﻿22 
  INTRODUCTION 
  TO 
  BOTANY 
  

  

  plant 
  consists 
  of 
  hundreds 
  of 
  thousands 
  or 
  millions 
  of 
  cooper- 
  

   ating 
  cells, 
  which 
  together 
  carry 
  on 
  the 
  work 
  of 
  the 
  plant. 
  

  

  Tin- 
  root-hair 
  cell 
  shows 
  its 
  life 
  most 
  clearly 
  by 
  growing 
  and 
  

   making 
  its 
  way 
  around 
  obstacles 
  (fig. 
  6). 
  Other 
  plant 
  cells 
  

   often 
  give 
  much 
  more 
  striking 
  evidence 
  of 
  life 
  in 
  the 
  move- 
  

   ments 
  which 
  arc 
  executed 
  by 
  single 
  cells 
  or 
  by 
  organs 
  built 
  up 
  

   of 
  great 
  numbers 
  of 
  cells. 
  Many 
  such 
  movements 
  will 
  be 
  

   described 
  ill 
  subsequent 
  chapters. 
  In 
  this 
  place 
  it 
  is 
  sufficient 
  

   to 
  call 
  the 
  student's 
  attention 
  to 
  the 
  fact 
  that 
  the 
  protoplast 
  

   is 
  alive 
  in 
  just 
  the 
  same 
  sense 
  that 
  any 
  minute 
  animal 
  is 
  alive. 
  

   Whatever 
  any 
  living 
  organism 
  can 
  do 
  it 
  does 
  by 
  virtue 
  of 
  the 
  

   energy 
  of 
  its 
  protoplasts. 
  

  

  The 
  most 
  remarkable 
  and 
  peculiar 
  of 
  the 
  characteristics 
  of 
  

   protoplasm 
  are 
  due 
  to 
  its 
  possessing 
  irritability. 
  By 
  this 
  is 
  

   meant 
  the 
  power 
  to 
  respond 
  in 
  some 
  definite 
  way 
  to 
  any 
  suit- 
  

   able 
  x//////////x, 
  or 
  exciting 
  cause, 
  acting 
  from 
  within 
  or 
  without 
  

   the 
  plant 
  body. 
  Some 
  of 
  the 
  principal 
  stimuli 
  are 
  gravity, 
  

   heat, 
  light, 
  chemical 
  substances, 
  and 
  contact 
  with 
  solid 
  objects. 
  

   When 
  protoplasts, 
  either 
  singly 
  or 
  combined 
  into 
  some 
  organ 
  

   of 
  the 
  plant, 
  are 
  acted 
  upon 
  by 
  any 
  stimulus 
  to 
  which 
  they 
  

   are 
  sensitive, 
  there 
  is 
  usually 
  no 
  immediate 
  visible 
  response. 
  

   If, 
  for 
  example, 
  a 
  young 
  seedling 
  with 
  a 
  stout 
  taproot 
  is 
  pinned 
  

   hori/ontally 
  to 
  a 
  piece 
  of 
  cork 
  which 
  lines 
  the 
  vertical 
  side 
  

   of 
  a 
  glass 
  jar 
  containing 
  moist 
  air, 
  no 
  change 
  is 
  at 
  first 
  noticed. 
  

   In 
  a 
  few 
  hours, 
  however, 
  the 
  part 
  of 
  the 
  root 
  for 
  a 
  short 
  

   distance 
  back 
  of 
  the 
  tip 
  will 
  be 
  found 
  bending 
  vertically 
  

   downward. 
  This 
  movement 
  is 
  a 
  response 
  to 
  the 
  stimulus 
  of 
  

   gravity, 
  acting 
  upon 
  the 
  very 
  sensitive 
  young 
  root 
  and 
  caus- 
  

   ing 
  unequal 
  growth 
  in 
  the 
  upper 
  and 
  lower 
  sides. 
  It 
  is 
  not 
  a 
  

   mere 
  bending, 
  like 
  that 
  of 
  an 
  unsupported 
  piece 
  of 
  wet 
  string, 
  

   for 
  the 
  moving 
  end-portion 
  of 
  the 
  root 
  will 
  be 
  found 
  to 
  push 
  

   downward 
  with 
  a 
  force 
  of 
  more 
  than 
  ten 
  times 
  its 
  own 
  weight. 
  

  

  19. 
  Turgidity. 
  Hoot, 
  hairs 
  and 
  other 
  cells 
  of 
  plants 
  usually 
  

   take 
  up 
  water 
  until 
  the 
  cell 
  walls 
  are 
  distended 
  with 
  water 
  

   and 
  protoplasm. 
  The 
  outward 
  pressure 
  which 
  distends 
  and 
  

   stretches 
  the 
  walls 
  is 
  called 
  //////<//, 
  and 
  the 
  resulting 
  condition 
  

  

  