﻿POLLINATION 
  AND 
  FERTILIZATION 
  

  

  153 
  

  

  of 
  the 
  most 
  effectual 
  means 
  of 
  preventing 
  self-pollination 
  in 
  

   bisexual 
  flowers 
  is 
  the 
  maturing 
  of 
  the 
  stamens 
  at 
  a 
  different 
  

   time 
  from 
  the 
  pistils 
  ; 
  this 
  is 
  known 
  as 
  dichogamy. 
  In 
  some 
  

  

  flowers, 
  as 
  in 
  the 
  figwort 
  and 
  some 
  

   plantains 
  (fig. 
  136), 
  the 
  pistils 
  ma- 
  

   ture 
  first. 
  In 
  such 
  cases 
  the 
  pollen 
  

   from 
  older 
  flowers 
  (in 
  the 
  stami- 
  

   nate 
  condition) 
  is 
  transferred 
  to 
  the 
  

   stigmas 
  of 
  recently 
  opened 
  flowers 
  

   (in 
  the 
  pistillate 
  condition). 
  

  

  Usually, 
  as 
  in 
  some 
  mallows 
  

   and 
  in 
  the 
  gentians 
  and 
  fireweeds 
  

   (EpiloTrium) 
  (fig. 
  137), 
  the 
  stamens 
  

   mature 
  first. 
  An 
  insect 
  visitor 
  to 
  

   a 
  newly 
  opened 
  flower 
  in 
  the 
  stami- 
  

   nate 
  condition 
  becomes 
  somewhat 
  

   covered 
  with 
  pollen. 
  Then, 
  flying 
  to 
  an 
  older 
  flower 
  in 
  the 
  

   pistillate 
  condition, 
  the 
  insect 
  is 
  almost 
  sure 
  to 
  leave 
  pollen 
  

   on 
  the 
  stigmas 
  and 
  thus 
  insure 
  cross-pollination. 
  

  

  It 
  is 
  common 
  to 
  find 
  the 
  stamens 
  of 
  a 
  flower 
  maturing 
  a 
  

   few 
  at 
  a 
  time, 
  as 
  in 
  ct 
  nasturtium," 
  buckwheat, 
  and 
  many 
  other 
  

   flowers. 
  This 
  gives 
  

   more 
  opportunities 
  

   for 
  insects 
  to 
  carry 
  

   away 
  the 
  pollen 
  

   than 
  would 
  be 
  pos- 
  

   sible 
  if 
  it 
  all 
  ma- 
  

   tured 
  at 
  once. 
  

  

  145. 
  Prevention 
  

  

  FIG. 
  138. 
  Lengthwise 
  section 
  of 
  dimorphous 
  

  

  flower 
  of 
  bluets 
  

  

  Fir;. 
  137. 
  Dichogamous 
  flowers 
  

   of 
  fireweed 
  (Epilobium) 
  

  

  At 
  the 
  left, 
  earlier 
  stage 
  with 
  

   stamens 
  mature 
  hut 
  the 
  stigmas 
  

   uiiited 
  into 
  a 
  club-shaped 
  mass. 
  

   At 
  the 
  right, 
  later 
  stage 
  with 
  

   stamens 
  withering 
  but 
  the 
  stig- 
  

   mas 
  spread 
  apart 
  and 
  ready 
  for 
  

   pollination 
  

  

  A, 
  long-styled 
  form 
  ; 
  B, 
  short-styled 
  form 
  ; 
  a, 
  anthers 
  ; 
  

   s, 
  stigmas. 
  About 
  3 
  times 
  natural 
  size 
  

  

  of 
  self-pollination 
  ; 
  

   dimorphism. 
  A 
  

  

  means 
  of 
  prevent- 
  

   ing 
  self-pollination 
  

  

  which 
  is 
  even 
  more 
  effective 
  than 
  dichogamy 
  is 
  found 
  in 
  the 
  

   structure 
  of 
  flowers 
  in 
  which 
  some 
  have 
  a 
  long 
  pistil 
  and 
  short 
  

   stamens, 
  others 
  a 
  short 
  pistil 
  and 
  long 
  stamens. 
  This 
  condition 
  

  

  