﻿POLLINATION 
  AND 
  FERTILIZATION 
  

  

  135 
  

  

  extremely 
  slender, 
  sticky 
  threads. 
  The 
  mature 
  grain 
  contains 
  

   two 
  nuclei, 
  a 
  generative 
  nucleus 
  and 
  a 
  tube 
  nucleus. 
  Contact 
  of 
  

   the 
  pollen 
  grain 
  with 
  the 
  moist 
  stigmatic 
  

   surface 
  brings 
  about 
  osmosis, 
  which 
  results 
  

   in 
  distention 
  of 
  the 
  pollen 
  grain. 
  At 
  some 
  

   point 
  the 
  outer 
  coat 
  becomes 
  ruptured 
  and 
  

   the 
  inner 
  thin 
  coat 
  protrudes 
  through 
  the 
  

   opening 
  and 
  is 
  the 
  beginning 
  of 
  the 
  pollen 
  

   tube. 
  Then 
  a 
  portion 
  of 
  the 
  contents 
  of 
  

   the 
  grain 
  passes 
  out 
  

   into 
  the 
  pollen 
  tube, 
  

   which 
  is 
  developed 
  

   from 
  the 
  thin 
  inner 
  

   coat 
  of 
  the 
  pollen 
  

   grain 
  (fig. 
  118, 
  A}. 
  

   The 
  pollen 
  tube 
  con- 
  

   tains 
  a 
  tube 
  nucleus 
  

   () 
  near 
  its 
  tip 
  and 
  

   a 
  generative 
  cell 
  (cf) 
  

   somewhere 
  farther 
  

   back. 
  At 
  length 
  the 
  gen- 
  

   erative 
  cell 
  divides 
  into 
  

   two 
  male 
  nuclei, 
  these 
  

   develop 
  into 
  male 
  cells 
  

   (fig. 
  118, 
  B}, 
  and 
  the 
  tube 
  

   nucleus 
  sooner 
  or 
  later 
  

   disappears. 
  

  

  126. 
  Course 
  of 
  the 
  pol- 
  

   len 
  tube. 
  The 
  pollen 
  tube 
  

   readily 
  makes 
  its 
  way 
  

   between 
  the 
  exterior 
  cells 
  

   of 
  the 
  stigma 
  and 
  passes 
  

   onward 
  to 
  the 
  ovary. 
  

   Sometimes 
  it 
  traverses 
  a 
  

  

  tubular 
  passage 
  and 
  sometimes 
  it 
  penetrates 
  the 
  tissues, 
  aided 
  

   by 
  the 
  corroding 
  action 
  of 
  ferments 
  which 
  it 
  secretes. 
  It 
  is 
  

  

  t 
  

  

  FIG. 
  118. 
  Germination 
  of 
  the 
  pollen 
  grain 
  

   of 
  a 
  dicotyledon 
  

  

  A, 
  an 
  early 
  stage 
  in 
  the 
  germination 
  ; 
  B, 
  later 
  

   stage, 
  with 
  the 
  tube 
  rather 
  fully 
  developed 
  ; 
  

   #, 
  generative 
  cell; 
  t, 
  tube 
  nucleus 
  ; 
  s^Sj.male 
  

   cells 
  formed 
  from 
  the 
  generative 
  cell. 
  It 
  is 
  ap- 
  

   parent 
  that 
  when 
  the 
  growth 
  of 
  the 
  tube 
  is 
  far 
  

   advanced, 
  the 
  tube 
  nucleus 
  (t) 
  almost 
  disappears. 
  

   Much 
  magnified. 
  After 
  Bonnier 
  and 
  Sablon 
  

  

  