﻿POLLINATION 
  AND 
  FERTILIZATION 
  

  

  141 
  

  

  self-fertilization, 
  for 
  several 
  generations 
  at 
  any 
  rate, 
  produces 
  

   better 
  results 
  than 
  cross-fertilization. 
  1 
  

  

  Whenever 
  cross-pollination 
  by 
  the 
  wind 
  or 
  by 
  the 
  agency 
  

   of 
  animals 
  is 
  impossible, 
  it 
  is 
  evident 
  that 
  self-pollination 
  

   would 
  be 
  advantageous, 
  since 
  it 
  is 
  infinitely 
  better 
  than 
  no 
  

   pollination 
  at 
  all. 
  Many 
  highly 
  successful 
  weeds 
  owe 
  their 
  

   predominance 
  partly 
  to 
  the 
  

   fact 
  that 
  they 
  produce 
  good 
  

   seed 
  after 
  self-pollination. 
  

  

  Since 
  cross-fertilization 
  at 
  

   intervals 
  appears 
  to 
  be 
  suffi- 
  

   cient 
  to 
  keep 
  up 
  the 
  strength 
  

   and 
  fertility 
  of 
  many 
  kinds 
  

   of 
  plants, 
  there 
  might 
  be 
  some 
  

   advantage 
  in 
  uniting 
  the 
  cer- 
  

   tainty 
  which 
  characterizes 
  self- 
  

   pollination 
  with 
  the 
  renewal 
  

   of 
  strength 
  which 
  comes 
  from 
  

   cross-pollination. 
  Violets 
  and 
  

   many 
  other 
  less 
  familiar 
  plants 
  

  

  unite 
  the 
  two 
  methods 
  by 
  pro- 
  

   , 
  . 
  ,. 
  FIG. 
  124. 
  A 
  violet 
  with 
  cleistoo;amous 
  

  

  during 
  ordinary 
  showy 
  flowers 
  flowerg 
  ag 
  seen 
  in 
  late 
  July 
  or 
  early 
  

  

  and 
  also 
  inconspicuous 
  closed, 
  August, 
  after 
  the 
  conspicuous 
  flowers 
  

   or 
  deistogamous, 
  flowers. 
  In 
  

   violets 
  the 
  latter 
  are 
  borne 
  

   on 
  flower 
  stalks 
  close 
  to 
  the 
  

   ground 
  (fig. 
  124) 
  and 
  usually, 
  

  

  before 
  maturing, 
  become 
  partially 
  buried 
  in 
  the 
  earth. 
  Pol- 
  

   lination 
  occurs 
  within 
  the 
  closed 
  flower, 
  the 
  pollen 
  tubes 
  

   developing 
  within 
  the 
  anthers 
  and 
  making 
  their 
  way 
  to 
  the 
  

   stigma. 
  The 
  deistogamous 
  flowers 
  produce 
  many 
  more 
  seeds 
  

   than 
  the 
  showy 
  ones, 
  but 
  the 
  latter 
  insure 
  at 
  least 
  occasional 
  

   cross-pollination 
  since 
  they 
  are 
  freely 
  visited 
  by 
  bees 
  and 
  

   other 
  flying 
  insects. 
  

  

  1 
  See 
  "Tobacco 
  Breeding," 
  Bulletin 
  96, 
  Bureau 
  of 
  Plant 
  Industry, 
  U.S. 
  

   Dept. 
  Agr., 
  1907. 
  

  

  have 
  disappeared 
  

  

  cl, 
  deistogamous 
  flowers; 
  caps, 
  cap- 
  

   sules 
  produced 
  by 
  earlier 
  flowers 
  of 
  

   the 
  same 
  sort 
  

  

  