﻿Brown: 
  The 
  refraction 
  of 
  light 
  in 
  plant 
  tissues 
  

  

  all 
  of 
  which 
  have 
  little 
  significance 
  in 
  anatomical 
  work 
  unless 
  the 
  

  

  F 
  

  

  index 
  of 
  refraction 
  of 
  the 
  tissue 
  substances 
  is 
  also 
  given. 
  In 
  as 
  

   much 
  as 
  many, 
  if 
  not 
  most, 
  of 
  the 
  ultra-microscopic 
  characters 
  of 
  

   cell 
  membranes 
  are 
  to 
  a 
  greater 
  or 
  less 
  extent 
  associated 
  with 
  the 
  

   refractive 
  powers 
  of 
  these 
  membranes, 
  the 
  laws 
  of 
  refraction 
  have 
  

   their 
  special 
  application 
  in 
  the 
  investigation 
  of 
  the 
  physical 
  or 
  

   chemical 
  composition 
  as 
  well 
  as 
  in 
  the 
  anatomical 
  characters 
  of 
  cell 
  

   membranes. 
  Such 
  laws, 
  therefore, 
  in 
  their 
  application 
  to 
  plant 
  

   tissues, 
  come 
  well 
  within 
  the 
  scope 
  of 
  botanical 
  research, 
  and, 
  as 
  

   such, 
  open 
  a 
  wide 
  field 
  for 
  investigation, 
  particularly 
  when 
  more 
  

   than 
  ordinary 
  precision 
  is 
  required. 
  

  

  Owing, 
  apparently, 
  to 
  chemical 
  change, 
  the 
  refraction 
  of 
  a 
  plant 
  

   membrane 
  is 
  peculiarly 
  liable 
  to 
  actual 
  variation 
  during 
  growth; 
  

   still 
  more 
  characteristic 
  of 
  the 
  plant 
  membrane, 
  due 
  to 
  its 
  hygro- 
  

   scopic 
  qualities, 
  are 
  the 
  apparent 
  fluctuations 
  (deviations) 
  in 
  re- 
  

   fraction 
  by 
  virtue 
  of 
  which 
  it 
  absorbs 
  (imbibes) 
  water 
  and 
  other 
  

   refractiv^e 
  liquids 
  in 
  which 
  it 
  may 
  be 
  immersed. 
  

  

  A 
  study 
  of 
  the 
  refraction 
  of 
  light 
  in 
  plant 
  tissues 
  and 
  of 
  some 
  of 
  

   the 
  conditions 
  affecting 
  or 
  apparently 
  affecting 
  its 
  stability 
  was 
  

   carried 
  on 
  by 
  the 
  writer 
  at 
  intervals 
  during 
  the 
  years 
  1917-1919, 
  

   at 
  the 
  Osborn 
  Botanical 
  Laboratory, 
  The 
  writer 
  expresses 
  

   indebtedness 
  to 
  Professor 
  A. 
  W. 
  Evans 
  and 
  other 
  members 
  of 
  the 
  

   Department 
  of 
  Botany 
  and 
  to 
  the 
  members 
  of 
  the 
  School 
  of 
  Fores- 
  

   try, 
  for 
  every 
  encouragement 
  throughout 
  the 
  investigation; 
  also, 
  

   to 
  Professor 
  C. 
  S. 
  Hastings 
  of 
  the 
  Department 
  of 
  Physics 
  for 
  

   many 
  valuable 
  suggestions, 
  and 
  for 
  verification 
  of 
  the 
  work 
  with 
  

   refractive 
  liquids. 
  The 
  methods 
  described 
  constitute 
  a 
  supple- 
  

   ment 
  to 
  the 
  more 
  general 
  technique 
  for 
  woody 
  tissues 
  given 
  in 
  an 
  

   earlier 
  paper 
  ('19, 
  p. 
  138). 
  

  

  V 
  

  

  II. 
  ISIETHODS 
  OF 
  MEASURING 
  REFRACTION 
  WITH 
  

  

  THE 
  MICROSCOPE 
  

  

  ■ 
  

  

  r 
  

  

  The 
  methods 
  by 
  which 
  it 
  is 
  possible 
  to 
  determine 
  the 
  refraction 
  

   of 
  an 
  object 
  (under 
  the 
  microscope) 
  depend 
  on 
  the 
  use 
  of 
  mount- 
  

   ing 
  media 
  of 
  known 
  refractive 
  powers. 
  With 
  a 
  gradient 
  series 
  of 
  

   such 
  media, 
  the 
  index 
  of 
  refraction 
  of 
  an 
  object 
  may 
  be 
  measured 
  

   with 
  accuracy 
  to 
  the 
  second 
  decimal 
  place. 
  

  

  