lecture 9

• Plant organs


  • Roots
  • Stem
  • Leaf
• Building a sporophyte


  1. Shoot system

  2. Root system
• Apical bud


  Growing tip of stem
• Node


  Site where structures extend from stem
• Internode


  Span of stem between nodes
• Axillary bud


  Buds that grow from axils (intersections where leaf meets stem)
• Leaf


  Composed of a blade and a petiole (stem-like structure supporting the blade)
• Primary tissues in plant organs


  • Dermal tissue
  • Vascular tissue
  • Ground tissue
• Dermal tissue


  Covers and protects the plant
• Vascular tissue


  Transports water, minerals, and sugars throughout the plant
• Ground tissue


  Anything not dermal or vascular, can serve as site of photosynthesis, provide supporting matrix for vascular tissue, and help store water and sugars
• Parenchyma


  • Alive at maturity
  • Simple cell walls
  • Can perform diverse functions like photosynthesis, starch storage, water movement, may divide and differentiate into other cell types

  
• Collenchyma


  • Alive at maturity
  • Similar to parenchyma but with thick cell walls, particularly at the corners
  • Often parallel the stem surface
  • Function to provide structural support

  
• Sclerenchyma


  • Name derived from scleros = "hard"
  • Have lignin embedded in very thick cell walls
  • Function in support, especially in protection
  • May be alive or dead (mostly dead)

  
• Sclerenchyma


  • Sclereids (small with thick walls, often found in fruit walls like nut shells or in seed coats)
  • Fibers (elongated and found in bunches, function in structural support)

  
• Epidermis


  • Layer of tightly packed cells that forms the outermost tissue of all plant organs
  • Must protect the plant from all environmental insults
  • Leaf and stem epidermal cells coated in a waxy cuticle that helps prevent desiccation
• Trichomes


  • Hairlike outgrowths on stems, leaves, or flowers that come in diverse forms
  • Can be glandular (able to manufacture and secrete chemicals) or non-glandular
  • Function in defense, thermoregulation, reflection, reducing evaporation, reaction triggers
• Trichomes


  • Helianthus annuus (common sunflower)
  • Helianthus argophyllus (silverleaf sunflower endemic to S. Texas)
  • Drosera (sundew)
• Root hairs


  Increase surface area for absorption of water and minerals
• Xylem


  Vascular plant tissue that conducts water and minerals upward from the roots to the rest of the plant
• Xylem cell types


  • Tracheids (relatively narrow, dead when functional with totally open interiors and lignified cell walls, no openings at end, water moves laterally by pits)
  • Vessel elements (also open, dead cells with lignified cell walls, but much wider, open end to end allowing formation of long vessels with free water movement)
• Vessel element differentiation


  1. Elongation

  2. Lignin deposition

  3. Programmed cell death

  4. Enzymatic digestion of end walls
• Phloem


  Vascular plant tissue consisting of living cells that transports sugars and other organic nutrients to the rest of the plant
• Phloem cell types


  • Sieve tube elements
  • Companion cells
• Xylem


  Vascular plant tissue
• Xylem cell types


  • Tracheids
  • Vessel elements
• Phloem cell types


  • Sieve tube elements
  • Companion cells
• Sieve tube elements


  • Connected end to end allowing formation of long sieve tubes
  • Ends have sieve plates with many pores
  • No nucleus and barely any cytoplasmic contents
• Companion cells


  Non-conducting cells connected to sieve-tube elements via plasmodesmata to pass sugars, nutrients, and large molecules necessary to keep sieve-tube elements alive
• Phloem cell differentiation


  1. Unequal cell division

  2. Larger cell loses nucleus and organelles over time

  3. Sieve plate pores also develop
• If companion cell dies, so does its associated sieve-tube element
• Root


  • Organ with multiple tissues/cell types that function together
  • Functions: anchorage, support, (sometimes) storage organs, symbiosis with soil microbes, absorption of water and minerals
• Taproot


  Main vertical root in most eudicots that gives rise to secondary branches called lateral roots
• Fibrous roots


  Shallow network of roots that establish in most monocots, with new roots coming off the stem and no one branch predominating
• Root diversity


  • Climbing and clasping plants
  • Epiphytes - plant that grows on another plant
  • Brace roots (or prop roots) that may support top-heavy plants like maize
• Root anatomy


  • Root cap
  • Zone of cell division
  • Zone of elongation
  • Zone of differentiation
  • Epidermis
  • Cortex
  • Endodermis
  • Vascular cylinder
  • Pericycle
  • Xylem
  • Phloem
• Lateral roots arise from meristematically active regions in the pericycle layer
• Key terms


  • node / internode
  • blade / petiole / axil
  • dermal / vascular / ground tissue
  • parenchyma
  • collenchyma
  • sclerenchyma (sclereids and fibers)
  • epidermis
  • pavement cells
  • guard cells
  • trichomes
  • root hairs
  • xylem
  • tracheids
  • vessel elements / vessels
  • phloem
  • sieve-tube elements / sieve plates
  • companion cells
  • taproot
  • lateral roots
  • fibrous roots
  • root cap
  • zones of cell division / cell elongation / cell differentiation
  • cortex
  • endodermis
  • pericycle