution pattern of bundle scars Secondary Growth in Monocot Stems • Most monocots do not form secondary xylem and secondary phloem • Palm trees – Exhibit diffuse secondary growth – Some thickening of stem from division and enlargement of parenchyma cells – Not true secondary growth because cambium is lacking Secondary Growth in Monocot Stems • Some monocots exhibit true secondary growth • Examples – Yucca, Agave (century plant), Dracaena (dragon’s blood tree) • Produce stems that are thin at top, thick at base • Cambium primarily forms parenchyma cells • Xylem surrounds phloem in vascular bundle Stem Modifications • Rhizomes – Underground stem – Internodes and nodes – Sometimes small, scale-like leaves • Leaves do not grow • Leaves are not photosynthetic – Buds in axils of scale leaves elongate, produce new branches which form new plants Stem Modifications • Tubers – Enlarged terminal portion of underground rhizome – Example: potato plant – Eyes of tuber - lateral buds Stem Modifications • Corms and bulbs – Corm • Short, thickened underground stem with thin, papery leaves • Central portion accumulates stored food to be used at time of flowering • New corms can form from lateral buds on main corm • Example: Gladiolus Stem Modifications • Corms and bulbs – Bulbs • Small stem portion • At least one terminal bud (produces new, upright leafy stem) • Lateral bud (produces new bulb) • Stores food in specialized fleshy leaves – Food used during initial growth spurt • Example: Allium cepa (table onion) Stem Modifications • Cladophylls – Also called cladodes – Flattened, photosynthetic stems that function as and resemble leaves – Develop from buds in axils of small, scale-like leaves – Example: Ruscus aculeatus (Butcher’s broom) Stem Modifications • Thorns – – – – Originate from axils of leaves Help protect plant from predators May have leaves growing on them Spines and prickles • Not modified stems • Spines – Modified leaves • Prickles – modified clusters of epidermal hairs Economic Value of Woody Stems • Forests – Home to many plants and animals – Source of raw materials for many useful products – Purify air – Keep soil from washing away – Affect weather patterns Economic Value of Woody Stems • Renewable resources – Harvesting of product from plant without destroying plant – Natural rubber, chewing gum, turpentine • Nonrenewable resources – actual harvesting and use of entire plant • Recycling – Example: recycling paper products – Helps preserve natural tree resources The Shoot System II: The Form and Structure of Leaves Chapter 6 Functions of Leaves • Photosynthesis – Release oxygen, synthesize sugars • Transpiration – Evaporation of water from leaf surface • Specialized functions – Water storage – Protection Comparison of Monocot and Dicot Leaves Type Monocot Dicot Shape of blade Venation Description Strap-shaped *blade Leaf bases usually wrap around stem Parallel vascular bundles Thin, flat blade Netted pattern of vascular bundles *blade – portion of leaf that absorbs light energy Petiole holds blade away from stem Leaf Blade • Broad, flat surface for capturing light and CO2 • Two types of leaves – Simple leaves – Compound leaves Leaf Blade • Simple leaves – Leaves with a single blade – Examples • Poplar • Oak • Maple Leaf Blade • Compound leaves – Blade divided into leaflets – Two types • Palmately compound – Leaflets diverge from a single point – Example: red buckeye • Pinnately compound – Leaflets arranged along an axis – Examples: black locust, honey locust Leaf Blade – Advantages of compound leaves • Spaces between leaflets allow better air flow over surface – May help cool leaf – May improve carbon dioxide uptake Petiole • • • • Narrow base of most dicot leaves Leaf without petiole – sessile Vary in shape Improves photosynthesis – Reduces extent to which leaf is shaded by other leaves – Allows blade to move in response to air currents Sheath • Formed by monocot leaf base wrapping around stem • Ligule – Keeps water and dirt from getting between stem and leaf sheath • Auricles – In some grass species – Two flaps of leaf tissue – Extend around stem at juncture of sheath and blade Sheath Why does grass need mowing so often? • Grass grows from base of sheath • Intercalary meristem • Allows for continued growth of mature leaf • Stops dividing when leaf reaches certain age or length Leaf Veins • Vascular bundles composed of xylem and phloem Type of venation Example Description Monocots •Several major veins running parallel from base to tip of leaf •Minor veins perpendicular to major veins Netted Dicots •Major vein (midvein or midrib) runs up middle of leaf •Lateral veins branch from midvein Open dichotomous Ferns and some gymnosperms Parallel Y-branches with no small interconnecting veins Epidermis • Covers entire surface of blade, petiole, and leaf sheath • Continuous with stem epidermis • Usually a single layer of cells • Cell types – – – – Epidermal cells Guard cells Subsidiary cells Trichomes Epidermal Cells • Appear flattened in cross-sectional view • Outer cell wall somewhat thickened • Covered by waxy cuticle – Inhibits evaporation through outer epidermal cell wall Stomatal Apparatus • Cuticle blocks most evaporation • Opening needed in epidermis for controlled gas exchange • Two guard cells + pore stoma • Subsidiary cells – Surround guard cells – May play role in opening and closing pore Stomatal Apparatus • Guard cells + subsidiary cells stomatal apparatus • Functions of stoma – Allows entry of CO2 for photosynthesis – Allows loss of water vapor by transpiration • Cools leaf by evaporation • Pulls water up from roots Stomatal Apparatus • Stomata usually more numerous on bottom of leaf • Stomata also found in – Epidermis of young stem – Some flower parts Trichomes • Secretory – Stalk with multicellular or secretory head – Secretion often designed to attract pollinators to flowers • Short hairs – Example: saltbush (Atriplex) – Hairs store water, reflect sunlight, insulate leaf against extreme desert heat Trichomes • Mat of branched hairs – Example: olive tree (Olea europea) – Act as heat insulators • Specialized trichomes – Leaves modified to eat insects as food Mesophyll • Two distinct regions in dicot leaf – Palisade mesophyll – Spongy mesophyll • Substomatal chamber – Air space just under stomata Mesophyll Type Cell type Location Description Palisade parenchyma, tightly packed, Palisade column shaped, mesophyll oriented at right angles to leaf surface Usually on upper surface Cells tightly packed, absorb sunlight more efficiently Spongy parenchyma Spongy cells, irregularly mesophyll shaped, abundant air spaces Usually located on bottom surface Irregular cell shape, abundant air spaces allow more efficient air exchange Mesophyll • Dicot midrib (midvein) – Xylem in upper part of bundle – Phloem in lower part of bundle • Bundle sheath – Single layer of cells surrounding vascular bundle – Loads sugars into phloem – Unloads water and minerals out of xylem Formation of New Leaves • Originate from meristems • Leaf primordia – early stages of development Formation of New Leaves • Steps in leaf formation – Initiated by chemical signal – Location in leaf depends on plant’s phyllotaxis – Cells at location begin dividing • Becomes leaf primordium – Shape of new leaf determined by how cells in primordium divide and enlarge Cotyledons • Seed leaves – Primarily storage organs – Slightly flattened, often oval shaped – Usually wither and die during seedling growth • Example of exception – bean plant • Cotyledons enlarge and conduct photosynthesis Heterophylly • Different leaf shapes on a single plant • Types of heterophylly – Related to age of plant • Example: ivy (Hedera helix) – Juvenile ivy leaves – three lobes to leaves – Adult ivy leaves – leaves are not lobed Heterophylly – Environment to which shoot apex is exposed during leaf development • Example: marsh plants – Water leaves » Leaves developing underwater are thin with deep lobes – Air leaves » Shoot tip above water in summertime develops thicker leaves with reduced lobing Heterophylly – Position of leaf on tree • Shade leaves – Develop on bottom branches of tree – Mainly exposed to shade – Leaves are thin with large surface area • Sun leaves – Develop near top of same tree – Exposed to more direct sunlight – Leaves are thicker and smaller Adaptations for Environmental Extremes • Xerophytes – Grow in dry climates – Leaves designed to conserve water, store water, insulate against heat • Sunken stomata • Thick cuticle • Sometimes multiple layers to epidermis Adaptations for Environmental Extremes • Xerophytes – Abundance of fibers in leaves • Help support leaves • Help leaf hold shape when it dries – Examples • Oleander (Nerium oleander) • Fig (Ficus) • Jade plant (Crassula argentea) Adaptations for Environmental Extremes • Hydrophytes – Grow in moist environments – Lack characteristics to conserve water – Leaves • Thin • Thin cuticle • Often deeply lobed • Mesophytes – Grow in moderate climates Leaf Modifications • Spines – Cells with hard cell wall – Pointed and dangerous to potential predators • Tendrils – Modified leaflets – Wrap around things and support shoot Leaf Modifications • Bulbs – Thick leaves sometimes referred to as bulb scales • Store food and water – Modified branches with short, thick stem and short, thick storage leaves Leaf Modifications • Plantlets – Leaves have notches along margins – Meristem develops in bottom of each notch that produce a new plantlet – Plantlet falls off leaf and roots in soil – Form of vegetative (asexual) reproduction – Example • Air-plant (Kalanchoe pinnata) Leaf Abscission • Abscission – separation • Result of differentiation and specialization at region at base of petiole called abscission zone – Weak area due to • Parenchyma cells in abscission zone are smaller and may lack lignin in cell walls • Xylem and phloem cells are shorter in vascular bundles at base of petiole • Fibers often absent in abscission zone Leaf Abscission • • • • Abscission zone weakens Cells in vascular bundles become plugged Leaf falls off Leaf scar – Scar that remains when leaf falls off – Sealed over with waxy materials which block entrance of pathogens Environmental Abscission Controls • Cold temperatures • Short days – Induce hormonal changes that affect formation of abscission zone – Leaves move nutrients back into stem – Leaves lose color – Leaves fall off tree – Leaves decompose and recycle nutrients