﻿THE 
  SEED 
  PLANTS 
  299 
  

  

  The 
  essential 
  structures 
  of 
  a 
  flower 
  are 
  stamens 
  (microsporo- 
  

   pliylls) 
  and 
  pistils, 
  or 
  carpels 
  (megasporophylls*). 
  A 
  carpel, 
  

   strictly 
  speaking, 
  is 
  one 
  megasporophyll, 
  and 
  when 
  several 
  

   carpels 
  are 
  united, 
  the 
  result 
  is 
  known 
  as 
  a 
  compound 
  carpel. 
  

   The 
  name 
  pistil 
  is 
  used 
  without 
  discrimination 
  for 
  either 
  a 
  

   simple 
  or 
  a 
  compound 
  carpel. 
  

  

  The 
  flower 
  and 
  the 
  resulting 
  seed 
  that 
  characterize 
  this 
  

   group 
  of 
  plants 
  has 
  often 
  led 
  people 
  to 
  give 
  to 
  the 
  angio- 
  

   sperms 
  names 
  which 
  suggest 
  these 
  characters. 
  The 
  most 
  com- 
  

   mon 
  of 
  these 
  names 
  are 
  flowering 
  plants 
  and 
  seed 
  plants. 
  The 
  

   name 
  phanerogam, 
  meaning 
  " 
  plants 
  with 
  visible 
  reproduction," 
  

   was 
  applied 
  when 
  botanists 
  knew 
  less 
  of 
  the 
  intricacies 
  of 
  the 
  

   reproduction 
  of 
  angiosperms 
  than 
  is 
  now 
  known. 
  In 
  the 
  same 
  

   way 
  cryptogam, 
  which 
  means 
  tc 
  plants 
  with 
  hidden 
  reproduc- 
  

   tion," 
  was 
  applied 
  collectively 
  to 
  the 
  pteridophytes, 
  bryophytes, 
  

   and 
  thallophytes. 
  These 
  names 
  are 
  still 
  used 
  by 
  many 
  people, 
  

   but 
  it 
  is 
  evident 
  that 
  an 
  interchange 
  of 
  the 
  names 
  would 
  bet- 
  

   ter 
  fit 
  the 
  facts 
  of 
  reproduction 
  in 
  the 
  groups 
  of 
  the 
  plant 
  

   kingdom. 
  

  

  284. 
  Stamens 
  and 
  pollen. 
  In 
  connection 
  with 
  the 
  discussion 
  

   of 
  the 
  gymnosperms 
  the 
  structures 
  of 
  the 
  stamen 
  are 
  fully 
  

   illustrated 
  (fig. 
  225). 
  The 
  parts 
  of 
  the 
  angiosperm 
  stamen 
  - 
  

   the 
  anther 
  and 
  the 
  filament 
  are 
  similar 
  to 
  the 
  same 
  structures 
  

   in 
  the 
  gymnosperms, 
  though 
  of 
  course 
  many 
  variations 
  appear. 
  

   In 
  the 
  young 
  angiosperm 
  anther 
  there 
  are 
  four 
  sporangia, 
  and 
  

   these, 
  when 
  they 
  ripen 
  their 
  spores, 
  unite 
  in 
  pairs, 
  so 
  that 
  

   two 
  pollen 
  sacs 
  are 
  formed 
  from 
  the 
  four 
  sporangia 
  (fig. 
  108). 
  

   The 
  anthers 
  of 
  the 
  angiosperms 
  may 
  open 
  in 
  a 
  variety 
  of 
  ways, 
  

   the 
  method 
  of 
  opening 
  being 
  called 
  the 
  clehiscence. 
  

  

  Since 
  the 
  pollen 
  grains 
  are 
  formed 
  by 
  the 
  division 
  of 
  cells 
  

   in 
  a 
  sporangium, 
  it 
  is 
  evident 
  that 
  they 
  are 
  asexual 
  spores. 
  

   When 
  mature 
  each 
  pollen 
  grain 
  consists 
  of 
  a 
  heavy 
  outer 
  wall, 
  

   an 
  inner 
  wall, 
  cytoplasm, 
  and 
  nucleus 
  (fig. 
  118). 
  Often 
  there 
  

   are 
  starch 
  and 
  oil 
  foods 
  in 
  the 
  pollen 
  grains. 
  The 
  single- 
  

   celled 
  pollen 
  grain 
  sometimes 
  begins 
  to 
  germinate 
  before 
  it 
  

   leaves 
  the 
  anther 
  in 
  which 
  it 
  was 
  formed, 
  and 
  when 
  this 
  has 
  

  

  