﻿Systematic 
  Atavism 
  651 
  

  

  curve 
  the 
  longitudinal 
  ribs. 
  The 
  straighter 
  

   and 
  steeper 
  the 
  screw 
  becomes, 
  the 
  more 
  the 
  

   ribs 
  will 
  become 
  twisted. 
  That 
  this 
  happens 
  in 
  

   the 
  opposite 
  direction 
  is 
  obvious, 
  without 
  fur- 
  

   ther 
  discussion. 
  The 
  twisting 
  is 
  the 
  inevitable 
  

   consequence 
  of 
  the 
  reversal 
  of 
  the 
  screw. 
  

  

  Two 
  points 
  remain 
  to 
  be 
  dealt 
  with. 
  One 
  is 
  

   the 
  direct 
  proof 
  of 
  the 
  reversal 
  of 
  the 
  screw, 
  the 
  

   other 
  the 
  discussion 
  of 
  its 
  cause. 
  The 
  first 
  may 
  

   be 
  observed 
  by 
  a 
  simple 
  experiment. 
  Of 
  course 
  

   it 
  proceeds 
  only 
  slowly, 
  but 
  all 
  that 
  is 
  necessary 
  

   is 
  to 
  mark 
  the 
  position 
  of 
  one 
  of 
  the 
  younger 
  

   leaves 
  of 
  a 
  growing 
  stem 
  of 
  a 
  twisting 
  indi- 
  

   vidual 
  and 
  to 
  observe 
  the 
  change 
  in 
  its 
  posi- 
  

   tion 
  in 
  a 
  few 
  hours. 
  It 
  will 
  be 
  seen 
  to 
  have 
  

   turned 
  some 
  way 
  around 
  the 
  stem, 
  and 
  finally 
  

   may 
  be 
  seen 
  to 
  make 
  a 
  complete 
  revolution 
  in 
  

   the 
  direction 
  opposite 
  to 
  the 
  screw, 
  and 
  there- 
  

   by 
  demonstrating 
  the 
  fact 
  of 
  its 
  uncurling. 
  

  

  The 
  cause 
  of 
  this 
  phenomenon 
  is 
  to 
  be 
  sought 
  

   in 
  the 
  intimate 
  connection 
  of 
  the 
  basal 
  parts 
  of 
  

   the 
  leaves, 
  which 
  we 
  have 
  detailed 
  above. 
  The 
  

   fibrovascular 
  strands 
  constitute 
  a 
  strong 
  rope, 
  

   which 
  is 
  twisted 
  around 
  the 
  stem 
  along 
  the 
  

   line 
  on 
  which 
  the 
  leaves 
  are 
  inserted. 
  The 
  

   strengthening 
  of 
  the 
  internodes 
  may 
  stretch 
  this 
  

   rope 
  to 
  some 
  extent, 
  but 
  it 
  is 
  too 
  strong 
  to 
  be 
  

   rent 
  asunder. 
  Hence 
  it 
  opposes 
  the 
  normal 
  

   growth, 
  and 
  the 
  only 
  manner 
  in 
  which 
  the 
  inter- 
  

  

  