﻿28 
  HE. 
  A. 
  C. 
  SEWAED 
  ON 
  CYCADEOIDEA 
  GIGANTEA. 
  [Feb. 
  1 
  89 
  7, 
  

  

  secondary 
  tissue, 
  while 
  the 
  absence 
  of 
  such 
  infilling-material 
  in 
  the 
  

   cork 
  tissue 
  is 
  in 
  accordance 
  with 
  air-containing 
  periderm-cells. 
  The 
  

   epidermal 
  cells 
  are 
  shown 
  at 
  e 
  in 
  PL 
  III. 
  fig. 
  1, 
  and 
  what 
  is 
  probably 
  

   the 
  remains 
  of 
  the 
  cuticle 
  at 
  c. 
  In 
  fig. 
  3 
  the 
  cork-cambium 
  cells 
  

   are 
  shown 
  under 
  a 
  higher 
  power. 
  From 
  the 
  relative 
  position 
  of 
  the 
  

   partially 
  disorganized 
  epidermis 
  and 
  its 
  underlying 
  periderm, 
  it 
  would 
  

   seem 
  that 
  the 
  phellogen 
  arose 
  in 
  the 
  subepidermal 
  cell-layer. 
  In 
  

   PL 
  III. 
  fig. 
  2 
  is 
  shown, 
  on 
  the 
  same 
  scale 
  as 
  fig. 
  1, 
  a 
  section 
  through 
  

   the 
  peripheral 
  portion 
  of 
  a 
  petiole 
  of 
  Macrozamia 
  Denisoni, 
  Moor 
  & 
  

   MuelL, 
  cut 
  from 
  the 
  persistent 
  base 
  of 
  a 
  leaf-stalk 
  which 
  has 
  re- 
  

   mained 
  attached 
  to 
  the 
  stem 
  after 
  the 
  fall 
  of 
  the 
  frond. 
  The 
  larger 
  

   phelloderm-cells 
  in 
  the 
  lower 
  part 
  of 
  the 
  section, 
  jphd., 
  pass 
  gradu- 
  

   ally 
  into 
  the 
  parenchyma 
  and 
  the 
  ground-tissue, 
  g, 
  and 
  on 
  the 
  left 
  

   of 
  the 
  figure 
  are 
  seen 
  four 
  thick- 
  walled 
  sclerenchymatous 
  cells, 
  s. 
  

   External 
  to 
  the 
  phelloderm 
  there 
  is 
  the 
  cork-cambium, 
  followed 
  by 
  

   the 
  regular 
  rows 
  of 
  flattened 
  and 
  somewhat 
  crushed 
  elements 
  of 
  the 
  

   cork, 
  ck 
  ; 
  finally 
  the 
  torn 
  epidermis 
  and 
  thick 
  cuticle 
  are 
  found 
  in 
  

   the 
  petiole- 
  surface 
  (e 
  & 
  c). 
  In 
  another 
  part 
  of 
  the 
  longitudinal 
  

   section, 
  shown 
  in 
  PL 
  V. 
  fig. 
  14, 
  the 
  epidermis 
  of 
  the 
  petioles 
  has 
  

   been 
  bent 
  in 
  such 
  a 
  manner 
  as 
  to 
  appear 
  in 
  some 
  places 
  in 
  surface- 
  

   view, 
  in 
  precisely 
  the 
  same 
  way 
  as, 
  in 
  a 
  badly-cut 
  transverse 
  section 
  

   of 
  a 
  fresh 
  leaf 
  or 
  twig, 
  the 
  epidermal 
  layer 
  is 
  often 
  bent 
  over 
  instead 
  

   of 
  being 
  cut 
  through 
  transversely. 
  "Where 
  the 
  epidermis 
  has 
  been 
  

   thus 
  folded 
  over, 
  the 
  component 
  cells 
  are 
  seen 
  to 
  be 
  polygonal 
  or 
  

   square 
  in 
  form 
  and 
  to 
  have 
  straight 
  walls, 
  as 
  in 
  most 
  recent 
  Cycads. 
  1 
  

   Several 
  stomata 
  stand 
  out 
  clearly, 
  their 
  position 
  being 
  readily 
  recog- 
  

   nized 
  by 
  the 
  occurrence 
  of 
  comparatively 
  large 
  and 
  dark-coloured 
  

   guard-cells. 
  In 
  PL 
  V. 
  fig. 
  15 
  a 
  stoma 
  and 
  some 
  of 
  the 
  neighbouring 
  

   epidermal 
  cells 
  are 
  represented 
  ; 
  the 
  two 
  conspicuous 
  guard-cells, 
  gd, 
  

   •055 
  mm. 
  in 
  length, 
  have 
  somewhat 
  shrunken 
  cell-contents, 
  and 
  

   the 
  oval 
  brown-coloured 
  bodies 
  in 
  the 
  lower 
  cell 
  bear 
  a 
  striking 
  

   resemblance 
  to 
  the 
  chloroplasts 
  in 
  the 
  guard-cells 
  of 
  recent 
  stomata. 
  

   It 
  is 
  by 
  no 
  means 
  impossible 
  that 
  the 
  substance 
  of 
  the 
  chlorophyll- 
  

   corpuscles 
  has 
  been 
  preserved 
  by 
  the 
  mineralizing 
  material 
  in 
  such 
  

   a 
  way 
  as 
  to 
  retain 
  in 
  the 
  fossil 
  condition 
  the 
  original 
  form 
  and 
  

   arrangement 
  of 
  these 
  constituents 
  of 
  the 
  living 
  cell. 
  

  

  The 
  forms 
  assumed 
  by 
  siliceous 
  or 
  calcareous 
  substances 
  in 
  the 
  

   tissues 
  of 
  fossil 
  plants 
  are, 
  however, 
  often 
  difficult 
  to 
  interpret, 
  and 
  

   one 
  cannot 
  always 
  determine 
  how 
  far 
  the 
  original 
  cell-contents 
  have 
  

   been 
  the 
  cause 
  of 
  the 
  form 
  assumed 
  by 
  the 
  material 
  in 
  the 
  petrified 
  

   cells. 
  Between 
  the 
  two 
  guard- 
  cells, 
  a 
  small 
  elliptical 
  median 
  

   aperture 
  is 
  clearly 
  seen, 
  especially 
  on 
  focussing 
  down 
  to 
  a 
  slightly 
  

   lower 
  level 
  ; 
  this 
  is 
  the 
  entrance 
  to 
  the 
  respiratory 
  cavity. 
  On 
  

   either 
  side 
  of 
  this 
  there 
  appears 
  to 
  be 
  a 
  subsidiary 
  cell 
  (s). 
  As 
  

   no 
  stoma 
  is 
  cut 
  through 
  vertically, 
  it 
  is 
  difficult 
  to 
  be 
  quite 
  certain 
  

   as 
  to 
  the 
  structures 
  observed 
  in 
  surface-view 
  ; 
  possibly 
  the 
  apparently 
  

   lower 
  cells, 
  s, 
  are 
  really 
  part 
  of 
  the 
  inwardly 
  curved 
  guard-cells. 
  

  

  1 
  See 
  Bornemarm, 
  ' 
  Uebev 
  organische 
  Reste 
  der 
  Lettenkohlengruppe 
  Thiirin- 
  

   •gens,' 
  Leipzig, 
  1856 
  ; 
  and 
  Kraus, 
  Pringsh. 
  Jabrb. 
  vol. 
  iv. 
  (1865-66). 
  

  

  