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Chapter 24 Notes

by: Min-Young Kim

Chapter 24 Notes BIOL 3040

Marketplace > Clemson University > Biology > BIOL 3040 > Chapter 24 Notes
Min-Young Kim
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About this Document

These notes cover Chapter 24 in our textbook for Dr. Wells' class.
Biology of Plants
Christina Wells
Class Notes
biology of plants
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This 4 page Class Notes was uploaded by Min-Young Kim on Thursday March 10, 2016. The Class Notes belongs to BIOL 3040 at Clemson University taught by Christina Wells in Spring 2016. Since its upload, it has received 16 views. For similar materials see Biology of Plants in Biology at Clemson University.


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Date Created: 03/10/16
Chapter  24  Notes  –  The  Root:  Structure  and  Development     -­‐ First  structure  to  emerge  from  germinating  seed  is  root   • Anchorage  and  absorption   • Storage  and  conduction   • Foods  manufactured  aboveground  in  the  photosynthesizing  portions   of  the  plant  move  down  through  the  phloem  to  the  storage  tissues  of   the  root   • In  biennial  plants,  large  food  reserves  accumulate  in  the  storage   regions  of  the  root  during  the  first  year.  Reserves  are  used  during  the   second  year  to  produce  flowers,  fruits,  seeds   • Water  and  minerals  move  through  xylem  to  aerial  parts   • Hormones  synthesized  in  meristematic  regions  of  roots  transported   upward  in  xylem  to  aerial  parts  (stimulate  growth  and  development)   • Clonal  regeneration:  roots  of  certain  eudicots  produce  buds  that  can   develop  into  new  shoots   -­‐ Root  Systems   o Primary  root:  first  root  of  plant  that  originates  in  embryo   o Taproot:  primary  root  in  all  seed  plants  except  monocots;  grows   directly  downward,  giving  rise  to  lateral  roots   o Taproot  system:  root  system  formed  from  strongly  developed   primary  root  and  its  branches   o In  monocots:  main  root  system  develops  from  adventitious  roots   (arise  from  stem)  and  lateral  roots  give  rise  to  fibrous  root  system   o Developmental  plasticity  of  root  system  is  its  response  to  uneven   distribution  of  nitrogen  and  inorganic  phosphate   o Taproot  systems  generally  penetrate  deeper  than  fibrous  root   systems   o The  bulk  of  most  fine  roots,  or  feeder  roots,  actively  engaged  in  the   uptake  of  water  and  minerals   o Many  fine  roots  infected  with  mycorrhizal  fungi   -­‐ In  growing  plant,  balance  maintained  between  total  surface  area  available  for   manufacture  of  food  and  surface  area  available  for  absorption  of  water  and   minerals   -­‐ Growth  of  many  roots  is  continuous  process  that  stops  only  under  such   adverse  conditions  as  drought  and  low  temperatures   -­‐ Root  cap:  thimble-­‐like  mass  of  living  parenchyma  cells  that  protects  apical   meristem  behind  it  and  aids  root  in  its  penetration  of  the  soil   o As  root  grows  longer,  root  cap  pushed  forward,  large  amounts  of   mucilage  (highly  hydrated  polysaccharide)  lubricates  root  during   passage  through  soil   o Border  cells:  root  cap  cells  that  are  programmed  to  separate  from  the   root  cap  and  from  each  other  as  they  reach  the  root  cap  periphery.  As   border  cells  are  released,  new  cells  are  added  to  the  root  cap   § Functions:  protection  of  apical  meristem  from  infection,   maintenance  of  intimate  root  soil  contact,  mobilization  of   essential  elements  for  uptake  by  roots,  short-­‐term  protection   from  drying  out  (desiccation),  and  specific   attraction/repulsion  of  bacteria;  decrease  frictional  resistance   for  root   o Rootcap  senses,  processes,  and  transmits  signals  to  meristem  and   elongation  region  of  root,  controls  direction   § Central  column  of  cells,  columella,  lateral  rootcap   -­‐ Root  apical  meristem  –  region  of  actively  dividing  cells   o Promeristem  –  least  differentiated  part  of  apical  meristem;   composed  of  initials  and  immediate  derivatives   o Two  main  types  of  apical  organization:   § “Closed  type”:  rootcap,  vascular  cylinder,  cortex  have  own   initials   § “Open  type”:  all  regions  arise  from  one  group  of  initials   o Most  cell  divisions  occur  a  short  distance  beyond  quiescent  initials   § Quiescent  center:  relatively  inactive  region  of  apical   meristem;  does  not  include  initials  of  rootcap   o Region  of  cell  division  =  apical  meristem   o Region  of  elongation  –  increase  in  length  of  root,  only  near  root  tip   o Region  of  maturation  –  differentiation;  primary  tissues  mature;  root   hairs  produced     o Processes  overlap   o Protophloem  sieve  tubes  –  first-­‐formed  primary  phloem  elements;   reach  maturity  nearer  root  tip  than  protoxylem  elements   -­‐ In  most  roots,  vascular  tissues  form  a  solid  cylinder,  but  many  roots  have   pith  or  pithlike  region  in  center   -­‐ The  epidermis  in  young  roots  absorbs  water  and  minerals   o Root  hairs  uptake  water  and  minerals;  tubular  extensions  of   epidermal  cells,  increase  surface  and  absorptive  area;  generally  short   lived     -­‐ Cortex  occupies  greatest  area  of  primary  body     o Cortical  tissue  contains  numerous  intercellular  spaces  –  air  spaces   essential  for  aeration  of  root  cells   o Aerenchyma  –  term  applied  to  parenchyma  tissue  with  large  and   abundant  intercellular  spaces   -­‐ Endodermis  characterized  by  presence  of  Casparian  strips  (wall   thickening;  bandlike  portion  of  primary  wall  and  middle  lamella  impregnated   with  suberin  and  sometimes  lignin)  in  anticlinal  walls   o Suberin  and  lignin  infiltrate  spaces  in  wall  usually  occupied  by  water   o Apoplastic  movement  of  water  and  solutes  across  endodermis  is   blocked  by  strips   -­‐ In  older  roots  where  cortex  is  retained,  suberin  lamella  is  eventually   deposited  internally  over  all  wall  surfaces  of  endodermis.  This  is  followed  by   deposition  of  cellulose,  which  may  become  lignified   -­‐ Passage  cells:  endodermal  cells  remaining  thin-­‐walled  and  retaining   Casparian  strips   -­‐ Roots  of  angiosperms  have  second  compact  layer  of  cells  with  Casparian   strips  called  exodermis.  Followed  by  deposition  of  suberin  lamella  and   cellulosic  layer;  reduces  water  loss  and  provides  defense  against  attack  by   microorganisms   -­‐ Vascular  cylinder  of  root  consists  of  primary  vascular  tissues  and  one  or   more  layers  of  nonvascular  cells  that  constitute  the  pericycle,  completely   surrounds  vascular  tissues   o Pericycle  originates  from  procambium;  young  root  composed  of   parenchyma  cells  with  primary  walls,  but  may  develop  secondary   walls   o Lateral  roots  arise  in  pericycle   o Center  of  vascular  cylinder  occupied  by  solid  core  of  primary  xylem   from  which  projections  extend  toward  pericycle   o Number  of  ridges  of  primary  xylem  differs  from  species  to  species     § Proto-­‐primary  xylem  elements  located  next  to  pericyle,  and   tips  called  protoxylem  poles.     § Metaxylem  is  part  of  primary  xylem  that  differentiates  after   protoxylem  and  occupies  inner  portions  of  ridges  and  center  of   vascular  cylinder   -­‐ Roots  of  some  angiosperms  have  pith  or  pithlike  region   -­‐ Secondary  growth  in  roots  and  stems  consists  of:   o Secondary  vascular  tissues  from  a  vascular  cambium   o Periderm  (mostly  cork  tissue)  from  a  cork  cambium   -­‐ Formation  of  vascular  cambium  is  initiated  by  divisions  of  procambial  cells   that  remain  meristematic  between  primary  xylem  and  primary  phloem  in   portions  of  root  that  are  no  longer  elongating.     o Pericycle  cells  divide;  inner  sister  cells  contribute  to  vascular   cambium   o Vascular  cambium  opposite  phloem  strands  produce  secondary  xylem   toward  inside;  primary  phloem  separated  from  primary  xylem   o Repeated  divisions  toward  inside  and  outside;  increases  in  width  of   secondary  xylem  and  phloem,  primary  phloem  almost  obliterated   o Periderm  replaces  epidermis;  cork  cambium  arises  in  outer   pericycle,  producing  cork  toward  outer  surface  and  phelloderm   toward  inner  surface   o Cork,  cork  cambium,  phelloderm  make  up  periderm   o Lenticels  are  regions  of  periderm  that  allow  gas  exchange  between   root  and  soil  atmosphere   -­‐ At  end  of  first  year’s  growth,  these  tissues  are  found  in  woody  root  (outside   to  inside):  possible  remnants  of  epidermis  and  cortex;  periderm;  pericycle;   primary  phloem  (if  present  fibers),  secondary  phloem;  vascular  cambium;   secondary  xylem;  primary  xylem   -­‐ Endogenous:  lateral  roots  originate  deep  within  parent  root   -­‐ Root  primordium:  young  lateral  root;  pushes  through  cortex,  secreting   enzymes   -­‐ Prop  roots:  aerial  roots  of  some  plants  used  for  support,  produced   aboveground;  stem-­‐borne   -­‐ Still  roots:  produced  from  stem  and  branches  of  many  tropical  trees   -­‐ Air  roots,  or  pneumatophores:  negative  gravitropic  extensions  that  grow   out  of  mud  and  provide  adequate  aeration  via  numerous  lenticels  and  broad   cortex  of  aerenchyma  tissue   -­‐ Most  roots  are  storage  organs;  some  are  fleshy  because  of  storage   parenchyma  (found  in  carrot,  sweet  potato,  sugar  beet)  


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