﻿\ 
  

  

  Brown: 
  The 
  refraction 
  of 
  light 
  in 
  plant 
  tissues 
  251 
  

  

  r 
  

  

  take 
  lip 
  relatively 
  more 
  of 
  the 
  liquid 
  and 
  show 
  a 
  correspondingly 
  

   wide 
  deviation 
  in 
  refraction, 
  

  

  m 
  , 
  

  

  The 
  imbibition 
  of 
  liquid 
  is 
  also 
  indicated 
  by 
  the 
  swelling 
  of 
  the 
  

   membrane, 
  and 
  those 
  membranes 
  which 
  have 
  the 
  widest 
  deviation 
  

   in 
  refraction, 
  as, 
  for 
  example, 
  the 
  mucilaginous 
  membranes 
  in 
  

   woody 
  fibers, 
  are 
  the 
  ones 
  which 
  swell 
  the 
  most. 
  Not 
  infrequently 
  

   such 
  membranes, 
  as 
  shown 
  by 
  an 
  accurate 
  projection 
  published 
  

   in 
  a 
  previous 
  paper 
  ('19, 
  p. 
  143), 
  may 
  swell 
  to 
  almost 
  double 
  

   thickness 
  when 
  dehydrated 
  sections 
  are 
  mounted 
  in 
  water, 
  

   glycerine, 
  or 
  McLean's 
  solution. 
  But 
  refraction 
  is 
  a 
  more 
  delicate 
  

   index 
  than 
  swelling 
  in 
  that 
  it 
  serves 
  to 
  differentiate 
  the 
  particular 
  

   portions 
  of 
  a 
  membrane 
  which 
  take 
  up 
  the 
  liquid. 
  

  

  Some 
  extremely 
  hygroscopic 
  membranes 
  are 
  invisible. 
  . 
  or 
  

   nearly 
  so, 
  in 
  liquids 
  differing 
  in 
  refraction 
  from 
  on^ 
  another 
  by 
  

   more 
  than 
  .055. 
  There 
  thus 
  is 
  an 
  apparently 
  close 
  relation 
  between 
  

   imbibition 
  and 
  refraction. 
  But 
  since 
  visibility 
  of 
  outline 
  is 
  

   partly 
  dependent 
  ujwn 
  other 
  factors, 
  such 
  as 
  the 
  relative 
  light 
  

   dispersion 
  of 
  the 
  liquid 
  and 
  the 
  object, 
  the 
  deviation 
  in 
  refraction 
  

   •may 
  not 
  be 
  exactly 
  proportional 
  to 
  the 
  imbibition 
  of 
  liquid. 
  

  

  3. 
  Change 
  in 
  refraction 
  with 
  growth 
  

  

  Deviations 
  or 
  apparent 
  fluctuations 
  in 
  refraction, 
  such 
  as 
  

   those 
  above 
  described,, 
  are 
  not 
  associated 
  with 
  any 
  actual 
  change 
  in 
  

   refraction 
  of 
  the 
  membrane 
  substance; 
  on 
  removing 
  the 
  imbibition 
  

   liquid 
  from 
  the 
  tissue, 
  the 
  refraction 
  of 
  the 
  membrane 
  is 
  found 
  

   unchanged. 
  But, 
  during 
  growth, 
  actual 
  changes 
  in 
  refraction 
  

   may 
  occur, 
  as, 
  for 
  example, 
  in 
  the 
  middle 
  lamella 
  of 
  wood 
  cells. 
  

   In 
  the 
  early 
  stages 
  of 
  cambial 
  growth, 
  such 
  as 
  may 
  be 
  found 
  in 
  

   the 
  stems 
  of 
  woody 
  angiosperms, 
  the 
  membrane 
  which 
  subse- 
  

   quently 
  becomes 
  the 
  middle 
  lamella 
  has 
  a 
  refractive 
  index 
  of 
  

   about 
  1.53,' 
  or 
  the 
  average 
  for 
  plant 
  tissues. 
  In 
  subsequent 
  

   stages, 
  such 
  as 
  occur 
  in 
  the 
  narrow 
  zone 
  of 
  undiflFerentiated 
  tissue 
  

   next 
  to 
  the 
  true 
  cambium, 
  where 
  the 
  thin-walled 
  immature 
  cells 
  are 
  

   In 
  various 
  stages 
  of 
  elongation, 
  the 
  refraction 
  of 
  this 
  membrane 
  

   increases 
  with 
  progressive 
  differentiation. 
  Thus, 
  as 
  soon 
  as 
  

   growth 
  in 
  length 
  of 
  the 
  wood 
  cells 
  has 
  ceased 
  and 
  the 
  walls 
  

   begin 
  to 
  thicken 
  prominently 
  by 
  apposition— 
  the 
  stage 
  in 
  which 
  

   the 
  first 
  secondary 
  thickenings 
  of 
  the 
  fibers 
  appear 
  and 
  the 
  tissue 
  

  

  