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Plant Form Chapter 36 Plant Body Organization A vascular plant consists of: 1. Root system, which is underground -Anchors the plant, and is used to absorb water and minerals 2. Shoot system, which is above ground -Consists of supporting stems, photosynthetic leaves and reproductive flowers Each has an apex that extends growth 2 3 1 Plant Body Organization Plant cell walls consist of cellulose -Primary cell wall -Found in all cells -Cellulose fibers parallel to microtubules -Secondary cell wall -Found in some cells -Additional layers of cellulose and lignin -Increase mechanical strength of wall 4 Plant Body Organization Roots, shoots and leaves contain three basic tissue systems: -Dermal tissue – For protection -Wax and bark -Ground tissue – For storage, photosynthesis and secretion -Vascular tissue – For conduction -Xylem – Water and dissolved minerals -Phloem – Nutrient-containing solution 5 Plant Body Organization Meristems are clumps of small cells with dense cytoplasm and large nuclei They act as stem cells do in animals -One cell divides producing a differentiating cell and another that remains meristematic 6 2 Plant Body Organization Apical meristems are located at the tips of stems and roots -Give rise to primary tissues which are collectively called the primary plant body -Three primary meristems -Protoderm à Epidermis -Procambium à 1o vascular tissue -Ground meristem à Ground tissue 7 Plant Body Organization Lateral meristems are found in plants that exhibit secondary growth -Give rise to secondary tissues which are collectively called the secondary plant body -Woody plants have two types -Cork cambium à Outer bark -Vascular cambium à 2o vascular tissue 8 Dermal Tissue Guard cells are paired sausage-shaped cells -Flank a stoma, which is the passageway for oxygen and carbon dioxide Guard cell formation is the result of an asymmetrical cell division that produces: -A guard cell -A subsidiary cell -Aids in stoma opening and closing 9 3 Dermal Tissue Roots hairs are tubular extensions of individual epidermal cells -Greatly increase the root’s surface area and efficiency of absorption 10 Vascular Tissue Xylem -Constitutes the main water- and mineralconducting tissue -Vessels: Continuous tubes of dead cylindrical cells arranged end-to-end -Tracheids: Dead cells that taper at the end and overlap one another -Vessels are shorter & wider than tracheids -And conduct water more efficiently 11 Vascular Tissue Xylem -Also conducts inorganic ions such as nitrates, and supports the plant body -Typically includes parenchyma cells in horizontal rows called rays -Function in lateral conduction and food storage Note: The diffusion of water vapor from a plant is termed transpiration 12 4 Vascular Tissue Phloem -Constitutes the main food-conducting tissue in vascular plants -Contains two types of elongated cells: sieve cells and sieve tube members -Living cells that contain clusters of pores called sieve areas or sieve plates -Sieve-tube members are more specialized -Associated with companion cells 13 Roots Roots have a simpler pattern of organization and development than stems Four regions are commonly recognized: -Root cap -Zone of cell division -Zone of elongation -Zone of maturation 14 15 5 Roots Root cap -Functions mainly in protection of the delicate tissues behind it -Also in the perception of gravity 16 Roots Zone of cell division -Contains mostly cuboidal cells, with small vacuoles and large central nuclei -Derived from rapid divisions of the root apical meristem 17 Roots Zone of elongation -Roots lengthen because cells become several times longer than wide -No further increase occurs above this zone 18 6 Roots Zone of maturation -The elongated cells become differentiated into specific cell types -Epidermal cells: Have very thin cuticle -Include root hair and nonhair cells -Cortex: Interior to the epidermis -Parenchyma cells used for storage 19 Roots Zone of maturation -Endodermis: Single-layered cylinder -Impregnated with bands of suberin called the Casparian strips -Pericycle: Multiple-layered cylinder -Gives rise to lateral (branch) roots or the two lateral meristems 20 21 7 22 Modified Roots Most plants produce either/or: -Taproot system: Single large root with small branch roots -Fibrous root system: Many small roots of similar diameter 23 Stems Like roots, stems contain the three types of plant tissue -Also undergo growth from cell division in apical and lateral stems Shoot apical meristem initiates stem tissue and intermittently produces primordia -Develop into leaves, other shoots and even flowers 24 8 Stems Leaves may be arranged in one of three ways 25 Stems The spiral (alternate) arrangement is the most common -Sequential leaves tend to be placed 137.5o apart -This is termed phyllotaxy -May optimize the exposure of leaves to the sun 26 External Stem Structure Node = Point of attachment of leaf to stem Internode = Area of stem between two nodes Blade = Flattened part of leaf Petiole = Stalk of leaf Axil = Angle between petiole/blade and stem Axillary bud = Develops into branches with leaves or may form flowers Terminal bud = Extends the shoot system during the growing season 27 9 28 Internal Stem Structure Monocot vascular bundles are usually scattered throughout ground tissue system Eudicot vascular tissue is arranged in a ring with internal ground tissue (pith) and external ground tissue (cortex) 29 30 10 31 Internal Stem Structure Vascular tissue arrangement is directly related to the stem’s ability for secondary growth -In eudicots, a vascular cambium develops between the primary xylem and phloem -Connects the ring of primary vascular bundles -In monocots, there is no vascular cambium -Therefore, no secondary growth 32 33 11 34 Internal Stem Structure Rings in the stump of a tree reveal annual patterns of vascular cambium growth -Cell size depends on growth conditions In woody eudicots and gymnosperms, the cork cambium arises in the outer cortex -Produces boxlike cork cells on outside and parenchyma-like phelloderm cells on inside -Collectively called the periderm 35 Internal Stem Structure 36 12 Internal Stem Structure 37 Internal Stem Structure Cork tissue cells get impregnated with suberin shortly after they are formed -They then die and constitute the outer bark 38 Leaves Leaves are the main site of photosynthesis -They are determinate structures whose growth stops at maturity Exist in two morphologies -Microphyll = Have one vein which does not extend the full length of the leaf -Found mainly in the phylum Lycophyta -Megaphylls = Have several to many veins 39 13 Leaves Veins consist of both xylem and phloem and are distributed throughout the leaf blades -Monocot leaves have parallel veins -Eudicot leaves have netted or reticulate veins 40 Leaves Leaf blades come in a variety of forms -Simple leaves contain undivided blades -May have teeth, indentations or lobes -Compound leaves have blades that are divided into leaflets -Pinnate = Leaflets in pairs along an axis -Palmate = Leaflets radiate out from a common point 41 Leaves The leaf’s surface is covered by transparent epidermal cells, most having no chloroplasts Epidermis has a waxy cuticle -The lower epidermis contains numerous mouth-shaped stomata flanked by guard cells 42 14 Leaves The mesophyll is the tissue between the upper and lower epidermis -Most eudicot leaves have two types -Palisade mesophyll = Usually two rows of tightly packed chlorenchyma cells -Spongy mesophyll = Loosely arranged cells with many air spaces in between -Monocot leaves mesophyll is usually not differentiated into palisade/spongy layers 43 Leaves 44 Leaves (Cont.) 45 15