﻿\ 
  

  

  y 
  

  

  258. 
  Brown: 
  The 
  refraction 
  of 
  light 
  in 
  plant 
  tissues 
  

  

  w 
  

  

  media; 
  but 
  if 
  the 
  section 
  is 
  mounted 
  in 
  a 
  medium 
  whose 
  index 
  of 
  

   refraction 
  (about 
  1.526) 
  approaches 
  sufficiently 
  close 
  to 
  thdt 
  of 
  

   the 
  inner 
  and 
  outer 
  thickenings 
  (/ 
  and 
  0, 
  Fig. 
  3), 
  then 
  the 
  middle 
  

   lamella, 
  whose 
  index 
  of 
  refraction 
  is 
  higher 
  ( 
  + 
  ) 
  than 
  this, 
  stands 
  

   out 
  plainly 
  (as 
  Indicated 
  in 
  Fig. 
  3, 
  C) 
  and 
  is 
  the 
  only 
  portion 
  of 
  

   the 
  wall 
  which 
  can 
  be 
  seen 
  even 
  under 
  the 
  most 
  favorable 
  con- 
  

   ditions 
  of 
  illumination. 
  In 
  alcohol 
  and 
  quinidine, 
  dehydration 
  

   may 
  cause 
  shrinkage 
  of 
  the 
  tissues 
  to 
  such 
  an 
  extent 
  that 
  not 
  as 
  

   good 
  results 
  are 
  obtained 
  as 
  would 
  be 
  expected 
  from 
  the 
  refractive 
  

  

  ++ 
  1 
  

  

  o 
  

  

  :l 
  ' 
  

  

  + 
  ++^ 
  

  

  B 
  

  

  ," 
  .-J' 
  

  

  P 
  W 
  20 
  30 
  40 
  5 
  

  

  Fig 
  3. 
  Appearance 
  of 
  a 
  single 
  section 
  of 
  collenchyma 
  of 
  Pyrus 
  communis 
  L. 
  

   mounted 
  m 
  three 
  different 
  refractive 
  media. 
  A, 
  in 
  Canada 
  balsam, 
  n 
  = 
  1.52; 
  there 
  

   IS 
  httle 
  or 
  no 
  definition 
  of 
  minute 
  detail. 
  B, 
  the 
  same 
  section 
  as 
  A. 
  but 
  mounted 
  in 
  

   glycerine. 
  „ 
  = 
  1.46; 
  the 
  pits 
  are 
  clearly 
  deHned. 
  C, 
  again 
  the 
  same 
  section, 
  but 
  

   mounted 
  m 
  a 
  mixture 
  of 
  castor 
  oil. 
  2 
  volumes 
  and 
  clove 
  oil 
  8 
  volumes, 
  n 
  = 
  1.526- 
  in 
  

  

  ^rn^t""" 
  °' 
  ^^'V^;''^^^^^"' 
  the 
  middle 
  lamella 
  (M) 
  is 
  clearly 
  defined, 
  but 
  the 
  other 
  

   membranes 
  (/ 
  and 
  O) 
  are 
  invisible. 
  The 
  tube 
  is 
  slightly 
  raised 
  from 
  sharp 
  focus; 
  

   the 
  mner 
  th.ckcnxng 
  (/) 
  of 
  the 
  collenchyma 
  walls 
  is 
  less 
  refractive 
  than 
  the 
  outer 
  

   thickening 
  (O) 
  and 
  appears 
  less 
  brightly 
  illuminated 
  in 
  A 
  and 
  B. 
  

  

  M 
  

  

  properties 
  of 
  these 
  media. 
  Glycerine, 
  on 
  the 
  other 
  hand 
  causes 
  

   a 
  slight 
  swelling 
  of 
  the 
  walls, 
  but 
  this 
  is 
  of 
  advantage, 
  in 
  this 
  case, 
  

   in 
  bringing 
  out 
  the 
  details 
  sought. 
  

  

  membrane 
  

  

  per 
  

  

  of 
  

  

  pits, 
  as 
  in 
  case 
  of 
  the 
  bordered 
  pits 
  of 
  conifers, 
  can 
  be 
  readily 
  

   demonstrated 
  in 
  the 
  following 
  manner: 
  place 
  a 
  few 
  drops 
  of 
  a 
  

  

  