Plant Form Chapter 36

2 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

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4 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

5 Plant Body Organization

6 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

7 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

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9 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  Epidermis -Procambium  1 o vascular tissue -Ground meristem  Ground tissue

10 Plant Body Organization

11 Plant Body Organization (Cont.)

12 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  Outer bark -Vascular cambium  2 o vascular tissue

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14 Plant Tissues As mentioned earlier, plants contain three main types of tissue -Dermal -Ground -Vascular

15 Dermal Tissue Forms the epidermis, which is usually one cell layer thick Covered with a fatty cutin layer constituting the cuticle Contains special cells, including guard cells, trichomes and root hairs

16 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

17 Dermal Tissue

18 Dermal Tissue (Cont.)

19 Dermal Tissue (Cont.)

20 Dermal Tissue Trichomes are cellular or multicellular hairlike outgrowths of the epidermis -Keep leaf surfaces cool and reduce evaporation by covering stomatal openings -Some are glandular, secreting substances that deter herbivory Trichome patterning is under genetic control

21 Dermal Tissue

22 Dermal Tissue (Cont.)

23 Dermal Tissue Roots hairs are tubular extensions of individual epidermal cells -Greatly increase the root’s surface area and efficiency of absorption

24 Ground Tissue Consist of three types of cells -Parenchyma -Collenchyma -Sclerenchyma

25 Ground Tissue Parenchyma cells are the most common type of plant cell -May live for many years, functioning in storage, photosynthesis and secretion -Some contain chloroplasts and are called chlorenchyma Collenchyma cells provide support for plant organs, allowing bending but not breaking -Have living protoplasts and may live for many years

26 Ground Tissue Sclerenchyma cells have tough thick walls -Lack living walls at maturity -Two general types -Fibers: Long, slender cells that are usually grouped in strands -Sclereids: Variable shape; branched; may occur singly or in groups -Both strengthen tissues

27 Ground Tissue

28 Ground Tissue (Cont.)

29 Vascular Tissue Xylem -Constitutes the main water- and mineral- conducting 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

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31 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

32 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

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34 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

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36 Roots Root cap -Contains two types of cells that are formed continuously by the root apical meristem -Columella cells: Inner -Root cap cells: Outer and lateral -Functions mainly in protection of the delicate tissues behind it -Also in the perception of gravity

37 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 -Quiescent center cells divide very infrequently -Apical meristem daughter cells soon subdivide into the three primary tissues

38 Roots Zone of cell division -Patterning of these tissues begins in this zone -WEREWOLF (WER) gene -Suppresses root hair development -SCARECROW (SCR) gene -Necessary for differentiation of endodermal and ground cells

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41 Roots Zone of elongation -Roots lengthen because cells become several times longer than wide -No further increase occurs above this zone

42 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

43 Roots Zone of maturation -Endodermis: Single-layered cylinder -Impregnated with bands of suberin called the Casparian strips -Stele: All tissues interior to endodermis -Pericycle: Multiple-layered cylinder -Gives rise to lateral (branch) roots or the two lateral meristems

44 Roots

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47 Plant Tissue Differentiation

48 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 Some plants, however, produce modified roots with specific functions -Adventitious roots arise from any place other than the plant’s root

49 Modified Roots Prop roots: Keep the plant upright Aerial roots: Obtain water from the air Pneumatophores: Facilitate oxygen uptake Contractile roots: Pull plant deeper into soil Parasitic roots: Penetrate host plants Food storage roots: Store carbohydrates Water storage roots: Weigh 50 or more kg Buttress roots: Provide considerable stability

50 Prop rootsAerial roots

51 PneumatophoresWater storage roots

52 Buttress roots

53 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

54 Stems

55 Stems Leaves may be arranged in one of three ways

56 Stems The spiral (alternate) arrangement is the most common -Sequential leaves tend to be placed o apart -This is termed phyllotaxy -May optimize the exposure of leaves to the sun

57 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

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59 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)

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62 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

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65 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

66 Internal Stem Structure

67 Internal Stem Structure

68 Internal Stem Structure Cork tissue cells get impregnated with suberin shortly after they are formed -They then die and constitute the outer bark The cork cambium also produces unsuberized cells called lenticels -Permit gas exchange to continue

69 Internal Stem Structure

70 Modified Stems Bulbs = Swollen underground stems, consisting of fleshy leaves Corms = Superficially resemble bulbs, but have no fleshy leaves Rhizomes = Horizontal underground stems, with adventitious roots Runners and stolons = Horizontal stems with long internodes that grow along the surface of the ground

71 Modified Stems Tubers = Swollen tips of rhizomes that contain carbohydrates Tendrils = Twine around supports and aid in climbing Cladophylls = Flattened photosynthetic stems resembling leaves

72 Modified Stems

73 Modified Stems (Cont.)

74 Modified Stems (Cont.)

75 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

76 Leaves The flattening of the leaf blade reflects a shift from radial to dorsal-ventral symmetry -It increases the photosynthetic surface The mechanism of this shift is becoming clearer through the analysis of mutants that lack distinct tops and bottoms

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79 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

80 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

81 Leaves

82 Leaves (Cont.)

83 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

84 Leaves

85 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

86 Leaves

87 Leaves (Cont.)

88 Modified Leaves Floral leaves (bracts) = Surround true flowers and behave as showy petals Spines = Reduce water loss and may deter predators Reproductive leaves = Plantlets capable of growing independently into full-sized plant Window leaves = Succulent, cone-shaped leaves that allow photosynthesis underground

89 Modified Leaves Shade leaves = Larger in surface area but with less mesophyll than sun-lit leaves Insectivorous leaves = Trap insects -Pitcher plants have cone-shaped leaves that accumulate rainwater -Sundews have glands that secrete sticky mucilage -Venus flytrap have hinged leaves that snap shut