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  Browx: 
  The 
  refraction 
  of 
  light 
  in 
  plant 
  tissues 
  

  

  pits 
  may 
  be 
  split 
  in 
  a 
  manner 
  similar 
  to 
  that 
  observed 
  by 
  Neeff 
  

  

  growing 
  

  

  The 
  treatment 
  by 
  which 
  the 
  middle 
  lamella 
  was 
  softened 
  is 
  as 
  

   follows: 
  thin 
  sections 
  (3 
  M 
  thick) 
  of 
  the 
  wood 
  of 
  Dracaena 
  were 
  

  

  placed 
  for 
  tiA'-enty-four 
  hours 
  in 
  a 
  25 
  per 
  cent 
  alcoholic 
  solution 
  of 
  

   hydrochloric 
  acid, 
  then 
  for 
  twelve 
  hours 
  in 
  20 
  per 
  cent 
  animonia. 
  

   After 
  such 
  treatment 
  the 
  primary 
  wall 
  and 
  particularly 
  the 
  middle 
  

   lamella 
  were 
  etched 
  and 
  softened 
  to 
  a 
  degree 
  that 
  permitted 
  

   cells 
  to 
  glide 
  upon 
  one 
  another, 
  the 
  closing 
  membranes 
  of 
  pits 
  

   were 
  divided 
  along 
  the 
  middle 
  lamella, 
  so 
  that 
  one 
  part 
  moved 
  

   upon 
  another. 
  By 
  this 
  treatment 
  with 
  acid 
  alcohol, 
  according 
  

   to 
  Mangin 
  (Haas 
  and 
  Hill, 
  '13, 
  p. 
  127), 
  the 
  combined 
  pectin 
  is 
  

   freed 
  from 
  its 
  bases 
  (e.g. 
  calcium) 
  and 
  is 
  then 
  dissolved 
  by 
  the 
  

   ammonia. 
  It 
  is 
  of 
  interest 
  to 
  note 
  that 
  an 
  effect 
  similar 
  to 
  that 
  

   obtained 
  by 
  hydrochloric 
  acid 
  and 
  ammonia 
  (Fig. 
  2) 
  was 
  obtained 
  

   by 
  placing 
  sections, 
  2.5 
  n 
  thick, 
  for 
  twenty-four 
  hours 
  upon 
  wet 
  

   Easter 
  lily 
  pollen 
  moistened 
  with 
  water, 
  the 
  membranes, 
  in 
  this 
  

   case, 
  being 
  acted 
  upon 
  by 
  an 
  enzyme. 
  In 
  sections 
  of 
  Tecoma, 
  

  

  the 
  middle 
  lamella 
  and 
  primary 
  walls 
  were 
  similarly 
  etched 
  and 
  

   softened. 
  

  

  Not 
  only 
  the' 
  middle 
  lamella 
  and 
  primary 
  wall 
  but 
  also 
  all 
  

   other 
  tissue 
  elements 
  have, 
  with 
  possibly 
  a 
  few 
  exceptions, 
  a 
  

   characteristic 
  refractive 
  index; 
  it 
  is 
  remarkable 
  to 
  what 
  an 
  extent 
  

   the 
  various 
  membranes 
  become 
  differentiated 
  in 
  their 
  refractive 
  

   properties 
  during 
  growth. 
  As 
  will 
  be 
  shown 
  presently, 
  a 
  mem- 
  

   brane 
  which 
  can 
  hardly 
  be 
  differentiated 
  by 
  reagents 
  may 
  be 
  

   clearly 
  defined 
  by 
  its 
  refractive 
  properties. 
  

  

  IV. 
  DEFINITION 
  OF 
  ANATOMICAL 
  DETAILS 
  

  

  Success 
  In 
  obtaining 
  clear 
  microscopic 
  definition 
  is 
  in 
  no 
  small 
  

   measure 
  dependent 
  upon 
  the 
  refraction 
  of 
  light 
  by 
  the 
  mounting 
  

   medium. 
  It 
  is 
  of 
  the 
  utmost 
  importance 
  that 
  the 
  index 
  of 
  refrac- 
  

   tion 
  of 
  the 
  tissue 
  should 
  bear 
  a 
  definite 
  relation 
  to 
  that 
  of 
  the 
  

   medium 
  in 
  which 
  it 
  is 
  mounted. 
  

  

  I. 
  Refraction 
  of 
  mounting 
  media 
  

  

  When 
  the 
  refractive 
  properties 
  of 
  a 
  tissue 
  are 
  known, 
  definite 
  

   refractive 
  efTects 
  may 
  be 
  obtained 
  by 
  selecting 
  media 
  having 
  the 
  

  

  The 
  following 
  list 
  (Table 
  II) 
  

  

  necessary 
  

  

  includes 
  some 
  of 
  the 
  refractive 
  media 
  most 
  useful 
  for 
  this 
  purpose 
  : 
  

  

  