﻿Double 
  Adaptations 
  455 
  

  

  thelier 
  with 
  Berberis, 
  Lycium 
  and 
  other 
  species, 
  

   which 
  lose 
  their 
  spines 
  in 
  damp 
  air. 
  

  

  In 
  some 
  conifers 
  the 
  evolution 
  of 
  horizontal 
  

   branches 
  may 
  be 
  modified 
  by 
  simply 
  turning 
  

   the 
  buds 
  upside 
  down. 
  Or 
  the 
  lateral 
  branches 
  

   can 
  be 
  induced 
  to 
  become 
  erect 
  stems 
  by 
  

   cutting 
  off 
  the 
  normal 
  summit 
  of 
  a 
  tree. 
  Nu- 
  

   merous 
  organs 
  and 
  functions 
  lie 
  dormant 
  un- 
  

   til 
  aroused 
  by 
  external 
  agencies, 
  and 
  many 
  

   other 
  cases 
  could 
  be 
  cited, 
  showing 
  the 
  wide 
  

   occurrence 
  of 
  double 
  adaptation. 
  

  

  There 
  are, 
  however, 
  two 
  points, 
  which 
  should 
  

   not 
  be 
  passed 
  over 
  without 
  some 
  mention. 
  One 
  

   of 
  them 
  is 
  the 
  influence 
  of 
  sun 
  and 
  shade 
  on 
  

   leaves, 
  and 
  the 
  other 
  the 
  atavistic 
  forms, 
  often 
  

   exhibited 
  during 
  the 
  juvenile 
  period. 
  

  

  The 
  leaves 
  of 
  many 
  plants, 
  and 
  especially 
  

   those 
  of 
  some 
  shrubs 
  and 
  trees, 
  have 
  the 
  ca- 
  

   pacity 
  of 
  adapting 
  themselves 
  either 
  to 
  intense 
  

   or 
  to 
  diffuse 
  light. 
  On 
  the 
  circumference 
  of 
  the 
  

   crown 
  of 
  a, 
  tree 
  the 
  light 
  is 
  stronger 
  and 
  the 
  

   leaves 
  are 
  small 
  and 
  thick, 
  with 
  a 
  dense 
  tissue. 
  

   In 
  the 
  inner 
  parts 
  of 
  the 
  crown 
  the 
  light 
  is 
  

   weak 
  and 
  the 
  leaves 
  are 
  broader 
  in 
  order 
  to 
  

   get 
  as 
  much 
  of 
  it 
  as 
  possible. 
  They 
  become 
  

   larger 
  but 
  thinner, 
  consisting 
  often 
  of 
  a 
  small 
  

   number 
  of 
  cell 
  layers. 
  The 
  definitive 
  forma- 
  

   tion 
  is 
  made 
  in 
  extreme 
  youth, 
  often 
  even 
  

   during 
  the 
  previous 
  summer, 
  at 
  the 
  time 
  of 
  the 
  

  

  