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  4 
  

  

  Brown: 
  The 
  refraction 
  of 
  light 
  in 
  plant 
  tissues 
  253 
  

  

  Tecoma), 
  the 
  true 
  middle 
  lamella 
  is 
  nearly 
  as 
  sharply 
  defined 
  as 
  

   in 
  coUenchyma 
  (C, 
  Fig. 
  2). 
  

  

  ■ 
  

  

  The 
  observed 
  differences 
  in 
  refraction 
  here 
  are 
  probably 
  asso- 
  

   ciated 
  with 
  differences 
  in 
  mineral 
  content 
  of 
  the 
  membrane 
  

   studied. 
  Of 
  the 
  mineral 
  elements 
  which 
  enter 
  into 
  the 
  composition 
  

   of 
  wood}' 
  tissues, 
  calcium, 
  especially 
  in 
  the 
  form 
  of 
  calcium 
  car- 
  

   bonate, 
  would 
  seem 
  to 
  be 
  the 
  only 
  one 
  likely 
  to 
  bring 
  about 
  an 
  

   increase 
  in 
  refraction 
  as 
  great 
  

  

  i 
  

  

  as 
  that 
  noted 
  in 
  the 
  middle 
  

   lamella 
  and 
  primary 
  wall. 
  

  

  The 
  presence 
  of 
  silica, 
  for 
  ex- 
  

   ample, 
  in 
  portions 
  of 
  the 
  sec- 
  

   ondary 
  layer 
  (Fig. 
  i, 
  S) 
  is 
  

   associated 
  with 
  a 
  much 
  lower 
  

   refractive 
  index.* 
  The 
  rise 
  in 
  

   refraction 
  of 
  the 
  middle 
  lam- 
  

   ella 
  and 
  primary 
  wall 
  during 
  

   development 
  finds 
  its 
  most 
  

   reasonable 
  explanation 
  in 
  the 
  

   supposition 
  that 
  in 
  these 
  

  

  .0 
  

  

  / 
  

  

  I 
  

  

  V! 
  

  

  \ 
  I 
  

  

  w 
  

  

  c 
  

  

  Fig. 
  2. 
  Longitudinal 
  section 
  of 
  a 
  wall 
  

  

  pectic 
  membranes 
  the 
  relative 
  between 
  two 
  thick-walled 
  parenchyma 
  cells 
  

  

  r 
  

  

  in 
  the 
  stem 
  of 
  Dracaena 
  aiirea, 
  after 
  twenty- 
  

  

  i 
  

  

  four 
  hours 
  treatment 
  in 
  Easter 
  lily 
  pollen 
  

  

  (P) 
  is 
  shown 
  stained 
  deeply 
  with 
  haema- 
  

   xyl 
  

  

  proportion 
  of 
  calcium 
  is 
  

  

  greatly 
  increased 
  as 
  soon 
  as 
  enzyme 
  solution. 
  The 
  midd'ie 
  lamdiri) 
  

  

  the 
  cells 
  have 
  attained 
  their 
  has 
  softened, 
  allowing 
  the 
  cells 
  to 
  glide 
  upon 
  

  

  mature 
  size 
  and 
  the 
  mechan- 
  ^"^^^*'''' 
  '^^ 
  ^^^'*^^ 
  membrane 
  (C) 
  of 
  a 
  

  

  . 
  , 
  . 
  • 
  , 
  . 
  . 
  simple 
  pit 
  has 
  separated 
  longitudinallyalong 
  

  

  ICal 
  tissues 
  begm 
  to 
  acquire 
  ^^e 
  middle 
  lamella 
  and 
  one 
  part 
  has 
  glided 
  

  

  hardness 
  and 
  strength. 
  Ap- 
  upon 
  the 
  other, 
  in 
  places, 
  the 
  primary 
  wall 
  

  

  parently, 
  these 
  membranes 
  

  

  acquire 
  their 
  mechanical 
  rigi- 
  ^^fddi^ 
  lameHa. 
  

   dity 
  as 
  a 
  result, 
  in 
  part 
  at 
  

  

  least, 
  of 
  the 
  addition 
  of 
  calcium, 
  possibly 
  in 
  combined 
  form, 
  to 
  

   the 
  pectin; 
  and 
  it 
  is 
  of 
  interest 
  to 
  add 
  that 
  both 
  by 
  chemical 
  

   treatment 
  and 
  by 
  the 
  action 
  of 
  certain 
  enzymes, 
  as 
  will 
  be 
  de- 
  

   scribed 
  presently, 
  the 
  primary- 
  wall 
  may 
  be 
  softened 
  and 
  the 
  middle 
  

   lamella 
  reduced 
  to 
  a 
  jelly-like 
  consistency, 
  so 
  that 
  cells 
  glide 
  upon 
  

   one 
  another 
  as 
  during 
  development; 
  even 
  closing 
  membranes 
  of 
  

  

  * 
  The 
  presence 
  of 
  highly 
  silicified 
  longitudinal 
  tracts 
  in 
  the 
  secondary 
  layer 
  of 
  

   wood 
  fibers 
  of 
  Tecoma 
  has 
  been 
  demonstrated 
  by 
  the 
  writer 
  and 
  will 
  be 
  published 
  in 
  a 
  

  

  subsequent 
  paper 
  on 
  fiber 
  skeletons. 
  

  

  