Roots

• Functions of Roots


  Main functions:

  • Anchorage
  • Absorption
  • Conduction

  Other functions:

  • Storage
• Characteristics of roots


  • Non-green due to absence of chlorophyll
  • Not divided into nodes and internodes
  • Absence of leaves and buds
  • Positively geotropic (grow towards gravity)
  • Positively hydrotropic (grow towards water)
  • Negatively phototropic (grow away from light)
• Root Systems


  • Taproot system
  • Fibrous root system
• Taproot system


  • A root system consisting of one prominent main root with smaller lateral roots branching from it
  • Provide very strong anchorage (able to go deep)
  • Main roots of dicot plants
• Fibrous root system


  • A root system consisting of several adventitious roots of approximately equal size that arise from the base of the stem
  • Do not branch profusely, are shallow and spread horizontally hence, cannot provide strong anchorage
  • Main root system of monocots
• Root cap


  • A covering of cells over the root tip that protects delicate meristematic tissue directly behind it
  • Contains statocytes (cells) involved in gravity perception
• Root hair


  An extension of an epidermal cell of a root that increases absorptive capacity of the root
• Root cap


  1. Each root tip has one

  2. A protective thimblelike layer

  3. Many cells thick

  4. Covers delicate root apical meristem

  5. May orient root so it grows downward

  6. Contains statocytes which are involved in gravity perception in plants

  7. If the cap is carefully removed the root will grow randomly

  8. Secretes mucilage to ease the movement of the root through soil

  9. Involved in communication with the soil microbiota
• Root hairs


  • Short-lived, unicellular extensions of epidermal cells near the growing root tip
  • Increase surface area of root in contact with moist soil, increasing root's absorptive capacity
• Structures Unique to Roots

  • Epidermis
  • Cortex
  • Pith
  • Xylem
  • Phloem
• Epidermis


  • Protects the root
  • Root hairs help absorb water and dissolved minerals
• Cortex


  • Consists of parenchyma cells
  • Usually stores starch
• Endodermis


  • Innermost layer of the cortex of the root that prevents water and dissolved materials from entering the xylem by passing between cells
  • Casparian strip: A band of waterproof material around the radial and transverse cells of the endodermis
  • Ensures that water and minerals enter the xylem only by passing through the endodermal cells
• Pericycle


  • A layer of cells just inside the endodermis of the root
  • Gives rise to lateral roots
• Xylem


  Conducts water and dissolved minerals
• Phloem


  Conducts dissolved sugar
• Monocot roots often have a pith in the center of the root
• In herbaceous eudicot roots, xylem and phloem form a solid mass in center of root
• Monocot roots lack a vascular cambium and do not have secondary growth
• Pathway of water from the soil through the various root tissues

  Root hair -> epidermis -> cortex (symplast or apoplast pathway) -> endodermis -> pericycle -> xylem of root
• Symplast


  A transport continuum consisting of the cytoplasm of many plant cells, connected from one cell to the next by plasmodesmata
• Apoplast


  A transport continuum consisting of the interconnected, porous plant cell walls, along which water moves freely
• Types of modified roots

  • Propagative roots
  • Pneumatophores
  • Aerial Roots
  • Photosynthetic roots of some orchids
  • Contractile roots some herbaceous dicots and monocots
  • Buttress roots
  • Parasitic roots
  • Symbiotic roots (mycorrhizae or "fungus roots")
  • Legumes (e.g., pea, beans, peanuts) and bacterium form root nodules
• Prop root


  An adventitious root that arises from the stem and provides additional support for the plant
• Contractile root


  A specialized root, often found on bulbs or corms, that contracts and pulls the plant to a desirable depth in the soil
• Pneumatophore


  • A specialized aerial root produced by certain trees living in swampy habitats
  • May facilitate gas exchange between the atmosphere and submerged roots
• Buttress roots


  • Swollen bases or braces that hold trees upright
  • Aid in extensive distribution of shallow roots
  • Found in some tropical rainforest trees
• Suckers


  • Aboveground stems that develop from adventitious buds on the roots
  • Asexual reproduction method of some roots
• Mycorrhiza


  A mutually beneficial association between a fungus and a root that helps the plant absorb essential minerals from the soil
• Nodule


  A small swelling on the root of a leguminous plant in which beneficial nitrogen-fixing bacteria (Rhizobium) live
• Roots which store the products of photosynthesis are important sources of food for human consumption (root crops)
• Some roots are used as flavorings, e.g. root beer flavoring (dried greenbrier roots)
• Root beer is a sweet North American beverage traditionally made using the root bark of the sassafras tree (Sassafras albidum)
• Root Crops


  • Carrots
  • Beets
  • Sugar beets
  • Parsnips
  • Turnips
  • Rutabagas (Swedish turnip)
  • Radishes
  • Sweet potatoes
  • Cassava
• • Water is transported upward through root xylem into stem xylem and rest of plant