﻿THE 
  SEED 
  PLANTS 
  

  

  301 
  

  

  forces 
  its 
  way 
  through 
  the 
  central 
  softer 
  tissues 
  of 
  the 
  style. 
  It 
  

   does 
  not 
  make 
  a 
  passageway 
  by 
  forcing 
  the 
  tissue 
  aside, 
  but 
  

   by 
  means 
  of 
  its 
  own 
  secretions 
  (enzymes) 
  it 
  breaks 
  down 
  these 
  

   tissues, 
  and 
  they 
  doubtless 
  furnish 
  nourishment 
  to 
  the 
  grow- 
  

   ing 
  pollen 
  tube 
  (fig. 
  230). 
  When 
  the 
  tube 
  reaches 
  the 
  cavity 
  

  

  P 
  

  

  0,1 
  

  

  FIG. 
  230. 
  Germinating 
  

   pollen 
  grains 
  

  

  The 
  pollen 
  grains 
  (g) 
  have 
  been 
  

   deposited 
  upon 
  the 
  stigma. 
  The 
  

   roughened 
  surface 
  of 
  the 
  stigma 
  

   is 
  made 
  by 
  cell 
  extensions, 
  or 
  

   papillae 
  (p). 
  Pollen 
  tubes 
  (?) 
  grow 
  

   from 
  the 
  grains 
  through 
  the 
  tis- 
  

   sue 
  or 
  along 
  the 
  central 
  canal 
  (c) 
  

   until 
  they 
  reach 
  the 
  ovule. 
  Only 
  

   a 
  small 
  part 
  of 
  the 
  stigma 
  and 
  

   style 
  are 
  shown 
  in 
  this 
  cut 
  

  

  FIG. 
  231. 
  

   angiospermous 
  

  

  Diagram 
  

  

  of 
  the 
  ovule 
  of 
  an 
  

   plant, 
  showing 
  the 
  parts 
  

   of 
  the 
  ovule 
  

  

  The 
  outer 
  integument 
  (01) 
  ; 
  the 
  inner 
  integu- 
  

   ment 
  (i); 
  the 
  micropyle, 
  which 
  is 
  the 
  opening 
  

   between 
  the 
  parts 
  of 
  the 
  inner 
  integument 
  ; 
  

   the 
  pollen 
  tube 
  (pt) 
  , 
  which 
  has 
  grown 
  through 
  

   the 
  micropyle 
  

  

  which 
  contains 
  the 
  ovules, 
  it 
  turns 
  across 
  to 
  the 
  micropyle 
  of 
  

   the 
  ovule. 
  It 
  then 
  grows 
  through 
  the 
  tissue 
  at 
  the 
  tip 
  of 
  the 
  

   ovule 
  to 
  the 
  end 
  of 
  the 
  embryo 
  sac 
  (fig. 
  231). 
  In 
  some 
  cases 
  

   (elm 
  and 
  walnut) 
  the 
  pollen 
  tube 
  grows 
  down 
  to 
  the 
  base' 
  of 
  

   the 
  ovule, 
  then 
  up 
  through 
  it, 
  and 
  finally 
  reaches 
  the 
  egg. 
  

  

  