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Meristems, Tissues

meristems - determines how the plant body develops after germination

  • acts like stem cells in animals
  • divides >> 1 meristematic cell, 1 cell able to differentiate
  • apical meristems - elongates roots/shoots
    • located at tips of stems/roots, behind root cap
    • produces primary growth, primary tissues (xylem/phloem)
    • constantly divides >> adds cells to tips of plant body >> size lengthens
    • root cap cells, epicotyl/hypocotyls protect root/shoot meristems
    • primary meristems - protoderm (epidermis), procambium (vascular tissues), ground meristem (ground tissue)
  • lateral meristems - peripheral cylinders of meristematic tissue, increases girth
    • produces secondary growth (sometimes not found in herbaceous, fleshy plants)
    • secondary growth >> treelike plants
    • 2 lateral meristems in woody plants - cork cambium (produces cork in outer bark), vascular cambium (produces secondary vascular tissue, between xylem/phloem)
    • secondary tissues - secondary xylem (main wood component), secondary phloem (near outer surface)

plant body organization - 4 types

  • root system - anchors the plant, penetrates soil to absorb water/nutrients
  • shoot system - stems for positioning leaves (site for photosynthesis)
    • produces flowers, fruits, seeds
    • axillary buds - apical meristems that replaces the main shoot when it gets eaten
  • dermal tissue - epidermis covered by fatty cutin layer in young plants, bark in plants w/ secondary growth
  • ground tissue - consists of mainly parenchyma cells (storage, photosynthesis, secretion)
  • vascular tissue - xylem (transfers water/minerals), phloem (transfers carbohydrates, nutrients)

dermal tissue - epidermal cells, from the protoderm

  • guard cells - cell pairs around a stoma (epidermal opening)
    • contain chloroplasts
    • stomata mostly on lower epidermis >> minimizes water loss
    • forms due to asymmetrical cell division
  • trichomes - hairlike growths from epidermis
    • keeps leaf surfaces cool, reduces evaporation
    • can secrete toxic substances to deter herbivores
  • root hairs - tubular extensions of epidermal cells
    • increases root surface area >> higher absorption efficiency

ground tissue - from ground meristem

  • parenchyma cells - large vacuoles, thin walls
    • most common type of plant cell
    • have only primary walls
    • used to store food/water
    • can remain alive for over 100 years even after fully maturing
    • chlorenchyma - parenchyma cells w/ chloroplasts
    • aerenchyma - loose parenchyma cells, stores O2
  • collenchyma cells - provides mechanical support for plant organs
    • lets plant bend w/o breaking
    • forms continuous cylinders beneath leaf petioles (stalks)
  • sclerenchyma cells - have lignin in secondary cell walls
    • in leaf veins/stems, seed coverings
    • strengthens tissues
    • fibers - long/slender cells grouped in strands
    • sclereids - branched shape

vascular tissue - xylem (inside) / phloem (outside)

  • water mov’t - gravity/atmospheric pressure moves water down
    • capillary action can’t move water more than 1 meter
    • tensile strength - tendency for water molecules to stick together
    • water evaporates at the top >> pulls adjacent molecules up
    • osmotic potential in roots + atmospheric pressure + negative pressure in stomata >> transpiration
  • xylem - main water-conducting plant tissue
    • combination of vessels elements (formed from dead/hollow cylindrical cells) and tracheids (overlapping dead cells)
    • vessels conduct water better than tracheids (favored by natural selection)
    • transpiration - diffusion of water vapor from plant
    • primary xylem - from procambium
    • secondary xylem - from vascular cambium
  • phloem - found near outer part of roots/stems
    • main food-conducting plant tissue (moves food slower than xylem moves water)
    • girdled (removing strips of bark) >> takes away phloem >> plant dies from starvation
    • sieve cells - found in seedless vascular plants, gymnosperms
    • sieve-tube members - found in angiosperms
    • sieve areas - clusters of pores, connects protoplasts in adjoining cells
    • sieve plates - larger sieve areas in sieve-tube members
    • sieve tubes - series of sieve-tube members connected end to end; no nucleus, mitochondria, chloroplasts
    • companion cell - adjacent specialized parenchyma cell associated w/ each sieve-tube member; supplies the sieve tubes w/ nutrients
    • plasmodesmata - cytoplasmic connections between adjacent cells
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