﻿SEED 
  DISTRIBUTION 
  

  

  167 
  

  

  usually 
  remain 
  from 
  year 
  to 
  year 
  without 
  great 
  changes 
  ex- 
  

   cept 
  those 
  which 
  are 
  brought 
  about 
  by 
  human 
  interference. 
  

   This 
  fact 
  is 
  evidence 
  enough 
  that 
  seeds 
  in 
  unimaginable 
  num- 
  

   bers 
  must 
  be 
  scattered 
  in 
  such 
  a 
  way 
  as 
  to 
  make 
  good 
  the 
  

   losses 
  in 
  the 
  plant 
  population 
  of 
  the 
  world 
  due 
  to 
  all 
  destruc- 
  

   tive 
  causes. 
  The 
  means 
  by 
  which 
  this 
  distribution 
  of 
  the 
  

   seeds 
  is 
  secured 
  will 
  be 
  taken 
  up 
  in 
  sections 
  162 
  and 
  163. 
  

  

  FIG. 
  147. 
  Dandelion 
  fruits 
  

  

  a, 
  akene 
  ; 
  b, 
  beak 
  of 
  pappus 
  ; 
  br, 
  bracts 
  ; 
  p, 
  pappus 
  (representing 
  the 
  limb 
  of 
  the 
  

   calyx) 
  ; 
  r, 
  common 
  receptacle 
  for 
  all 
  the 
  fruits. 
  Twice 
  natural 
  size 
  

  

  160. 
  The 
  struggle 
  for 
  existence. 
  Only 
  a 
  small 
  proportion 
  

   of 
  all 
  the 
  seeds 
  annually 
  produced 
  can 
  have 
  a 
  chance 
  to 
  grow. 
  

   The 
  resulting 
  contest 
  among 
  plants 
  for 
  a 
  foothold 
  and 
  for 
  the 
  

   means 
  of 
  subsistence 
  forms 
  one 
  portion 
  of 
  what 
  the 
  great 
  Eng- 
  

   lish 
  naturalist, 
  Charles 
  Darwin, 
  called 
  the 
  struggle 
  for 
  existence. 
  

   It 
  is 
  shown 
  by 
  careful 
  calculation 
  that 
  about 
  5,300,000 
  acres 
  

   of 
  land 
  could 
  be 
  sown 
  with 
  the 
  wheat 
  grown 
  at 
  the 
  end 
  of 
  

   fifteen 
  years 
  from 
  a 
  single 
  parent 
  kernel, 
  if 
  every 
  grain 
  were 
  

   to 
  grow 
  and 
  live. 
  But 
  the 
  wheat 
  plant 
  does 
  not 
  produce 
  a 
  

   very 
  large 
  number 
  of 
  seeds, 
  The 
  so-called 
  Russian 
  thistle 
  

  

  