﻿300 
  INTRODUCTION 
  TO 
  BOTANY 
  

  

  occurred, 
  two 
  cells, 
  and 
  not 
  one, 
  are 
  to 
  be 
  seen 
  within 
  the 
  wall 
  

   of 
  the 
  pollen 
  grain. 
  The 
  pollen 
  grains 
  must 
  be 
  placed 
  upon 
  

   the 
  tip 
  of 
  the 
  stigma 
  before 
  further 
  development 
  takes 
  place. 
  

   The 
  process 
  of 
  proper 
  placing 
  of 
  the 
  pollen 
  grain 
  is 
  known 
  

   as 
  pollination, 
  to 
  which 
  a 
  chapter 
  has 
  already 
  been 
  given 
  

   (Chapter 
  X). 
  

  

  285. 
  The 
  pistil. 
  The 
  pistil, 
  or 
  carpel, 
  consists 
  of 
  three 
  parts 
  : 
  

   the 
  enlarged 
  base, 
  which 
  is 
  called 
  the 
  ovary, 
  in 
  which 
  the 
  

   ovules 
  or 
  developing 
  seeds 
  are 
  borne; 
  the 
  elongated 
  portion 
  

   above 
  the 
  ovary 
  (the 
  style) 
  ; 
  the 
  tip 
  of 
  the 
  style, 
  usually 
  more 
  

   or 
  less 
  expanded 
  (the 
  $//////"/ 
  ). 
  The 
  stigma, 
  when 
  ripe, 
  is 
  ordi- 
  

   narily 
  covered 
  by 
  a 
  sticky 
  fluid 
  which 
  causes 
  the 
  pollen 
  to 
  

   adhere 
  to 
  it. 
  Through 
  the 
  style 
  the 
  pollen 
  tube 
  grows 
  to 
  the 
  

   ovules. 
  The 
  ovules 
  may 
  be 
  borne 
  singly 
  or 
  many 
  together 
  in 
  

   the 
  ovary. 
  They 
  may 
  be 
  attached 
  on 
  the 
  bottom, 
  at 
  the 
  sides, 
  

   at 
  the 
  top, 
  or 
  on 
  a 
  central 
  axis 
  of 
  the 
  ovary. 
  While 
  the 
  ovules 
  

   differ 
  so 
  much 
  in 
  their 
  position, 
  there 
  is 
  a 
  general 
  uniformity 
  

   in 
  their 
  individual 
  structure. 
  The 
  surface 
  of 
  the 
  ovule 
  consists 
  

   of 
  one 
  or 
  two 
  i/i/t't/ttiiirnts, 
  which 
  at 
  the 
  tip 
  do 
  not 
  quite 
  cover 
  

   the 
  inner 
  tissue 
  of 
  the 
  ovule. 
  This 
  open 
  tip 
  is 
  the 
  micropyle^ 
  

   which 
  means 
  "little 
  gate." 
  Similar 
  structures 
  of 
  the 
  ovules 
  of 
  

   gymnosperms 
  have 
  already 
  been 
  mentioned 
  (sect. 
  274). 
  AVithin 
  

   the 
  tissue 
  of 
  the 
  ovule 
  is 
  the 
  cmlryo 
  sac 
  (fig. 
  231). 
  When 
  fully 
  

   formed 
  the 
  embryo 
  sac 
  incloses 
  seven 
  cells. 
  At 
  the 
  micropy- 
  

   lar 
  end 
  of 
  the 
  sac 
  are 
  three 
  cells, 
  the 
  central 
  one 
  of 
  which 
  is 
  

   the 
  egg, 
  while 
  those 
  at 
  the 
  sides 
  of 
  the 
  egg 
  are 
  the 
  helper 
  cells, 
  

   or 
  sytii'ri/ti/x. 
  These 
  helper 
  cells 
  may 
  nourish 
  the 
  egg 
  or 
  pos- 
  

   siblv 
  may 
  assist 
  the 
  development 
  of 
  the 
  entering 
  pollen 
  tube. 
  

   In 
  the 
  end 
  of 
  the 
  sac 
  opposite 
  the 
  micropyle 
  are 
  the 
  antipodal 
  

   cells, 
  which 
  usually 
  disappear 
  soon 
  after 
  they 
  are 
  formed 
  ; 
  and 
  

   iu 
  the 
  central 
  part 
  of 
  the 
  sac 
  is 
  the 
  endosperm 
  cell, 
  which 
  

   later 
  grows 
  and 
  produces 
  the 
  food 
  or 
  endosperm 
  of 
  the 
  seed. 
  

  

  286. 
  The 
  pollen 
  tube 
  and 
  fertilization. 
  After 
  pollen 
  grains 
  

   have 
  fallen 
  upon 
  the 
  stigma, 
  the 
  outer 
  wall 
  of 
  the 
  pollen 
  grain 
  

   breaks, 
  and 
  from 
  the 
  inner 
  wall 
  the 
  beginning 
  of 
  the 
  pollen 
  

   tube 
  extrudes. 
  The 
  tube 
  tip 
  enters 
  the 
  stigmatic 
  tissue 
  and 
  

  

  