Chapter 35

• Plant organization


  Plants have organs composed of different tissues, which in turn are composed of different cell types
• Cell


  The fundamental unit of life
• Tissue


  A group of cells consisting of one or more cell types that together perform a specialized function
• Organ


  Consists of several types of tissues that together carry out particular functions
• Basic vascular plant organs

  • Roots, stems, and leaves
• Root system

  Roots that anchor the plant and absorb water and minerals
• Shoot system


  Stems and leaves that take up CO2 and light
• Roots


  • Anchor the plant
  • Absorb minerals and water
  • Store carbohydrates
• Primary root


  The first root to emerge
• Lateral roots


  Branches of the primary root that provide anchorage and water absorption
• Taproot system


  Tall plants with large shoot masses generally have this, with the primary root developing into a large taproot
• Fibrous root system


  Small or trailing plants generally have this, with the primary root dying early and not forming a taproot
• Root hairs

  Finger-like extensions of epidermal cells near the root tip that increase the absorptive surface
• Most root systems form mycorrhizal associations
• Stems

  • Consist of an alternating system of nodes (points of leaf attachment) and internodes (stem segments between nodes)
  • The growing shoot tip (apical bud) causes elongation of a young shoot
  • Axillary buds have the potential to form lateral branches, thorns, or flowers
• Primary function of stems

  To elongate and orient the shoot to maximize photosynthesis
• Modified stems


  • Rhizomes, stolons, tubers
• Leaves


  • The main photosynthetic organ, consisting of a flattened blade and a petiole stalk
  • Monocots have parallel leaf veins, eudicots have branching leaf veins
• Modified leaves


  • Serve various functions
• Dermal tissue system

  In non-woody plants, consists of the epidermis covered in a waxy cuticle<|>In woody plants, protective periderm replaces the epidermis in older regions
• Ground tissue system


  Tissue that is neither dermal nor vascular, including pith internally and cortex externally, with cells specialized for storage, photosynthesis, support, and transport
• Vascular tissue system


  Transports materials through the plant and provides mechanical support, consisting of xylem and phloem
• Xylem


  Conducts water and dissolved minerals upward from roots into the shoots
• Phloem


  Transports sugars from where they are made (primarily leaves) to storage or growth sites
• Stele


  The vascular tissue of a root or stem
• Vascular cylinder

  In angiosperms, the stele of the root is a solid central vascular cylinder divided into vascular bundles
• Water-conducting cells of the xylem

  • Tracheids and vessel elements, which are dead and lignified at maturity
  • Water moves between tracheids through pits
  • Vessel elements have perforation plates that allow water to flow freely
• Sugar-conducting cells of the phloem

  • In seedless vascular plants and gymnosperms, sugars are transported through sieve cells
  • In angiosperms, sugars are transported in sieve tubes, chains of sieve-tube elements connected to companion cells by plasmodesmata
• Primary growth


  Growth in length, produced by apical meristems at the tips of roots and shoots
• Secondary growth


  Growth in thickness, produced by lateral meristems like the vascular cambium and cork cambium
• Indeterminate growth


  A plant can grow throughout its life due to meristems
• Determinate growth


  Most animals and some plant organs cease to grow at a certain size
• Primary growth of roots

  • The root tip is covered by a root cap, with growth occurring in three zones: cell division, elongation, and differentiation/maturation
  • In eudicots, the xylem is star-shaped with phloem between the "arms"
  • In monocots, a core of ground tissue is surrounded by alternating rings of xylem and phloem
• Primary growth of shoots

  • The shoot apical meristem is a dome-shaped mass of dividing cells at the shoot tip, protected by leaves of the apical bud
  • Axillary buds develop from meristematic cells at the bases of leaf primordia
• Stem growth and anatomy

  • In eudicots, the vascular tissue consists of vascular bundles arranged in a ring
  • In monocot stems, the vascular bundles are scattered throughout the ground tissue
• Leaf growth and anatomy

  • Leaves develop from leaf primordia along the sides of the shoot apical meristem
  • The epidermis has stomata that allow gas exchange
  • The mesophyll ground tissue is between the upper and lower epidermis, with palisade and spongy layers in eudicots
  • Veins are the leaf's vascular bundles that function as the leaf's skeleton and are protected by bundle sheaths
• Secondary growth


  Growth in thickness produced by lateral meristems like the vascular cambium and cork cambium<|>Occurs in gymnosperms and many eudicots, but is rare in monocots<|>Occurs in stems and roots of woody plants, but rarely in leaves
• Vascular cambium and secondary vascular tissue

  • The vascular cambium is a ring of meristematic cells that adds secondary xylem (wood) to the inside and secondary phloem to the outside
  • Early wood has thin cell walls for water transport, late wood has thick walls for support
  • Tree rings indicate annual growth, with thick rings for warm/wet years and thin rings for cold/dry years
  • Heartwood no longer transports water/minerals, while sapwood still does
• Primary growth and secondary growth occur simultaneously