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Plant Structure and Growth 9.3 General Plant Structure • All plants have roots, stems and leaves – Roots: • absorb water and mineral ions • anchor plants to the ground • provide food storage – Stems: • provide attachment for leaves (at nodes) • transport nutrients (water and mineral ions from up from roots through xylem; organic nutrients / sugars through phloem) • provide storage and support – Leaves: • photosynthesis • gas exchange and transpiration Plant Classification Kingdom Plantae Non-vascular Plants Vascular Plants small; no conductive (vascular) tissue – ex: bryophytes (mosses) have conductive (vascular) tissue (xylem and phloem) Spermatophytes Filicophytes produce seeds have alternating life cycles (haploid and diploid) – ex: ferns Angiospermophytes Gymnospermophytes (“angiosperms”) flowering plants (“gymnosperms”) seeds are in cones (naked seeds) Monocots Dicots ONE seed leaf ex: grasses, orchids TWO seed leaves ex: roses Angiosperms: Monocotyledonous plants versus Dicotyledonous plants Structure MONOCOTS DICOTS Cotyledons (seed leaves) ONE TWO Leaf veins (venation) Parallel Netlike Vascular bundles in stems Arranged throughout entire stem (scattered) Arranged in a ring/ circle around edge of stem Roots Fibrous (groups of thin roots spread out into the soil) Taproot (one main vertical root with small branches off of it) Flowers Flower parts in multiples of 3’s Flower parts in multiples of 4 or 5 Dicot Tissues: Stems Structure Epidermis Xylem Phloem Function •Protection •Transport (water and mineral ions from roots up to rest of plant) •Structural/ mechanical support (lignin/ lignified in terrestrial plants) •Transport (organic nutrients – sugars – from “source to sink”) Cambium •Region of stem cells that separate xylem and phloem; differentiate into new xylem/ phloem cells Cortex •Support through turgid, fluid-filled cells (sometimes aids in storage) Pith A. Plan diagram of dicot stem tissue •Support through turgid, fluid-filled cells •Storage B. Actual image of dicot stem tissue Dicot Tissues: Leaves Structure Function Relationship between distribution of structure in leaf and its function Cuticle (waxy layer) and upper epidermis Protection Upper epidermis secretes waxy layer (cuticle) to protect leaf tissues from heat/ light damage and prevent water loss Palisade mesophyll Photosynthesis Cells are chloroplast-rich and tightly packed in upper half of leaf for maximum light absorption Vascular bundles Transport (xylem = water and mineral ions; phloem = sugars) Located in middle of leaf so all cells have access Spongy mesophyll Gas exchange Cells are loosely packed with spaces between them in lower half of leaf near stomata openings Lower epidermis (guard cells/ stomatal pores) Transpiration (water vapour loss) and gas exchange Lower temperatures/ less light on bottom of leaf so water loss is minimized; guard cells open and close stomatal pores for gas exchange and transpiration Modifications of Plant Roots, Stems, and Leaves • Many plants have modified roots, stems, and leaves in order to help them survive in different environments Modified Structure Function and Example(s) Storage roots Underground root cells that are specialized/ modified to store large quantities of water and carbohydrates Examples: Carrots/ beets Stem tubers Underground, horizontally-growing stems that are modified to store carbohydrates Example: Potatoes Bulbs Underground, small, vertical-growing stems attached to enlarged/ swollen leaf bases that are modified to store food Example: onions Tendrils Modified leaves that coil around objects (above ground) to aid in support, attachment and climbing Examples: Vines/ peas Plant Growth: Meristems • Plants ONLY grow at regions called meristems: areas of undifferentiated cells (stem cells) • Meristems can be apical (at the apex/ tip of a root or stem – to allow plants to grow taller or allow roots to extend throughout soil) or lateral (on the side (cambium) – to allow plants to grow outward/ become thicker) Apical and Lateral Meristems in Dicotyledonous Plants • Dicotyledonous plants contain both apical and lateral meristems • Both apical and lateral meristems rely on totipotent cell divisions for growth Apical Meristems (1° Growth) Lateral Meristems (2° Growth) Occur at tips of stems and roots Occur at cambium in woody trees/ shrubs: •vascular cambium produces secondary xylem (wood) and secondary phloem •cork cambium (in bark) produces cork cells of outer bark Add vertical growth to roots and stems (increased light/ CO2 and nutrient absorption) Add lateral growth to stems (widens/ thickens stem of plant) Produces new leaves and flowers Produces bark and wood on trees/ woody shrubs Develops into primary xylem and phloem Produces secondary xylem and phloem Lateral Meristem Growth (Bark and Wood) Plant Growth: Tropisms • A tropism is a directional response in a plant to an external stimulus (light, chemicals, gravity, touch). • Plant tropisms are either positive (toward stimulus) or negative (away from stimulus) • Phototropism is a positive tropism in plant stems – they grow toward the light Phototropism and Auxin • Auxin is a plant hormone that promotes growth by lengthening plant cells and altering gene expression (produced in coleoptiles = protective sheaths around apical meristems) • Normally, auxin is distributed evenly throughout plant cells in stems so plant stems grow evenly • If one side of a plant receives more light, auxin is redistributed so that the shaded side of the plant has MORE of it. More auxin = more growth/ cell elongation, so stem bends on shaded side (cells elongate) and plant stem grows toward light. – Auxin works by activating proton pumps in cells; H+ ions pumped to cell wall from cytoplasm, decreasing pH, loosening cellulose for stretching/ growth