﻿CHAPTER 
  III 
  

  

  V 
  

  

  ROOTS 
  AND 
  THEIR 
  RELATION 
  TO 
  THE 
  WORK 
  OF 
  PLANTS 
  

  

  16. 
  Structure 
  of 
  roots. 
  We 
  have 
  already 
  discussed 
  the 
  gen- 
  

   eral 
  nature 
  of 
  roots. 
  Careful 
  examination 
  of 
  a 
  cross 
  section 
  

   of 
  a 
  young 
  root 
  shows 
  that 
  there 
  is 
  a 
  definitely 
  organized 
  

  

  .y/////'/.r. 
  around 
  which 
  is 
  the 
  crt<'.i\ 
  both 
  well 
  shown 
  

   in 
  figure 
  15. 
  The 
  surface 
  layer 
  of 
  tin- 
  

   cortex 
  is 
  the 
  epil>'rinix 
  (fig. 
  16, 
  e~). 
  The 
  

   growing 
  root 
  tip 
  is 
  covered 
  with 
  several 
  

   layers 
  of 
  dead 
  or 
  dying 
  cells, 
  which 
  make 
  

   up 
  the 
  rootc'i/i. 
  As 
  the 
  root 
  pushes 
  its 
  way 
  

   1 
  1 
  1 
  rough 
  the 
  soil 
  the 
  rootcap 
  is 
  constantly 
  

   being 
  worn 
  away 
  on 
  the 
  outside. 
  During 
  

   the 
  seasons 
  of 
  activity 
  the 
  rootcap 
  is 
  con- 
  

   tinually 
  being 
  rebuilt 
  by 
  the 
  formation 
  of 
  

   new 
  layers 
  of 
  cells 
  on 
  its 
  inner 
  surface. 
  

   In 
  older 
  roots 
  the 
  epidermis 
  has 
  given 
  

   place 
  to 
  barh\ 
  which 
  is 
  composed 
  of 
  sev- 
  

   eral 
  layers 
  of 
  cells. 
  If 
  a 
  cross 
  section 
  of 
  

   a 
  very 
  young 
  root 
  of 
  a 
  dicotyledonous 
  

   plant 
  is 
  examined 
  with 
  a 
  moderate 
  mag- 
  

   nification, 
  it 
  will 
  be 
  seen 
  that 
  the 
  cortex 
  

   is 
  made 
  up 
  of 
  rather 
  loosely 
  aggregated 
  

   cells, 
  while 
  the 
  central 
  cylinder 
  is 
  more 
  

   closely 
  compacted. 
  In 
  the 
  cylinder 
  are 
  

  

  1( 
  ind 
  a 
  definite 
  number 
  of 
  radially 
  arranged 
  tibrovascular 
  

   bundles. 
  Tin- 
  woody 
  strands 
  of 
  these 
  bundles 
  alternate 
  with 
  

   strands 
  of 
  \\hat 
  are 
  kiio\\n 
  as 
  />,t*f 
  ///-,/*. 
  shown 
  in 
  figure 
  44. 
  

  

  17. 
  The 
  root-hair 
  cell. 
  The 
  general 
  tubular 
  structure 
  of 
  

   root 
  hairs 
  was 
  very 
  brietly 
  rxplained 
  in 
  section 
  8. 
  A 
  root 
  hair, 
  

  

  20 
  

  

  Kn.. 
  !">. 
  i 
  Irosa 
  -''! 
  i"i 
  i 
  

  

  cit 
  i-i-nl 
  nil 
  portion 
  of 
  

  

  root 
  of 
  sweet 
  

  

  ( 
  . 
  I 
  '-H/-//.S) 
  

  

  ', 
  cnrt.A 
  : 
  0, 
  mitrnnost 
  

   la\ 
  'T 
  of 
  cells 
  of 
  cent 
  nil 
  

  

  c\ 
  lin.icr; 
  o, 
  vessels 
  ..I" 
  

   \ 
  Hi-ions 
  sizes; 
  

   ccll.s. 
  M.-iu-nilic 
  

  

  AI't.T 
  lc 
  I'.ary 
  

  

  