Plant Organs: Roots

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Chapter 6

• Plant Organs Roots

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American Ginseng

• Parax quinquefolius in the eastern woodlands of
  North America
• Small herbaceous plant not very remarkable but
  almost harvested to extinction in 1700s
• Underground part is extraordinary thick root
  that resembles a man
• also similar to species in China and Korea
• Root used to make a tonics for number of
  disorders
• cancer, rheumatism, diabetes, impotence,
  sterility and aging even though no scientific
  evidence of benefits

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Root Structure and Function

• Plant roots are usually more extensive that the
  aerial system above ground
• corn can put out 2.5 m of root and spread out 1.2
  m from the stem
• desert plants may go as deep as 50 m
• rye plants can have total root length of 500 km
  or 310 miles
• Dependent on plant and soil conditions
• Grow down and in direction of gravity

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2 Types of Root Systems

• Taproot system and fibrous root system that both
  develop from embryonic root radicle from seed
• Both adapted to obtain water in a variety of ways

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Taproot System

• 1 main root with many smaller lateral or branch
  roots coming out usually in regular rows along
  the length of the main root
• eudicots (oak, cherry, beans and daisy) and
  gymnosperms (seeds/no flowers)
• most trees start with taproots when young but
  move on to shallow lateral roots that have
  vertical branches moving downward
• Get water from deep into underground sources

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Fibrous System

• Has several to many roots of same size, embryonic
  root is short lived
• Originate from base of embryonic tissue and later
  from stem
• adventitious because mot coming from root but
  stem tissue
• in unusual locations roots from stems, buds on
  roots
• onions, crabgrass and other monocots
• Grow shallowly to collect rain water at the
  surface

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Roots Have Many Functions

• Anchorage roots anchor plant to have a solid
  foundation in soil so can grow, important for
  survival by keeping stem upright and leaves
  oriented for maximum sunlight capture
• Absorption and Conduction absorbs water and
  dissolved minerals such as nitrates, phosphates,
  sulfates from soil use to make organic
  molecules for growth transported thru plant
  (conduction)
• Storage surplus carbohydrates made in leaves
  are moved by phloem to roots where converted into
  starch or complex sugar until needed, use it for
  roots energy needs but most is moved to where
  plant needs it

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Storage

• Taproot of beets, carrots, radishes and turnips
  are modified for storage
• Fibrous roots of sweet potatoes are modified for
  storage as well
• Usually they are biennials
• Other roots in very dry regions are modified for
  water storage
• Some plants have roots that are specialized to
  perform unusual functions

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Root Tips and Cap

• Root cap covers tip to protect it and the apical
  meristem
• Cap looses parenchyma cells as growing root
  pushes thru soil replaced by new cells from
  apical meristem
• Secretes lubrication (polysaccharides) that
  reduce some of the friction
• Cap may help orient root so that it grows
  downward (gravitropism)
• remove the cap, apical meristem makes a new one,
  but until it is back in place no response to
  gravity

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Root Hairs

• Short lived, unicellular extensions near growing
  tip
• Form continually from in the area of cell
  maturation closest to the root tip, those dying
  are closer to the more mature end of root
• Short, lt1 cm but numerous to increase the
  absorptive capacity of root and increase surface
  area

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Root Hairs and Soil

• Soil particles are coated with water and
  dissolved minerals
• Hairs meet soil and absorb much of water and
  minerals from the soil

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Missing Parts

• Roots lack nodes and internodes and usually have
  no leaves or buds except in special circumstances
• Tissues fond in the stem are present in the roots
  but are usually arranged slightly differently in
  roots than stems
• Roots all have protective coating (epidermis),
  cortex for storage of starch and other molecules
  and vascular tissue for conduction

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Herbaceous Eudicot Roots - Epidermis

• Central core is mostly vascular tissue
• Ranunculus or buttercup root is the
  representative organism with primary growth
• Contains a single layer of epidermis with the
  root hairs being modifications of the epidermis
  to increase water uptake
• doesnt secrete waxy cuticle like on stems and
  leaves
• most water moves into root and between cells as
  it is the path of least resistance
• cellulose of cell wall act as sponge to absorb
  water

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Herbaceous Eudicot Roots - Cortex

• Composed of loosely arranged parenchyma cell with
  large intercellular space major part of root
• Lacks collenchyma cells for support but as ages
  may developing supporting sclerenchyma cells
• Primary function is storage parenchyma cells
  contain numerous starch grains provides energy
  during winter and injury
• Intercellular air spaces provide pathway for
  water uptake and aeration of root need oxygen
  from spaces in soil into cortex spaces for use in
  cellular respiration

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Herbaceous Eudicot Roots - Endodermis

• Inner layer is endodermis controls amount and
  kinds of water and dissolved minerals that enter
  the xylem in the center of root
• Differs from cortex cells are tightly packed
  and each cell has a band-like region called the
  Casparian strip
• on radial (side) and transverse (upper and lower)
  walls
• contains suberin - fatty material that is
  waterproof

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Water and Mineral Movement

• Move from epidermis to cortex by 2 pathways until
  reach endodermis
• symplast continuum of cell cytoplasm to cell
  cytoplasm using plasmodesmata
• apoplast interconnected porous cell walls of
  plant water and inorganic minerals move freely
  doesnt ever enter a living cell
• Most water moves thru apoplast until reaches
  endodermis but the Casparian strip will prevent
  passive movement of water must enter cytoplasm
  by osmosis, minerals move by carrier proteins in
  the plasma membrane regulates what enters into
  endodermis

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Center of Root

• Cylinder of vascular tissues stele
• Outer most layer of cells is the pericycle just
  inside the endodermis parenchyma cells that
  remain meristematic to make lateral roots

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Lateral Roots

• Lateral roots push thru several layers of root
  tissue before entering the soil
• Has all parts of root and features of original
  root
• Pericycle is involved in forming lateral
  meristems that produce secondary growth in woody
  roots

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Xylem and Phloem

• Xylem is centermost tissue in the stele between
  2, 3, 4 or more arms stretching outward
• tracheids and vessel elements to move water and
  minerals
• Phloem located in patched between xylem arms
• sieve tube elements for moving sucrose

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Fluid Movement

• Root hair ? epidermis ? cortex (symplast/apoplast
  pathway) ? endodermis ? pericycle ? xylem of root
  
• Water now moves upward to rest of plant
• Flow in phloem is from leaves with sugar to roots
  for storage (starch) and then if needed elsewhere
  sugar is moved from root to area needed

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Vascular Cambium

• Gives us secondary tissue in woody plants is
  between xylem and phloem
• Primary eudicot lack pith ground tissue in
  centers of many stems and roots

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Monocot Roots

• Central tissue is the pith in monocots
• Root structure is more varied in internal
  structure than eudicot
• Greenbriar (Smilax) is representative monocot
  root
• Epidermis, cortex, endodermis and pericycle

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Xylem and Phloem

• Not a solid cylinder of vascular tissue but
  alternating phloem and xylem in strands around
  the central pith of parenchyma cells
• No vascular cambium in monocots
• long lived monocots like palms have thickened
  roots produced by modified form of primary growth
  
• parenchyma cells in cortex divide and enlarge
  cortex expansion

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Roots of Woody Plants

• Woody plants produce secondary growth in the root
  as well as the stem
• Growth is usually a good distance from the root
  tip in 2 lateral meristems
• vascular cambium
• cork cambium
• both make wood and bark
• in temperate climates see annual rings in stems
  and roots

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Roots with Unusual Functions

• Adventitious roots come from tissues not to be
  root tissue
• Many areal roots are adapted for functions other
  than normal root function

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Prop Roots

• Develop from branches or from vertical stem and
  grow into the soil to help support plant in
  upright position
• More common in monocots such as corn and sorghum
• Tropical and subtropical eudicot trees such as
  red mangrove and banyan have prop roots

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Buttress Roots

• Tropical rainforest trees have shallow roots
  concentrated near the surface in a mat
• Catch and absorb almost all inorganic minerals
  from leaves made by decomposers
• Have buttress roots swollen bases or braces to
  help hold tree upright

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Pneumatophores

• Occur in swampy and tidal environments where soil
  is flooded or water logged
• Some roots grow upward until above high tide
• allows for oxygen to be absorbed and gas exchange
• Also serve as anchor
• black and white mangrove

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Areal Roots

• Climbing plants and epiphytes (plants that grow
  on other plants) use areal roots to attach them
  to plant
• Some epiphytes have roots that function for
  something besides anchoring some orchids have
  photosynthetic roots and may absorb moisture
• Strangler fig drop down long roots from high up
  in tree growing on
• the support tree usually dies and fig becomes
  self-supporting

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Parasitic Plants

• Mistletoe is a parasite
• roots penetrate the host and absorbs water and
  minerals from host xylem but not sugar
• Other parasitic plants will also take sugar

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Corms and Bulbs

• These are underground stems or buds specialized
  for asexual reproduction
• Have wiry contractile roots contract to pull
  corm or bulb deeper into soil
• necessary as new growth is on the top of old
  growth and would end up on the top of the ground
• More common in monocots

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Suckers

• Some roots reproduce asexually by making suckers
  that are above ground stems that develop from
  adventitious buds on roots
• grow own roots and becomes independent plant
• black locust, pear, apple, cherry, red raspberry
  and blackberries
• Also seen in some weeds that are hard to get rid
  of

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Root Association with Other Species

• Roots of trees grow outward encounter other
  trees or other species
• may get a natural graft grow together like in
  birch and maple
• share water, minerals, sugars and other things
  like hormones
• Roots of most plant species form mutually
  beneficial relationships
• certain soil fungi mycorrhizae permit
  transfer from roots to fungus and fungus may
  provide phosphorous to plant

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Mycorrhizae

• Threadlike body of fungus partner extends thru
  soil extracting minerals well beyond the plants
  roots
• 2 types of structures
• ectomycorrhizae mycelium encircles root like a
  sheath
• endomycorrhizae mycelium penetrates the root
• Fungus/plant relationship is mutually beneficial
  without 1 the other doesnt grow as well

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Nitrogen-Fixing Bacteria and Plants

• Rhizobia form association with leguminous
  plants such as clover, peas and soybeans
• Swellings cause nodes to develop on roots and
  house millions of rhizobia
• bacteria gets products of photosynthesis from
  plant and bacteria produce NH3 from atmospheric
  N2 need to make important biomolecules like
  amino acids, nitrogenous bases