﻿136 
  

  

  INTRODUCTION 
  TO 
  BOTANV 
  

  

  supposed 
  that 
  the 
  pollen 
  tube 
  is 
  nourished 
  by 
  the 
  cells 
  that 
  

   are 
  broken 
  down 
  in 
  the 
  path 
  of 
  the 
  tube. 
  The 
  time 
  required 
  

   for 
  the 
  pollen 
  tube 
  to 
  reach 
  the 
  ovule 
  varies 
  in 
  different 
  plants, 
  

  

  ranging 
  from 
  a 
  few 
  hours 
  to 
  

   more 
  than 
  a 
  year. 
  Usually 
  

   the 
  tube 
  finds 
  its 
  way 
  into 
  

   the 
  ovule 
  through 
  a 
  minute 
  

   opening 
  known 
  as 
  the 
  > 
  i<! 
  <><>- 
  

   />//!<>, 
  (fig. 
  119, 
  w), 
  but 
  in 
  

   some 
  plants 
  it 
  grows 
  directly 
  

   through 
  the 
  substance 
  of 
  the 
  

   ovule. 
  

  

  127. 
  Fertilization. 
  ( 
  )n 
  en- 
  

   tering 
  the 
  ovule, 
  one 
  of 
  the 
  

   male 
  cells 
  unites 
  with 
  the 
  <//// 
  

   itinji'ii* 
  of 
  the 
  embryo 
  sac, 
  

   which 
  is 
  within 
  the 
  ovule, 
  and 
  

   the 
  other 
  male 
  cell 
  in 
  some 
  

   cases, 
  or 
  perhaps 
  usually, 
  

   unites 
  with 
  the 
  central 
  nu- 
  

   cleus 
  of 
  the 
  embryo 
  sac 
  to 
  

   form 
  the 
  endosperm 
  'nucleus 
  

   (fig. 
  119, 
  en). 
  The 
  nature 
  

   and 
  function 
  of 
  the 
  latter 
  

   union 
  are 
  not 
  as 
  yet 
  per- 
  

   fectly 
  understood. 
  The 
  fusion 
  

   of 
  egg 
  nucleus 
  and 
  male 
  cell 
  

   is, 
  however, 
  a 
  very 
  common 
  

   and 
  most 
  important 
  phenom- 
  

   enon, 
  occurring 
  in 
  many 
  of 
  

   the 
  simpler 
  plants 
  as 
  well 
  as 
  

   in 
  the 
  higher 
  ones. 
  In 
  general 
  it 
  may 
  be 
  said 
  th&t 
  fertilization 
  

   consists 
  in 
  the 
  union 
  of 
  tin- 
  nn>-li-u# 
  of 
  a 
  male 
  cell 
  with 
  the 
  nucleus 
  

   of 
  an 
  egg 
  cell 
  Other 
  illustrations 
  of 
  this 
  will 
  be 
  given 
  in 
  

   Chapters 
  XV-XVIII. 
  After 
  fertilization 
  the 
  egg 
  rapidly 
  

   grows 
  into 
  the 
  embryo 
  of 
  a 
  new 
  plant. 
  The 
  ovule 
  also 
  grows 
  

  

  FIG. 
  119. 
  Diagram 
  to 
  illustrate 
  course 
  

   of 
  the 
  pollen 
  tube 
  during 
  fertilization 
  

  

  p, 
  pollen 
  grains 
  ; 
  t, 
  pollen 
  tube 
  ; 
  n, 
  nucel- 
  

   lus, 
  or 
  body 
  of 
  tbe 
  ovule 
  ; 
  a, 
  <n/ti/><"/n/ 
  

   i-i'llft 
  of 
  emlu-yo 
  sac; 
  en, 
  endosperm 
  nu- 
  

   cleus 
  of 
  embryo 
  sac; 
  egg, 
  the 
  egg 
  ap- 
  

   paratus, 
  consisting 
  of 
  the 
  egg 
  cell 
  and 
  

  

  t\v< 
  operating 
  cells 
  ; 
  /i, 
  the 
  //ii<Ti>i>ii/i', 
  

  

  or 
  small 
  opening, 
  through 
  which, 
  in 
  most 
  

  

  ordinary 
  (lowering 
  plants, 
  the 
  pollen 
  

  

  tube 
  makes 
  its 
  way 
  to 
  the 
  e^g 
  at 
  the 
  tip 
  

  

  of 
  the 
  embryo 
  sac 
  

  

  