﻿Double 
  Adaptations 
  457 
  

  

  until 
  direct 
  experimental 
  investigations 
  have 
  

   been 
  made. 
  

  

  So 
  it 
  is 
  for 
  instance 
  with 
  the 
  submerged 
  leaves 
  

   of 
  water-plants. 
  As 
  a 
  rule 
  they 
  are 
  linear, 
  or 
  

   if 
  compound, 
  are 
  reduced 
  to 
  densely 
  branching 
  

   filiform 
  threads. 
  Hence 
  we 
  may 
  conclude 
  that 
  

   this 
  structure 
  is 
  of 
  some 
  use 
  to 
  them. 
  Now 
  

   two 
  European 
  and 
  some 
  corresponding 
  Amer- 
  

   ican 
  species 
  of 
  water-parsnip, 
  the 
  Slum 
  lati- 
  

   folium 
  and 
  Bemla 
  angustifolia 
  with 
  their 
  al- 
  

   lies, 
  are 
  umbellifers, 
  which 
  bear 
  pinnate 
  instead 
  

   of 
  bi- 
  or 
  tri-pinnate 
  leaves. 
  But 
  the 
  young 
  

   plants 
  and 
  even 
  the 
  young 
  shoots 
  when 
  devel- 
  

   oping 
  from 
  the 
  rootstocks 
  under 
  water 
  comply 
  

   with 
  the 
  above 
  rule, 
  producing 
  very 
  compound, 
  

   finely 
  and 
  pectinately 
  dissected 
  leaves. 
  From 
  a 
  

   systematic 
  point 
  of 
  view 
  these 
  leaves 
  indicate 
  

   the 
  origin 
  of 
  the 
  water-parsnips 
  from 
  ordinary 
  

   umbellifers, 
  which 
  generally 
  have 
  bi- 
  and 
  tri- 
  

   pinnate 
  leaves. 
  

  

  Similar 
  cases 
  of 
  double 
  adaptation, 
  depend- 
  

   ent 
  on 
  external 
  conditions 
  at 
  different 
  periods 
  

   of 
  the 
  evolution 
  of 
  the 
  plant 
  are 
  very 
  numerous. 
  

   They 
  are 
  most 
  marked 
  among 
  leguminous 
  

   plants, 
  as 
  shown 
  by 
  the 
  trifoliolate 
  leaves 
  of 
  the 
  

   thorn-broom 
  and 
  allies, 
  which 
  in 
  the 
  adult 
  state 
  

   have 
  green 
  twigs 
  destitute 
  of 
  leaves. 
  

  

  As 
  an 
  additional 
  instance 
  of 
  dimorphism 
  and 
  

   probable 
  double 
  adaptation 
  to 
  unrecognized 
  ex-* 
  

  

  