﻿644 
  Mutations 
  

  

  the 
  leaves, 
  which 
  has 
  been 
  replaced 
  in 
  the 
  teasel- 
  

   family 
  by 
  a 
  grouping 
  in 
  pairs. 
  In 
  order 
  to 
  

   prove 
  the 
  validity 
  of 
  this 
  assertion, 
  it 
  will 
  be 
  

   necessary 
  to 
  discuss 
  two 
  points 
  separately, 
  viz. 
  : 
  

   relative 
  positions 
  of 
  the 
  leaves, 
  and 
  the 
  manner 
  

   in 
  which 
  the 
  alternate 
  position 
  causes 
  the 
  stems 
  

   to 
  become 
  twisted. 
  

  

  Leaves 
  are 
  affixed 
  to 
  their 
  stems 
  and 
  branches 
  

   in 
  various 
  ways. 
  Among 
  them 
  one 
  is 
  of 
  

   wide 
  occurrence 
  throughout 
  the 
  whole 
  realm 
  

   of 
  the 
  higher 
  plants, 
  while 
  all 
  the 
  others 
  are 
  

   more 
  rare. 
  Moreover 
  these 
  subordinate 
  ar- 
  

   rangements 
  are, 
  as 
  a 
  rule, 
  confined 
  to 
  definite 
  

   systematic 
  groups. 
  Such 
  groups 
  may 
  be 
  large, 
  

   as 
  for 
  instance, 
  the 
  monocotyledons, 
  that 
  have 
  

   their 
  leaves 
  arranged 
  in 
  two 
  opposite 
  rows 
  in 
  

   many 
  families, 
  or 
  small, 
  as 
  genera 
  or 
  subdivi- 
  

   sions 
  of 
  genera. 
  Apart 
  from 
  these 
  special 
  

   cases 
  the 
  main 
  stem 
  and 
  the 
  greater 
  part 
  of 
  the 
  

   branches 
  of 
  the 
  pedigree 
  of 
  the 
  higher 
  plants 
  

   exhibit 
  a 
  spiral 
  condition 
  or 
  a 
  screw 
  arrange- 
  

   ment, 
  all 
  leaves 
  being 
  inserted 
  at 
  different 
  

   points 
  and 
  on 
  different 
  sides 
  of 
  the 
  stem. 
  This 
  

   condition 
  is 
  assumed 
  to 
  be 
  the 
  original 
  one, 
  

   from 
  which 
  the 
  more 
  specialized 
  types 
  have 
  

   been 
  derived. 
  As 
  is 
  usual 
  with 
  characters 
  in 
  

   general, 
  it 
  is 
  seen 
  to 
  vary 
  around 
  an 
  average, 
  

   the 
  spiral 
  becoming 
  narrower 
  and 
  looser. 
  A 
  

   narrow 
  spiral 
  condenses 
  the 
  leaves, 
  while 
  a 
  

  

  