﻿412 
  Brow^^: 
  Silicious 
  skeleton 
  

  

  but, 
  upon 
  standing 
  a 
  few 
  hours, 
  small 
  isolated 
  areas 
  (FiG, 
  i, 
  C) 
  

   may 
  be 
  distinguished. 
  These 
  areas 
  appear 
  to 
  be 
  slightly 
  more 
  

   refractive 
  than 
  the 
  material 
  of 
  which 
  the 
  rest 
  of 
  the 
  secondary 
  

   . 
  thickening 
  is 
  composed, 
  when 
  the 
  wall 
  has 
  taken 
  up 
  as 
  much 
  as 
  

   it 
  will 
  of 
  the 
  liquid 
  in 
  which 
  it 
  is 
  mounted. 
  They 
  are 
  circular 
  

   in 
  outline, 
  1-3 
  /x 
  in 
  diameter, 
  approximately 
  two 
  hundred 
  in 
  

   number 
  in 
  the 
  cylindrical 
  portion 
  of 
  the 
  fiber, 
  and 
  distributed 
  

   quite 
  evenly 
  through 
  the 
  secondary 
  thickening 
  of 
  the 
  wall. 
  They 
  

   are 
  without 
  order 
  of 
  arrangement 
  except 
  that 
  those 
  of 
  larger 
  

   diameter 
  appear 
  to 
  be 
  placed 
  near 
  the 
  outside. 
  The 
  exact 
  size 
  

   and 
  position 
  in 
  the 
  fiber 
  wall 
  of 
  areas 
  seen 
  with 
  sufficient 
  clearness 
  

   to 
  be 
  drawn 
  with 
  the 
  Abbe 
  camera 
  are 
  shown 
  in 
  Fig. 
  i, 
  C; 
  num- 
  

   erous 
  others 
  could 
  be 
  seen 
  distributed 
  evenly 
  through 
  this 
  mem- 
  

   brane 
  but 
  not 
  with 
  sufficient 
  clearness 
  to 
  be 
  projected. 
  

  

  One 
  effect 
  of 
  imbibition 
  upon 
  the 
  membrane-mass 
  would 
  be 
  

   to 
  lower 
  the 
  index 
  of 
  refraction 
  of 
  that 
  portion 
  which 
  takes 
  up 
  

   the 
  liquid. 
  From 
  the 
  fact 
  that 
  the 
  relatively 
  more 
  refractive 
  

   areas 
  appear 
  in 
  the 
  membrane 
  after 
  the 
  imbibition 
  of 
  water 
  or 
  

   glycerine, 
  it 
  is 
  indicated 
  that 
  these 
  portions 
  of 
  the 
  membrane 
  im- 
  

   bibe 
  little 
  or 
  no 
  liquid; 
  the 
  remainder 
  of 
  the 
  membrane, 
  on 
  the 
  

   other 
  hand, 
  is 
  evidently 
  composed 
  of 
  hygroscopic 
  material. 
  

   The 
  impermeable 
  areas, 
  in 
  all 
  probability, 
  represent 
  the 
  cut 
  ends 
  

   of 
  silicious 
  rods 
  which, 
  as 
  will 
  be 
  shown 
  presently, 
  form 
  a 
  skeleton- 
  

   like 
  structure 
  in 
  the 
  secondary 
  thickening 
  of 
  the 
  fiber. 
  

  

  3. 
  Histological 
  elements 
  of 
  the 
  secoxdary 
  thickening 
  

  

  The 
  secondary 
  thickening 
  of 
  fibers 
  of 
  the 
  xylem 
  of 
  Qtiercus, 
  

   Hicoriay 
  Caesarea, 
  Aspidosperma, 
  and 
  the 
  majority 
  of 
  woody 
  

   plants 
  forms 
  nearly 
  the 
  entire 
  cell-wall. 
  It 
  is 
  without 
  doubt 
  

   essential 
  to 
  the 
  mechanical 
  strength 
  of 
  these 
  sclerous 
  membranes 
  

   and 
  is 
  never 
  absent 
  in 
  fibers 
  the 
  longitudinal 
  dimensions 
  of 
  which 
  

   tend 
  to 
  remain 
  unchanged 
  under 
  different 
  conditions 
  of 
  moisture. 
  

  

  As 
  previously 
  mentioned, 
  small 
  isolated 
  portions 
  of 
  this 
  thick- 
  

   ening 
  of 
  the 
  fiber 
  in 
  Tecoma 
  are 
  composed 
  of 
  non-hygroscopic 
  

   material. 
  If 
  the 
  untreated 
  section 
  is 
  placed 
  upon 
  a 
  mica 
  slip 
  and 
  

   held 
  over 
  an 
  alcohol 
  flame 
  until 
  the 
  fibers 
  are 
  partly 
  burned 
  to 
  

   ash, 
  numerous 
  hyaline, 
  non-combustible, 
  rods 
  of 
  mineral 
  com- 
  

   position 
  are 
  left 
  partly 
  free 
  along 
  the 
  burned 
  margin 
  of 
  the 
  secon- 
  

  

  