﻿454 
  

  

  On 
  the 
  Fossil 
  Plants 
  of 
  the 
  Coal-measures. 
  

  

  [June 
  10_, 
  

  

  ture, 
  composed 
  of 
  a 
  double 
  row 
  of 
  what 
  have 
  evidently 
  been 
  colourless 
  

   cells, 
  and 
  which 
  are 
  elongated 
  vertically, 
  but 
  with 
  square 
  ends. 
  

  

  In 
  the 
  more 
  matured 
  stems, 
  the 
  central 
  vascular 
  axis 
  of 
  the 
  young 
  

   twigs 
  becomes 
  the 
  centre 
  whence 
  radiate 
  six 
  exogenously 
  developed 
  

   wedges 
  of 
  vascular 
  tissue, 
  each 
  of 
  which 
  enlarges 
  as 
  it 
  proceeds 
  out- 
  

   wards 
  aud 
  terminates 
  at 
  its 
  outer 
  extremity 
  in 
  a 
  slightly 
  rounded 
  con- 
  

   tour. 
  Each 
  wedge 
  consists 
  of 
  a 
  series 
  of 
  radiating 
  vascular 
  lamina?, 
  

   separated 
  by 
  numerous 
  medullary 
  rays, 
  which 
  latter 
  consist 
  of 
  long 
  and, 
  

   for 
  the 
  most 
  part, 
  single 
  vertical 
  rows 
  of 
  mural 
  cells. 
  These 
  six 
  exo- 
  

   genous 
  wedges 
  are 
  separated 
  from 
  each 
  other 
  by 
  a 
  large 
  wedge 
  of 
  cellular 
  

   cortical 
  parenchyma, 
  the 
  cells 
  of 
  which 
  are 
  elongated 
  radially 
  and 
  have 
  a 
  

   somewhat 
  mural 
  arrangement. 
  As 
  those 
  between 
  any 
  two 
  contiguous 
  

   wedges 
  proceed 
  outwards, 
  they 
  separate 
  more 
  or 
  less 
  definitely 
  into 
  two 
  

   series, 
  which 
  diverge 
  right 
  and 
  left 
  to 
  sweep 
  round 
  the 
  peripheral 
  extremity 
  

   of 
  each 
  nearest 
  exogenous 
  wedge, 
  meeting 
  and 
  blending 
  with 
  a 
  similar 
  set 
  

   coming 
  from 
  the 
  opposite 
  side 
  of 
  the 
  wedge. 
  In 
  doing 
  this 
  they 
  form 
  a 
  

   sort 
  of 
  loop, 
  enclosing 
  a 
  semilunar 
  mass 
  of 
  smaller 
  cells 
  interposed 
  

   between 
  the 
  loop 
  and 
  the 
  outer 
  end 
  of 
  the 
  exogenous 
  wedge. 
  The 
  author 
  

   demonstrates 
  that 
  this 
  enclosed 
  cellular 
  tissue 
  is 
  essentially 
  a 
  cambial 
  

   layer, 
  out 
  of 
  which 
  all 
  the 
  new 
  vessels 
  and 
  peripheral 
  extensions 
  of 
  

   the 
  medullary 
  rays 
  were 
  developed. 
  Young 
  vessels 
  are 
  seen 
  at 
  its 
  

   inner 
  surface 
  in 
  process 
  of 
  formation. 
  External 
  to 
  these 
  two 
  specialized 
  

   cortical 
  tissues 
  there 
  is, 
  in 
  these 
  matured 
  stems, 
  a 
  mass 
  of 
  the 
  primitive 
  

   cortical 
  parenchyma 
  seen 
  in 
  the 
  youngest 
  shoots, 
  enclosed, 
  as 
  before, 
  

   in 
  a 
  double 
  layer 
  of 
  epidermal 
  cells. 
  

  

  The 
  author 
  has 
  traced 
  the 
  development 
  of 
  branches 
  from 
  this 
  axis. 
  

   They 
  are 
  given 
  off 
  from 
  single 
  exogenous 
  wedges 
  in 
  a 
  very 
  peculiar, 
  but 
  

   eminently 
  exogenous 
  manner, 
  the 
  details 
  of 
  which 
  are 
  given 
  in 
  the 
  

   memoir. 
  But, 
  besides 
  these, 
  other 
  clusters 
  of 
  vessels 
  are 
  given 
  off 
  which 
  

   have 
  no 
  exogenous 
  development 
  or 
  radiating 
  arrangement. 
  It 
  is 
  not 
  yet 
  

   clear 
  what 
  these 
  secondary 
  vascular 
  bundles 
  signify. 
  

  

  The 
  author 
  points 
  out 
  the 
  general 
  resemblance 
  between 
  this 
  develop- 
  

   ment 
  of 
  the 
  detached 
  exogenous 
  wedges 
  and 
  that 
  of 
  the 
  4-partite 
  woody 
  

   axes 
  of 
  the 
  Bignonias 
  of 
  Brazil, 
  demonstrating 
  at 
  the 
  same 
  time 
  their 
  

   very 
  marked 
  differences. 
  

  

  Though 
  no 
  traces 
  of 
  leaves 
  have 
  yet 
  been 
  discovered 
  in 
  connexion 
  with 
  

   these 
  stems, 
  the 
  author 
  has 
  very 
  little 
  doubt 
  that 
  they 
  belong 
  to 
  some 
  

   Lycopodiaceous 
  plant. 
  The 
  nature 
  of 
  the 
  vessels 
  and 
  the 
  simplicity 
  of 
  

   their 
  arrangement 
  alike 
  indicate 
  cryptogamic 
  features, 
  at 
  the 
  same 
  time 
  

   that 
  their 
  mode 
  of 
  development 
  indicates, 
  with 
  remarkable 
  distinctness, 
  

   that 
  we 
  have 
  here 
  another 
  example 
  of 
  that 
  exogenous 
  mode 
  of 
  develop- 
  

   ment 
  of 
  which 
  the 
  author 
  has 
  already 
  described 
  so 
  many 
  modifications 
  

   amongst 
  the 
  fossil 
  stems 
  from 
  the 
  Coal-measures. 
  The 
  occurrence 
  of 
  this 
  

   physiological 
  process 
  of 
  exogenous 
  growth 
  in 
  a 
  stem 
  which, 
  when 
  

   matured, 
  was 
  little 
  more 
  than 
  one 
  tenth 
  of 
  an 
  inch 
  in 
  diameter, 
  shows 
  

  

  