﻿+ 
  .012 
  

  

  +.008 
  +.'35: 
  

  

  +.152 
  

  

  252 
  Browx: 
  The 
  refraction 
  of 
  light 
  in 
  plant 
  tissues 
  

  

  1 
  

  

  hardens— 
  there 
  is 
  a 
  rapid 
  rise 
  in 
  the 
  index 
  of 
  refraction 
  of 
  the 
  

   middle 
  lamella 
  to 
  1.59, 
  or 
  ,135 
  above 
  that 
  of 
  glycerine 
  (Fig. 
  i), 
  

   at 
  which 
  it 
  remains 
  nearly 
  constant. 
  Such 
  a 
  change 
  in 
  refraction 
  

  

  can 
  hardly 
  be 
  explained 
  except 
  ' 
  

   in 
  relation 
  to 
  a 
  change 
  in 
  the 
  

  

  chemical 
  composition 
  of 
  the 
  

   pectin 
  lamella, 
  as 
  a 
  result 
  of 
  

   which, 
  beginning 
  as 
  a 
  mem- 
  

   brane 
  of 
  average 
  refraction, 
  it 
  

   quickly 
  becomes 
  the 
  most 
  re- 
  

   fractive 
  of 
  all. 
  

  

  h 
  

  

  The 
  primary 
  thickening 
  of 
  

  

  wood 
  fibers 
  (P, 
  FiG. 
  i) 
  is 
  also 
  

   composed 
  of 
  a 
  highly 
  refrac- 
  

   tive 
  material. 
  In 
  xerophytic 
  

   species 
  of 
  Tecoma 
  and 
  a 
  num- 
  

   ber 
  of 
  other 
  woods, 
  this 
  mem- 
  

   brane 
  first 
  appears 
  as 
  a 
  collen- 
  

   chyma-like 
  thickening 
  upon- 
  

  

  01234-5 
  

  

  Fig. 
  I. 
  Abbe 
  camera 
  drawing 
  of 
  fibers 
  

  

  of 
  ipg 
  tabaco^(T..o,«a 
  sp.). 
  mounted 
  in 
  fhe 
  Sides 
  of 
  the 
  middle 
  lamella, 
  

  

  glycerine 
  (m 
  = 
  1.46). 
  All 
  membranes 
  are 
  ^^ 
  cells 
  which 
  He 
  near 
  the 
  true 
  

  

  + 
  with 
  reference 
  to 
  the 
  glycerine 
  medium. 
  camblum, 
  and 
  which 
  have 
  not 
  

  

  Lr:„t!:\!~"!: 
  'ri'Tl'!! 
  ."="'■ 
  ceased 
  to 
  elongate 
  and 
  to 
  glide 
  

  

  branes 
  appear 
  bright; 
  this 
  is 
  indicated 
  in 
  

   the 
  shading. 
  The 
  middle 
  lamella 
  (M) 
  is 
  

  

  upon 
  

  

  This 
  mem- 
  

  

  less 
  distinct 
  in 
  glycerine 
  than 
  indicated 
  in 
  brane, 
  though 
  much 
  thickened 
  

  

  the 
  drawing. 
  The 
  membranes 
  (M. 
  middle 
  in 
  species 
  of 
  Tecoma 
  and 
  cer- 
  

  

  lamella. 
  and 
  P. 
  primary 
  thickening 
  of 
  the 
  . 
  - 
  . 
  1 
  ' 
  

  

  fiber) 
  are 
  highly 
  refractive 
  (n 
  = 
  1.59) 
  and 
  ^^^^ 
  ^^^^^ 
  genera, 
  IS 
  ordmarily 
  

  

  appear 
  brightly 
  illuminated, 
  especially 
  the 
  ^hln 
  and, 
  because 
  of 
  similarity 
  

  

  middle 
  lamella. 
  The 
  secondary 
  layer 
  (5) 
  in 
  the 
  refractive 
  and 
  staining 
  

  

  less 
  

  

  1.54) 
  and 
  relatively 
  

  

  dark. 
  T, 
  tertiary 
  thickening. 
  qualities, 
  is 
  likely 
  to 
  be 
  mis- 
  

  

  taken 
  as 
  part 
  of 
  a 
  somewhat 
  

   thickened 
  middle 
  lamella. 
  Both" 
  membranes 
  stain 
  deeply 
  with 
  

   haematoxylin, 
  methylene 
  blue, 
  fuchsin, 
  and 
  6ther 
  stains 
  for 
  pectic 
  

   bodies, 
  and 
  together 
  constitute 
  by 
  far 
  the 
  most 
  refractive 
  (.132 
  

   -.135 
  above 
  glycerine) 
  membranes 
  in 
  the 
  mature 
  xylem. 
  But 
  the 
  

   refraction 
  of 
  the 
  primary 
  wall 
  'is 
  slightly 
  lower 
  (about 
  .003) 
  than 
  

   that 
  of 
  the 
  middle 
  lamella, 
  and, 
  when 
  observed 
  in 
  a 
  refractive 
  

   medium 
  at 
  which 
  the 
  former 
  becomes 
  invisible 
  (about 
  i 
  545 
  for 
  

  

  