﻿30 
  MR. 
  A. 
  C. 
  SEWARD 
  ON 
  CTCADEOIDEA 
  GIGANTEA. 
  [Feb. 
  1897, 
  

  

  sponding 
  to 
  those 
  on 
  the 
  outside 
  of 
  the 
  Bennettites-hxmdle 
  (PI. 
  IV.. 
  

   fig. 
  10). 
  The 
  dark 
  radial 
  lines, 
  m, 
  are 
  no 
  doubt 
  rows 
  of 
  medullary- 
  

   ray 
  parenchymatous 
  cells. 
  Below 
  the 
  radially 
  -disposed 
  tracheids 
  y 
  

   the 
  tissue 
  is 
  very 
  indistinct 
  and 
  disorganized, 
  but 
  probably 
  there 
  

   was 
  in 
  this 
  position 
  a 
  group 
  of 
  centripetal 
  tracheids 
  corresponding- 
  

   to 
  those 
  shown 
  at 
  cj> 
  in 
  fig. 
  10. 
  In 
  longitudinal 
  section 
  the 
  

   imperfect 
  xylem 
  elements 
  are 
  seen 
  to 
  have 
  scalariform 
  thickenings 
  

   on 
  their 
  walls. 
  

  

  It 
  has 
  already 
  been 
  pointed 
  out 
  that 
  the 
  spaces 
  between 
  the 
  

   petiole- 
  bases 
  are 
  occupied 
  by 
  a 
  mass 
  of 
  scaly 
  structures 
  or 
  ramenta. 
  

   In 
  transverse 
  section 
  the 
  ramenta 
  have 
  the 
  form 
  of 
  short 
  and 
  tapered 
  

   cell-rows, 
  more 
  or 
  less 
  closely 
  crowded 
  together 
  between 
  the 
  petioles. 
  

   Longitudinal 
  sections 
  through 
  the 
  petioles 
  show 
  the 
  ramenta 
  attached 
  

   here 
  and 
  there 
  to 
  the 
  epidermal 
  layer, 
  from 
  which 
  they 
  arise 
  a» 
  

   laminar 
  outgrowths. 
  In 
  PI. 
  Y. 
  fig. 
  14 
  the 
  ramenta 
  are 
  seen 
  to 
  curve 
  

   towards 
  the 
  leaf-stalk 
  surfaces 
  at 
  cc, 
  whereas 
  in 
  the 
  greater 
  part 
  of 
  

   the 
  space 
  between 
  the 
  petioles 
  they 
  occur 
  as 
  closely-crowded 
  

   laminae, 
  r, 
  parallel 
  to 
  the 
  petiole-bases. 
  In 
  PI. 
  III. 
  figs. 
  1, 
  4, 
  & 
  8, 
  

   some 
  ramenta 
  are 
  shown 
  in 
  transverse 
  section 
  ; 
  they 
  have 
  the 
  form 
  

   of 
  a 
  single 
  layer 
  of 
  cells, 
  or 
  in 
  the 
  median 
  portion 
  they 
  may 
  be 
  

   two, 
  three, 
  or 
  more 
  cells 
  thick. 
  The 
  largest 
  in 
  fig. 
  4 
  has 
  a 
  length 
  

   of 
  *75 
  mm. 
  and 
  a 
  breadth 
  in 
  the 
  middle 
  of 
  '1 
  mm. 
  In 
  the 
  longi- 
  

   tudinal 
  section 
  the 
  cells 
  of 
  the 
  ramenta 
  are 
  long 
  and 
  narrow, 
  with 
  

   fairly 
  square 
  ends. 
  Precisely 
  similar 
  structures 
  have 
  been 
  previously 
  

   described 
  and 
  figured 
  by 
  Carruthers 
  and 
  others, 
  as 
  forming 
  a 
  kind 
  

   of 
  interpetiolar 
  packing 
  in 
  Bennettites. 
  1 
  The 
  ramentum 
  shown 
  in 
  

   PI. 
  III. 
  fig. 
  7 
  was 
  drawn 
  from 
  a 
  section 
  of 
  B. 
  Gibsonianus 
  ; 
  it 
  is 
  

   shorter 
  and 
  broader 
  than 
  most 
  of 
  these 
  structures, 
  which 
  are 
  

   practically 
  identical 
  in 
  form 
  in 
  the 
  Portland 
  stem, 
  in 
  Bennettites 
  

   Gibsonianus, 
  and 
  B. 
  Morierei? 
  No 
  doubt 
  the 
  compact 
  and 
  crowded 
  

   arrangement 
  of 
  these 
  epidermal 
  structures 
  between 
  the 
  petiole-bases 
  

   has 
  been 
  partly 
  brought 
  about 
  by 
  the 
  development 
  of 
  cork, 
  and 
  the 
  

   consequent 
  increase 
  in 
  thickness 
  in 
  the 
  persistent 
  leaf 
  -bases. 
  The 
  

   projecting 
  network 
  shown 
  in 
  the 
  photograph, 
  PL 
  I. 
  and 
  PI. 
  II. 
  

   fig. 
  2, 
  is 
  chiefly 
  made 
  up 
  of 
  silicified 
  ramental 
  tissue, 
  and 
  exactly 
  the 
  

   same 
  is 
  the 
  case 
  in 
  Bennettites 
  Gibsonianus 
  and 
  other 
  fossil 
  trunks. 
  

   The 
  crowded 
  and 
  narrow 
  cellular 
  plates 
  apparently 
  absorbed 
  the 
  

   mineralizing-solution 
  more 
  readily 
  than 
  the 
  tissues 
  of 
  the 
  petioles, 
  

   and 
  so 
  resisted 
  decay, 
  while 
  the 
  substance 
  of 
  the 
  petioles 
  has 
  been 
  

   for 
  the 
  most 
  part 
  destroyed. 
  The 
  comparatively 
  loose 
  mass 
  of 
  

   ramenta 
  probably 
  acted 
  like 
  a 
  sponge, 
  and 
  sucked 
  up 
  the 
  siliceous 
  

   solutions 
  with 
  avidity. 
  In 
  speaking 
  of 
  these 
  scaly 
  outgrowths 
  

   covering 
  the 
  basal 
  portions 
  of 
  the 
  petioles 
  in 
  Bennettites, 
  Graf 
  Solms 
  3 
  

   has 
  called 
  attention 
  to 
  the 
  striking 
  resemblance 
  which 
  they 
  bear 
  to 
  

   the 
  ramenta 
  or 
  paleae 
  so 
  abundant 
  on 
  the 
  leaf-stalks 
  of 
  many 
  ferns. 
  

  

  1 
  Cp. 
  Carruthers's 
  figures 
  {op. 
  jam 
  cit.), 
  pi. 
  lx. 
  figs. 
  7 
  & 
  11. 
  

  

  2 
  Lignier, 
  op. 
  cit. 
  pi. 
  i. 
  figs. 
  10-13. 
  

  

  • 
  3 
  Solms-Laubach, 
  ' 
  On 
  the 
  Fructification 
  of 
  Bennettites 
  Gibsonianus, 
  Carr. 
  r 
  

   Annul. 
  Bot. 
  vol. 
  v. 
  (1890-91) 
  p. 
  423. 
  

  

  