﻿Retrograde 
  Varieties 
  143 
  

  

  jection 
  can 
  be 
  made 
  to 
  our 
  using 
  them 
  as 
  a 
  case 
  

   of 
  correlative 
  variability. 
  

  

  / 
  

  

  The 
  essential 
  character 
  of 
  the 
  purple 
  thorn- 
  

   apple 
  lies 
  in 
  the 
  color 
  of 
  the 
  flowers, 
  which 
  are 
  

   of 
  a 
  very 
  beautiful 
  pale 
  blue. 
  But 
  this 
  color 
  

   is 
  not 
  limited 
  to 
  the 
  corolla. 
  It 
  is 
  also 
  to 
  be 
  

   seen 
  in 
  the 
  stems 
  and 
  in 
  the 
  stalks 
  and 
  veins 
  of 
  

   the 
  leaves, 
  which 
  are 
  stained 
  with 
  a 
  deep 
  purple, 
  

   the 
  blue 
  color 
  being 
  added 
  to 
  the 
  original 
  green. 
  

   Even 
  on 
  the 
  surface 
  of 
  the 
  leaves 
  it 
  may 
  spread 
  

   into 
  a 
  purplish 
  hue. 
  On 
  the 
  stems 
  it 
  is 
  to 
  be 
  

   met 
  with 
  everywhere, 
  and 
  even 
  the 
  young 
  seed- 
  

   lings 
  show 
  it. 
  This 
  is 
  of 
  some 
  importance, 
  as 
  

   the 
  young 
  plants 
  when 
  unfolding 
  their 
  cotyle- 
  

   dons 
  and 
  primary 
  leaves, 
  may 
  be 
  distinguished 
  

   by 
  this 
  means 
  from 
  the 
  seedlings 
  of 
  the 
  white- 
  

   flowered 
  species. 
  In 
  crossing 
  experiments 
  it 
  is 
  

   therefore 
  possible 
  to 
  distinguish 
  the 
  whites 
  and 
  

   the 
  blues, 
  even 
  in 
  young 
  seedlings, 
  and 
  ex- 
  

   perience 
  shows 
  that 
  the 
  correlation 
  is 
  quite 
  

   constant. 
  The 
  color 
  can 
  always 
  be 
  relied 
  upon 
  ; 
  

   if 
  lacking 
  in 
  the 
  seedlings, 
  it 
  will 
  be 
  lacking 
  in 
  

   the 
  stems 
  and 
  flowers 
  also; 
  but 
  if 
  the 
  axis 
  of 
  

   the 
  young 
  plant 
  is 
  ever 
  so 
  slightly 
  tinged, 
  the 
  

   color 
  will 
  show 
  itself 
  in 
  its 
  beauty 
  in 
  the 
  later 
  

  

  *' 
  

  

  stages 
  of 
  the 
  life 
  of 
  the 
  plant. 
  

  

  This 
  is 
  what 
  we 
  term 
  correlation. 
  The 
  colors 
  

   of 
  the 
  different 
  organs 
  are 
  always 
  in 
  agreement. 
  

   It 
  is 
  true 
  that 
  they 
  require 
  the 
  concurrence 
  of 
  

  

  