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  Brown: 
  Silicious 
  skeleton 
  

  

  these 
  structures 
  are 
  independent 
  of 
  each 
  other 
  in 
  the 
  properties 
  

   which 
  they 
  impart 
  to 
  the 
  wood. 
  They 
  may 
  therefore 
  be 
  sep- 
  

   arately 
  considered. 
  

  

  a. 
  Silicious 
  skeleton 
  

  

  Structure. 
  — 
  ^The 
  longitudinal 
  rods, 
  of 
  which 
  the 
  siHcious 
  skeleton 
  

   of 
  tracheids 
  and 
  fibers 
  is 
  composed, 
  are 
  nearly 
  straight, 
  parallel, 
  

   and 
  extend 
  longitudinally 
  in 
  the 
  colloidal 
  matrix 
  from 
  one 
  end 
  of 
  

   the 
  tracheid 
  or 
  fiber 
  to 
  the 
  other. 
  

  

  The 
  number 
  of 
  vertical 
  rods 
  in 
  a 
  skeleton 
  varies 
  according 
  to 
  

   the 
  genus 
  or 
  species, 
  from 
  possibly 
  as 
  few 
  as 
  four 
  in 
  the 
  tracheids 
  

   of 
  Trochodendron 
  to 
  possibly 
  as 
  many 
  as 
  two 
  hundred 
  in 
  the 
  

   fibers 
  of 
  Tecoma, 
  

  

  These 
  rods 
  occasionally 
  branch 
  or 
  coalesce 
  — 
  more 
  frequently 
  in 
  

   some 
  species 
  than 
  In 
  others. 
  The 
  coalescence 
  probably 
  serves 
  to 
  

   reduce 
  the 
  number 
  of 
  vertical 
  rods 
  in. 
  the 
  tapering 
  ends 
  of 
  a 
  fiber 
  or 
  

   tracheid, 
  as 
  shown 
  diagrammatically 
  in 
  FiG. 
  2 
  ; 
  in 
  conifers 
  the 
  rods 
  

   branch 
  around 
  the 
  large 
  circular 
  pits 
  or 
  become 
  reticulate 
  where 
  

   the 
  pits 
  are 
  numerous; 
  in 
  Pinus 
  Strobus, 
  a 
  silicious 
  ring 
  encloses 
  

   the 
  extreme 
  border 
  of 
  each 
  large 
  pit. 
  Scalariform 
  pitting 
  in 
  

   particular 
  makes 
  necessary 
  a 
  great 
  reduction 
  in 
  the 
  number 
  of 
  

   vertical 
  rods 
  in 
  a. 
  tracheid. 
  Thus 
  in 
  Trochodendron 
  a 
  single 
  rod 
  

   of 
  comparatively 
  large 
  diameter 
  passes 
  to 
  each 
  side 
  of 
  a 
  vertical 
  

   series 
  of 
  the 
  transversely 
  elongated 
  pits 
  and 
  a 
  transverse 
  rod 
  of 
  

   smaller 
  diameter 
  passes 
  between 
  each 
  two 
  pits 
  of 
  the 
  series 
  and 
  

   joins 
  at 
  either 
  end 
  to 
  each 
  of 
  the 
  vertical 
  rods 
  like 
  the 
  rounds 
  of 
  a 
  

   ladder. 
  Hence, 
  judging 
  from 
  the 
  structure 
  of 
  the 
  skeleton, 
  the 
  

   pits 
  and 
  pit-areas 
  would 
  not 
  swell 
  or 
  shrink 
  in 
  either 
  the 
  longi- 
  

   tudinal 
  or 
  the 
  transverse 
  planes. 
  The 
  rods 
  are 
  filamentous 
  in 
  

   shape 
  and 
  are, 
  as 
  before 
  mentioned, 
  probably 
  less 
  than 
  3 
  m 
  in 
  

   diameter. 
  The 
  details 
  of 
  structure, 
  as 
  summarized 
  above, 
  may 
  

   be 
  observed 
  in 
  burned 
  preparations, 
  as 
  follows 
  : 
  from 
  small 
  trans- 
  

  

  J 
  

  

  verse 
  sections 
  of 
  the 
  xylem 
  of 
  Tecoma 
  15-20 
  ju 
  thick, 
  it 
  is 
  found 
  

   that 
  as 
  many 
  as 
  twenty 
  rods 
  may 
  be 
  seen 
  partly 
  exposed 
  along 
  

   the 
  burned 
  outer 
  margin 
  of 
  the 
  secondar>- 
  layer 
  (FiG. 
  3, 
  A). 
  

   On 
  burning 
  deeper 
  inward 
  toward 
  the 
  center 
  of 
  the 
  fiber, 
  other 
  

   rods 
  would 
  undoubtedly 
  be 
  exposed; 
  hence 
  it 
  may 
  be 
  assumed 
  

   that 
  not 
  less 
  than 
  a 
  hundred 
  rods 
  may 
  be 
  present 
  in 
  the 
  fiber, 
  of 
  

   which 
  only 
  twenty 
  are 
  exposed 
  at 
  the 
  margin. 
  In 
  unburned 
  

  

  