﻿418 
  Brown: 
  Silicious 
  skeleton 
  

  

  is 
  therefore 
  not 
  abundant 
  in 
  the 
  secondary 
  thickening 
  of 
  the 
  

  

  fiber 
  of 
  Tecoma, 
  but 
  if 
  concentrated 
  in 
  the 
  skeleton 
  rods 
  the 
  amount 
  

  

  would 
  be 
  more 
  than 
  sufficient 
  to 
  form 
  a 
  skeleton 
  with 
  two 
  hundred 
  

  

  vertical 
  rods 
  composed 
  either 
  of 
  silica 
  or 
  of 
  some 
  highly 
  silicified 
  

   material, 
  . 
  . 
  

  

  Various 
  evidences 
  make 
  it 
  almost 
  certain 
  that 
  the 
  'composi- 
  

   tion 
  of 
  the 
  rods 
  varies 
  in 
  different 
  families, 
  genera, 
  and 
  species, 
  

   though 
  silica 
  or 
  silicious 
  matter 
  may 
  always 
  be 
  present. 
  This 
  is 
  

   indicated 
  in 
  the 
  fact 
  that 
  the 
  effect 
  of 
  hydrofluoric 
  acid 
  upon 
  the 
  

   mechanical* 
  rigidity 
  of 
  the 
  fiber 
  is 
  quite 
  different 
  In 
  some 
  species 
  

   from 
  what 
  it 
  is 
  in 
  others. 
  Thus 
  in 
  Tecoma 
  and 
  several 
  other 
  

   genera, 
  as 
  will 
  be 
  described 
  more 
  fully 
  presently, 
  the 
  fibrous 
  tissues 
  

   of 
  the 
  xylem 
  suddenly 
  contract 
  more 
  than 
  20 
  per 
  cent 
  in 
  length 
  

   during 
  treatment 
  with 
  hydrofluoric 
  acid; 
  the 
  wood 
  of 
  Aspido- 
  

  

  • 
  sperma 
  and 
  of 
  numerous 
  other 
  genera, 
  on 
  the 
  other 
  hand, 
  is 
  

  

  • 
  merely 
  softened 
  by 
  this 
  treatment. 
  Also, 
  hydrochloric 
  acid 
  

   attacks 
  the 
  skeleton 
  rods 
  of 
  Guaiacum 
  sp. 
  and 
  softens 
  the 
  wood 
  

   sufficiently 
  to 
  permisectioning 
  in 
  the 
  microtome; 
  but 
  with 
  this 
  acid 
  

   there 
  is 
  no 
  material 
  contraction 
  of 
  the 
  fibers, 
  and 
  the 
  skeleton 
  rods 
  

   are 
  still 
  present 
  in 
  the 
  ash, 
  though 
  a 
  large 
  portion 
  of 
  the 
  outer 
  

   mineralized 
  film 
  has 
  apparently 
  been 
  dissolved 
  by 
  the 
  acid. 
  In 
  

   this 
  case 
  the 
  rods 
  may 
  be 
  composed 
  of 
  a 
  central 
  axis 
  of 
  silica 
  or 
  of 
  

   highly 
  silicified 
  material, 
  enclosed 
  by 
  a 
  thin 
  outer 
  covering 
  of 
  

   mineral 
  composition 
  which 
  is 
  partly 
  soluble 
  in 
  hydrochloric 
  acid. 
  

  

  That 
  the 
  skeleton 
  rods 
  are 
  not 
  composed 
  entirely 
  of 
  crystalline 
  

   'silica 
  is 
  evident 
  from 
  the 
  fact 
  that 
  when 
  the 
  macerated 
  fibers 
  of 
  

  

  dosp 
  

  

  moun 
  

  

  without 
  leaving 
  any 
  insolubl 
  eremains 
  which 
  might 
  be 
  positively 
  

   identified 
  as 
  part 
  of 
  a 
  skeleton 
  rod. 
  But 
  the 
  rods 
  seem 
  tc 
  

  

  be 
  

  

  matrix 
  

  

  quently 
  remain 
  visible 
  a 
  short 
  rime 
  in 
  the 
  acid 
  after 
  the 
  rest 
  of 
  

   the 
  wall 
  has 
  been 
  dissolved. 
  The 
  evidences 
  as 
  a 
  whole 
  therefore 
  

   favor 
  the 
  conclusion 
  that 
  the 
  skeleton 
  rods 
  are 
  composed 
  of 
  highly 
  

   silicified 
  material, 
  the 
  composition 
  of 
  which 
  varies 
  in 
  the 
  different 
  

   families, 
  genera, 
  and 
  species. 
  

  

  Physical 
  properties.— 
  As 
  before 
  mentioned, 
  the 
  rods 
  probably 
  

   imbibe 
  little 
  or 
  no 
  liquid 
  and 
  hence 
  under 
  ordinary 
  temperature 
  

  

  