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  4 
  

  

  The 
  refraction 
  of 
  light 
  in 
  plant 
  tissues* 
  

  

  Forest 
  B. 
  H. 
  Brown 
  

  

  I 
  

  

  (with 
  four 
  text 
  figures) 
  

  

  CONTENTS 
  

  

  I. 
  INTRODUCTION 
  243 
  

  

  II. 
  METHODS 
  OF 
  MEASURING 
  REFRACTION 
  WITH 
  THE 
  MICRO- 
  

   SCOPE. 
  . 
  244 
  

  

  1. 
  Refractive 
  liquids., 
  245 
  

  

  2. 
  Preparation 
  of 
  tissues 
  for 
  study 
  247 
  

  

  3. 
  Determination 
  of 
  relative 
  refraction 
  247 
  

  

  j4. 
  By 
  oblique 
  illumination 
  .■ 
  ; 
  

  

  , 
  • 
  

  

  ■' 
  • 
  247 
  

  

  J3. 
  By 
  direct 
  illumination 
  248 
  

  

  C. 
  Modifications 
  with 
  hygroscopic 
  tissues 
  248 
  

  

  D. 
  Relative 
  accuracy 
  of 
  the 
  methods 
  '. 
  , 
  249 
  

  

  III. 
  REFRACTION 
  OF 
  TISSUES 
  

  

  I. 
  Relative 
  refraction. 
  . 
  . 
  

  

  249 
  

   250 
  

  

  2. 
  Deviation 
  in 
  refraction 
  with 
  imbibition 
  250 
  

  

  3. 
  Change 
  in 
  refraction 
  with 
  growth 
  251 
  

  

  IV. 
  DEFINITION 
  OF 
  ANATOMICAL 
  DETAILS 
  254 
  

  

  I. 
  Refraction 
  of 
  mounting 
  media 
  

  

  254 
  

  

  2. 
  Pits 
  and 
  middle 
  lamella 
  in 
  collench\'ma 
  257 
  

  

  3. 
  Perforations 
  in 
  closing 
  membrane 
  of 
  pits 
  258 
  

  

  V. 
  SUMMARY 
  

  

  259 
  

  

  I. 
  INTRODUCTION 
  

  

  The 
  laws 
  of 
  refraction 
  have 
  an 
  interest 
  in 
  microscopy, 
  not 
  

   merely 
  in 
  connection 
  with 
  the 
  definition 
  of 
  anatomical 
  details, 
  

   but 
  also 
  in 
  the 
  determination 
  of 
  the 
  chemical 
  or 
  physical 
  qualities 
  

   of 
  any 
  part 
  or 
  product 
  of 
  the 
  cell. 
  Such 
  laws 
  are 
  of 
  universal 
  

   application, 
  have 
  the 
  advantage 
  of 
  great 
  precision, 
  and 
  serve 
  to 
  

   differentiate 
  many 
  characters 
  not 
  readily 
  brought 
  out 
  by 
  stains 
  or 
  

   chemical 
  reagents. 
  Furthermore, 
  a 
  knowledge 
  of 
  the 
  refractive 
  

   properties 
  of 
  the 
  various 
  tissues 
  is 
  indispensable 
  if 
  one 
  is 
  to 
  obtain 
  

   the 
  clearest 
  definition 
  of 
  tissue 
  outlines 
  by 
  aid 
  of 
  the 
  microscope. 
  

   Current 
  methods, 
  however, 
  go 
  little 
  further 
  than 
  listing 
  the 
  

   indices 
  of 
  refraction 
  of 
  media 
  in 
  which 
  tissues 
  may 
  be 
  mounted, 
  

  

  * 
  Contribution 
  from 
  the 
  Osborn 
  Botanical 
  Laboratory, 
  

  

  243 
  

  

  