﻿Double 
  Adaptations 
  433 
  

  

  choice 
  is 
  final, 
  and 
  a 
  further 
  change 
  does 
  not 
  oc- 
  

   cur 
  in 
  the 
  normal 
  course 
  of 
  things. 
  

  

  The 
  most 
  curious 
  and 
  most 
  suggestive 
  in- 
  

   stance 
  of 
  such 
  an 
  alternation 
  is 
  the 
  case 
  of 
  the 
  

   water-persicaria 
  or 
  Polygonum 
  ampliibium. 
  It 
  

   is 
  known 
  to 
  occur 
  in 
  two 
  forms, 
  one 
  aquatic 
  and 
  

   the 
  other 
  terrestrial. 
  These 
  are 
  recorded 
  in 
  

   systematic 
  works 
  as 
  varieties, 
  and 
  are 
  described 
  

   under 
  the 
  names 
  of 
  P. 
  ampliibium 
  var. 
  natans 
  

   Moench, 
  and 
  P. 
  ampliibium 
  var. 
  terrestre 
  Leers 
  

   or 
  P. 
  ampliibium 
  var. 
  terrestris 
  Moench. 
  Such 
  

   authorities 
  as 
  Koch 
  in 
  his 
  German 
  flora, 
  and 
  

   Grenier 
  and 
  Godron 
  in 
  their 
  French 
  flora 
  agree 
  

   in 
  the 
  conception 
  of 
  the 
  two 
  forms 
  as 
  varieties. 
  

  

  Notwithstanding 
  this, 
  the 
  two 
  varieties 
  may 
  

   often 
  be 
  observed 
  to 
  sport 
  into 
  one 
  another. 
  

   They 
  are 
  only 
  branches 
  of 
  the 
  same 
  plant, 
  

   grown 
  under 
  different 
  conditions. 
  The 
  aquatic 
  

   form 
  has 
  floating 
  or 
  submerged 
  stems 
  with 
  ob- 
  

   long 
  or 
  elliptic 
  leaves, 
  which 
  are 
  glabrous 
  and 
  

   have 
  long 
  petioles. 
  The 
  terrestrial 
  plants 
  are 
  

   erect, 
  nearly 
  simple, 
  more 
  or 
  less 
  hispid 
  

   throughout, 
  with 
  lanceolate 
  leaves 
  and 
  short 
  pe- 
  

   tioles, 
  often 
  nearly 
  sessile. 
  The 
  aquatic 
  form 
  

   flowers 
  regularly, 
  producing 
  its 
  peduncle 
  at 
  

   right 
  angles 
  from 
  the 
  floating 
  stems, 
  but 
  the 
  

   terrestrial 
  specimens 
  are 
  ordinarily 
  seen 
  with- 
  

   out 
  flower-spikes, 
  which 
  are 
  but 
  rarely 
  met 
  with, 
  

   at 
  least 
  as 
  far 
  as 
  my 
  own 
  experience 
  goes. 
  In- 
  

  

  