﻿424 
  

  

  Brown: 
  Silicious 
  skeleton 
  

  

  apart 
  or 
  crowded 
  together; 
  hence 
  the 
  swelling 
  and 
  shrinking 
  of 
  

   the 
  cell-wall 
  may 
  take 
  place 
  at 
  right 
  angles 
  to 
  the 
  axis 
  of 
  growth. 
  

  

  In. 
  the 
  vicinity 
  of 
  pits, 
  such 
  as 
  the 
  circular 
  pits 
  of 
  Finns 
  

   Strohus 
  or 
  the 
  scalariform 
  pits 
  of 
  Trachodendronj 
  the 
  vertical 
  

   system 
  of 
  rods 
  is 
  replaced 
  by 
  reticulate 
  branches 
  or 
  transverse 
  

   connections 
  which 
  would 
  probably 
  inhibit 
  swelling 
  or 
  shrinking 
  

   of 
  these 
  areas 
  in 
  all 
  directions 
  except, 
  possibly, 
  in 
  thickness. 
  

  

  The 
  skeleton 
  rods 
  are 
  composed 
  of 
  highly 
  mineralized 
  material, 
  

   the 
  composition 
  of 
  which 
  varies 
  in 
  the 
  different 
  families, 
  genera, 
  

   and 
  species; 
  but 
  silica 
  or 
  some 
  silicate 
  is 
  present 
  in 
  at 
  least 
  a 
  large 
  

  

  4 
  

  

  number 
  of 
  cases. 
  

  

  .f 
  

  

  