﻿Retrograde 
  Varieties 
  131 
  

  

  omilla}, 
  the 
  yarrow 
  (Achillea 
  Millefolium), 
  

   the 
  daisies, 
  the 
  Dahlia 
  and 
  many 
  others. 
  

   Species 
  occur 
  in 
  this 
  group 
  of 
  plants 
  from 
  time 
  

   to 
  time 
  that 
  lack 
  the 
  ray-florets, 
  as 
  in 
  the 
  tansy 
  

   (Tanacetum 
  vulgar 
  e] 
  and 
  some 
  artemisias. 
  

   And 
  the 
  genus 
  of 
  the 
  marigolds 
  or 
  Bidens 
  is 
  

   noted 
  for 
  containing 
  both 
  of 
  these 
  types. 
  The 
  

   smaller 
  and 
  the 
  three-toothed 
  marigold 
  (B. 
  

   cernua 
  and 
  B. 
  tripartita) 
  are 
  very 
  common 
  

   plants 
  of 
  wet 
  soil 
  and 
  swamps, 
  ordinarily 
  lack- 
  

   ing 
  the 
  ray-florets, 
  and 
  in 
  some 
  countries 
  they 
  

   are 
  very 
  abundant 
  and 
  wholly 
  constant 
  in 
  this 
  

   respect, 
  never 
  forming 
  radiate 
  flower-heads. 
  

   On 
  the 
  other 
  hand 
  the 
  white-flowered 
  and 
  the 
  

   purple 
  marigold 
  (B. 
  leucantha 
  and 
  B. 
  atropur- 
  

   purea) 
  are 
  cultivated 
  species 
  of 
  our 
  gardens, 
  

   prized 
  for 
  their 
  showy 
  flower-heads 
  with 
  large 
  

   white 
  or 
  deeply 
  colored, 
  nearly 
  black-purple 
  

   florets. 
  

  

  Here 
  we 
  have 
  opportunity 
  to 
  observe 
  positive 
  

   and 
  negative 
  varieties 
  of 
  the 
  same 
  character. 
  

   The 
  smaller, 
  and 
  the 
  three-toothed 
  marigold 
  

   occur 
  from 
  time 
  to 
  time, 
  provided 
  with 
  ray- 
  

   florets, 
  showing 
  a 
  positive 
  variation. 
  And 
  the 
  

   white 
  marigold 
  has 
  produced 
  in 
  our 
  gardens 
  a 
  

   variety 
  without 
  rays. 
  Such 
  varieties 
  are 
  quite 
  

   constant, 
  never 
  returning 
  to 
  the 
  old 
  species. 
  

  

  Positive 
  and 
  negative 
  varieties 
  of 
  this 
  kind 
  

   are 
  by 
  no 
  means 
  rare 
  among 
  the 
  compositae. 
  

  

  