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  INTRODUCTION 
  TO 
  BOTANY 
  

  

  During 
  the 
  growth 
  of 
  the 
  pollen 
  tube 
  the 
  single 
  cell 
  which 
  

   at 
  iirst 
  was 
  in 
  the 
  pollen 
  grain 
  has 
  developed 
  until 
  it 
  now 
  con- 
  

   sists 
  of 
  three 
  cells, 
  which 
  are 
  carried 
  near 
  the 
  tip 
  of 
  the 
  tube. 
  

   Two 
  of 
  these 
  cells 
  are 
  male 
  cells 
  and 
  can 
  serve 
  as 
  sperms, 
  al- 
  

   though 
  they 
  are 
  not 
  ciliated. 
  The 
  other 
  cell 
  is 
  a 
  nutritive 
  cell, 
  

   which 
  goes 
  forward 
  with 
  the 
  tip 
  of 
  the 
  pollen 
  tube 
  and 
  is 
  an 
  

   important 
  factor 
  in 
  its 
  growth. 
  These 
  cells 
  all 
  pass 
  out 
  of 
  the 
  

   tu 
  lie 
  through 
  an 
  opening 
  in 
  its 
  tip 
  when 
  it 
  has 
  reached 
  the 
  

   embryo 
  sac. 
  One 
  of 
  the 
  male 
  cells 
  unites 
  with 
  the 
  egg 
  cell, 
  

   and 
  in 
  many 
  cases, 
  perhaps 
  generally, 
  the 
  other 
  male 
  cell 
  passes 
  

   to 
  the 
  center 
  of 
  the 
  sac 
  and 
  unites 
  with 
  the 
  endosperm 
  cell. 
  

   The 
  first 
  union 
  of 
  cells 
  is 
  like 
  that 
  of 
  the 
  gymnosperms, 
  but 
  

   the 
  second 
  does 
  not 
  occur 
  except 
  in 
  the 
  angiosperms. 
  This 
  

   union 
  of 
  both 
  the 
  male 
  cells 
  with 
  other 
  cells 
  in 
  the 
  same 
  em- 
  

   bryo 
  sac 
  is 
  known 
  as 
  double 
  fertilization. 
  After 
  fertilization 
  

   the 
  egg 
  grows 
  into 
  a 
  new 
  plant, 
  and 
  the 
  endosperm 
  cell 
  grows 
  

   into 
  the 
  endosperm, 
  or 
  food 
  material, 
  that 
  is 
  stored 
  in 
  the 
  seed 
  

   around 
  the 
  embryo 
  plant. 
  Double 
  fertilization 
  gives 
  to 
  the 
  seed 
  

   outside 
  the 
  embryo, 
  as 
  well 
  as 
  to 
  the 
  embryo 
  itself, 
  the 
  charac- 
  

   ters 
  of 
  both 
  parent 
  plants. 
  In 
  this 
  way, 
  when 
  plants 
  such 
  as 
  

   different 
  kinds 
  of 
  corn 
  are 
  crossed, 
  the 
  ear 
  thereby 
  produced 
  

   will 
  have 
  some 
  grains 
  showing 
  the 
  characters 
  of 
  both 
  parents. 
  

  

  287. 
  The 
  new 
  plant. 
  The 
  way 
  in 
  which 
  the 
  fertilized 
  egg 
  

   grows 
  into 
  the 
  new 
  plant 
  is 
  striking. 
  The 
  egg 
  cell 
  first 
  divides 
  

   and 
  redivides 
  in 
  such 
  a 
  way 
  as 
  to 
  form 
  a 
  chain 
  of 
  cells. 
  One 
  

   end 
  of 
  this 
  chain 
  may 
  become 
  elongated 
  and, 
  as 
  the 
  suspensor, 
  

   may 
  attach 
  the 
  developing 
  embryo 
  to 
  the 
  wall 
  of 
  the 
  embryo 
  

   sac. 
  The 
  cells 
  at 
  the 
  other 
  end 
  of 
  the 
  chain 
  are 
  the 
  ones 
  that 
  

   produce 
  the 
  parts 
  of 
  the 
  new 
  plant-- 
  the 
  stem 
  tip, 
  leaves, 
  and 
  

   root 
  tip 
  (fig. 
  232). 
  The 
  embryo 
  in 
  some 
  kinds 
  of 
  angiosperms 
  

   has 
  but 
  one 
  leaf, 
  which 
  grows 
  at 
  the 
  end, 
  and 
  the 
  young 
  stem 
  

   is 
  at 
  the 
  side 
  of 
  the 
  embryo. 
  These 
  are 
  the 
  angiosperms 
  with 
  

   one 
  seed 
  leaf, 
  or 
  the 
  monocotyledons. 
  In 
  other 
  cases 
  the 
  embryo 
  

   may 
  have 
  two 
  (or 
  even 
  more) 
  seed 
  leaves, 
  and 
  the 
  stem 
  is 
  at 
  

   the 
  tip 
  and 
  the 
  leaves 
  are 
  on 
  the 
  sides 
  of 
  the 
  embryo. 
  These 
  

   are 
  the 
  dicotyledons, 
  or 
  plants 
  with 
  two 
  seed 
  leaves. 
  

  

  